I don’t buy this explanation.
The FM modulation uses a much higher bandwidth than AM. The distance between channels on FM radio is 200 kHz compared to only 9 kHz on AM. That’s more than 20x more bandwidth for FM. On AM, no matter how deeply you modulate the carrier, the bandwidth will not exceed twice the bandwidth of the input signal. On FM, the deeper you modulate it, the wider the output spectrum will be, and it can easily exceed the bandwidth of the input signal.
In addition to that, the whole FM band is much higher frequency, while I guess quite a lot of noise, especially burst noise caused by eg thunderstorms is relatively low frequency. So it’s not picked up because it’s out of band.
Any noise that falls inside the channel does get picked up by the receiver regardless of modulation. However because the available bandwidth is so much higher than the real bandwidth of the useful signal, there is actually way more information redundancy in FM encoding, so this allows to remove random noise as it will likely cancel out.
If I encoded the same signal onto 20 separate AM channels and then averaged the output from all of them (or better - use median filter) that would cancel most of random noise just as well.
Also another thing with modulation might be that if there is any narrow-band non-white noise happening to fall inside the channel (eg a distant sender on colliding frequency), on AM it will be translated as-is to the audible band and you’ll hear it as a single tone. On FM demodulation it will be spread across the whole output signal spectrum, so it will be perceived quieter and nicer by human ear, even if its total energy is the same. That’s why AM does those funny sounds when tuning, but FM does not.
The wider channels is the source of the available audio fidelity, but wider channels make you more exposed to noise, not less. A wider channel means listening to more noise sources, and having transmitter power stretched thinner for a much lower SNR.
In other words, the noise rejection of FM is what enabled the use of wider channels and therefore better audio quality. An analog answer before digital error correction.
In FM, the rejection is so strong that if you have two overlapping transmissions, you will only hear the stronger one assuming it is notably stronger. This in turn is why air traffic still use AM where you can hear both overlapping transmissions at once (possibly garbled if carrier wave was off), and react accordingly rather than being unaware that it happened.
Technology moved on from both plain AM and plain FM a long time ago, and modern “digital” modulation schemes have different approach to interference rejection.
Shannon theorem disagrees with you. The wider the channel, the MORE noise you can tolerate when transmitting signal at a given data rate.
In audio, the amount of information you need to transmit is naturally limited by the audio bandwidth (for FM truncated at about 15 kHz), so the useful signal bandwidth is fixed. Hence, if you transmit the same audio band over a broader channel of frequencies, you can tolerate more noise; or, for the same density of noise in the channel, you can get better SNR at the output. This is exactly what FM does. It uses the information multiplied in the most of that 200 kHz channel and projects it on 0-15 kHz band.
While you are right that a wider channel captures more noise in total, noise does not add up the same way as useful signal, because it’s random. Doubling the channel width only increases the amplitude of noise by sqrt(2).
There is no “magic noise rejection” coming from different ways of modulating the signal if all other things are the same. You can’t remove noise; you can’t magically increase SNR. If anything, FM makes the noise more pleasant to listen to and perceivably quieter by spreading non random, irregular noise over the whole band so it sounds more like white noise.
But it also allows to use wider channels, and increase the fidelity of the signal, including increasing SNR. But that’s thanks to using significantly wider channels than audio.
Also, it’s not like FM can use wider channels because of better SNR. FM can use wider channels because of how this modulation works - the spectrum of FM signal can be arbitrarily wide, depending on the depth of modulation. AM cannot do that. It only shifts the audio band up (and mirrors on both sides of the carrier). It can’t “spread it”.
Btw: this is a very similar phenomenon as when you average multiple shots of the same thing in photography, eg when photographing at night. By adding more frames (or using very long exposures) you obviously capture more total noise, but the amount of useful signal grows much faster because signal is correlated in time, but noise is not.
It’s not immediately clear that Shannon’s theorem is a good point of comparison here, since it’s only recently that coding schemes have really approached the Shannon limits, and FM and AM do not use these.
Even if one does assume a Shannon-perfect coding scheme, as the noise ratio gets greater the benefits of spreading a signal across a higher bandwidth fades. Furthermore, most coding schemes hit their maximum inefficiency as the signal to noise ratio decreases and messages start to be too garbled to be well decoded.
I’d additionally note that folks get near the Shannon noise limit _through_ ‘magic noise rejection’ (aka turbo and ldpc codes). It’s therefore not obvious that FM isn’t gaining clarity due to a noise rejection mechanic. The ‘capture effect’ is well described as an interference reducing mechanism.
Empirically, radio manufacturers who do produce sophisticated long range radio usually advertise a longer range when spreading available power across a narrower rather than wider bandwidth.
Well, yes and no. It's a bit more complex than just taking the numbers from Shannon-Hartley, but I admit that my original description was at best lacking, so thank you for pointing that out.
Shannon-Hartley describes that the theoretical information capacity of a signal given a bandwidth and an SNR. Doubling bandwidth halves your SNR (received noise increases, received signal does not), in turn reducing the bits gained per unit of bandwidth. At very high SNR, doubling bandwidth almost doubles capacity, but as SNR goes down, the benefit of additional bandwidth levels off until bandwidth no longer has any effect.
However, this provides the number achievable by a perfect modulation scheme using all available bandwidth and signal strength. AM and FM are both incredibly inefficient, and more importantly have very different reactions to noise - something Shannon-Hartley does not concern itself with.
With truly random noise, FM and AM noise both scale based on noise amplitude as you say. In AM, all noise overlapping with the band is played back verbatim, whereas in FM only the noise causing frequency variations in the carrier wave have any effect on the signal, and end up with a non-linear response to noise.
However, we do not deal with purely white noise, and FM has far superior handling of non-random noise. In order to have any effect, it need to either induce frequency shifts to the carrier wave, or have enough power to cause the interference to be captured instead. There's also the far higher power efficiency, as FM puts all its power into the signal, whereas traditional AM puts most of it into a useless carrier and wastes half the remaining power on the redundant sideband (yes, SSB is a thing). These were certainly also factors in FMs demise.
A simpler means to remove bandwidth from the equation would be to compare with a narrow-band FM transmission, or by multiplying the input waveform for an AM transmitter by some factor to fill the bandwidth. I believe FM should still handily beat it at least above its threshold. I don't see anyone giving exact numbers of this though, so I guess it could be a fun SDR project for someone wanting to prove either point. :)
(Neither AM nor FM is of anything but historic value at this point - their only redeeming quality is discrete circuit simplicity if you need to MacGyver one out of shoelace and bubblegum, but that's it.)
> …traditional AM puts most of it into a useless carrier and wastes half the remaining power on the redundant sideband.
I had not thought about this before, and I have no intuition as to what the answer is, but does the redundant sideband have any effect, positive or negative, on noise rejection?
You are applying Shannon theorem incorrectly. Both AM and FM modulations are nowhere even remotely close to using their bandwidth with 100% efficiency, due to technology costs, and the difference in modulation is crucial. The article is correct and the mathematical models of AM and FM are well understood since decades.
Where did I say AM and FM are close to 100% efficiency?
I was only replying to an obviously incorrect statement that by using more bandwidth you decrease SNR. If it were the case, Shannon theorem would not work.
It doesn’t matter how close to the limit your encoding is, whether it is 20% or 99% the relationship between the bandwidth, noise floor and how much data you can send stays the same - by increasing bandwidth you can usually send considerably more information even if your encoding is poor. Which in translates to either a wider useful bandwidth or lower noise floor or any combination of both.
A trivial thought experiment to illustrate this: For any analog encoding, if I double the transmission bandwidth by encoding the same signal over 2 channels instead of one, I can average the output signal coming out the receivers and get better SNR than using one channel and one receiver. That works regardless of AM, FM or whatever fancy encoding you could use.
That's not how this works. That's not how any of this works. Averaging a high SNR channel with a low SNR channel is likely to produce something less good than the high SNR channel. Could you get an improvement over the high SNR channel? Yes, and the limit of the improvement is related to the SNR of each and averaging the signals won't get you anywhere near that.
Averaging two noisy signals increases SNR. That’s not even a thought experiment, that’s a reality. This is a technique used by probably all modern smartphone cameras to do night photos, as well as a common technique used by astrophotographers. Instead of taking one picture, you take a series of pictures and then align them and average. This improves SNR dramatically. A very long time ago we used this technique to get razor sharp, low noise pictures of the Moon at 3k x 3k resolution using… a cheap VGA internet camera: https://astronet.pl/wydarzenia/n2309/ Note that cameras at those times were barely
capable of doing videoconferencing in artificial evening light - what you saw was mostly noise. Those sensors were really, really terrible.
What you seem to be missing is the fact we’re talking here about transferring the same fixed bandwidth signal over a wider channel, not transferring a wider bandwidth signal over a wider channel.
// edit: just noticed someone else gave another nice application of this phenomenon: GPS
> This is a technique used by probably all modern smartphone cameras to do night photos, as well as a common technique used by astrophotographers...
I think this is a lot simpler because each of your pixels is assumed to have a single, correct DC value. This doesn't hold for a time varying signal like AM/FM.
They're thinking about when you sample from the same noise distribution, averaging gives an unbiased estimator of the mean. But when you know one SNR is higher than the other, maybe this doesn't hold? But maybe if you transform the distributions to look the same, thus taking a weighted average? I'm not sure.
Where are you getting 1.41x? What you'd really like to increase is the SNR. As you open up the bandwidth, the amount of energy you can collect in your band increases, but there's no way to collect the energy from only the signal and not collect the energy from noise. So as you increase your bandwidth, your SNR stays the same.
Not all noise is gaussian. And the fact that the noise is random while the signal is not, is useful when you can average and drop your noise floor. But you need multiple measurements to do that.
> (...) use AM where you can hear both overlapping transmissions at once
Yes. Assuming signal strengths for both are comparable. Say, within 20 dB of each other.
> (possibly garbled if carrier wave was off)
Nah. If both stations have sufficient energy fall into receiver's bandwidth window (IF filter for analog receiver), no garbling. If one of stations has carrier sufficiently off to fall entirely outside IF, only other will be audible.
You are probably thinking about SSB, where two stations with carrier offset indeed produce weird sounding interference.
Interesting. However, if one of those stations runs on a slightly different frequency, I guess its output would be garbled, correct? Like I guess SSB receiver just shifts down the band by a constant?
If it's kind of close it just sounds like someone talking in a slightly lower or higher pitch. It can still be pretty intelligible with the frequency slightly off. Eventually it gets very distorted though and you start losing a big part of the waveform entirely. Try listening in to some websdrs and you'll see what it's like.
The wider channels is the source of the available audio fidelity, but wider channels make you more exposed to noise, not less.
From a signal:noise perspective, what matters is the ratio of bandwidth available in the transmission channel to the bandwidth of the content you are trying to send. Consider GPS, for instance, where the use of a 2 MHz channel to send 50 bps data provides an SNR advantage that would otherwise be achievable only through witchcraft.
FM has strong noise immunity advantages -- notably AM rejection and the capture effect -- but they don't provide additional sound quality by themselves. That's where the bandwidth helps. An FM channel that is only as wide as an AM channel would sound pretty awful.
> FM has strong noise immunity advantages -- notably AM rejection and the capture effect -- but they don't provide additional sound quality by themselves. That's where the bandwidth helps. An FM channel that is only as wide as an AM channel would sound pretty awful.
Comparing such low-modulation factor FM with traditional AM would be an interesting experiment.
It certainly wouldn't sound good, but I'm not sure it would sound worse than traditional AM at the same SNR. The NFM use-cases I'm familiar with tend to cap audio bandwidth, so they're not fair comparisons.
I'm mostly imagining what music would sound like via NBFM on a VHF amateur or public-safety radio channel. It's not an appealing thought... the words "toll quality" come to mind.
> This in turn is why air traffic still use AM where you can hear both overlapping transmissions at once (possibly garbled if carrier wave was off), and react accordingly rather than being unaware that it happened.
I’m not convinced this is the reason. The carrier wave is always off by a little. While you’re transmitting you hear nothing anyway. And when two parties are transmitting simultaneously, any third parties just hear very loud screeching. A 0.001% difference in carrier frequency would be more than enough to cause this effect in a VHF radio. Notably, this exact problem was a major contributing cause to the worst accident in aviation history. Using FM would have prevented it.
AM is used for two reasons - simplicity of transceivers
AND the fact that two simultaneous transmissions result in buzz instead of locking onto stronger signal. We WANT to know that there's a collision in transmission so that we know we need to retransmit. What would be the expected effect if two FM transmission on same channel were sent?
Fixing the "glitch" would result in way more problems than it solves. Interestingly, aviation authorities do not blame collission behaviour of AM radio for Tenerife, but instead corrected crew management procedures and pushed greater radio phraseology standardisation.
>We WANT to know that there's a collision in transmission so that we know we need to retransmit
Digital trunked public safety systems solved this problem decades ago. If you key up when the frequency is in use you get a distinct rejected tone. I'd think prevention is far preferable to sorting it out once everyone's finished walking on each other.
It also means you need to replace everyones radio at the same time because everyone needs to hear everyone on the channel.
Where new additional technologies are possible, they have been applied (digital packet networks, like with CPLDC - Controller-Pilot Data Link Communications).
Replacing A3E modulated VHF radio requires you replace it for literally everyone, because there are way more users at airport than you think.
> It also means you need to replace everyones radio at the same time because everyone needs to hear everyone on the channel.
In the public safety context it's not uncommon to phase in new systems (like digital trunked systems) incrementally. You accomplish that by simulcasting the dispatch audio over both systems, and monitoring incoming audio from both systems.
A common pattern for how this plays out would be something like this: all the fire departments and ems agencies in a given jurisdiction are dispatched using two-tone (eg, motorola) paging over a VHF frequency. New digital radios are introduced, and all the fire/ems personnel keep their existing pagers, and (some|most|all) are given the new digital radios. People without the new radios can still talk to dispatch using VHF. And of course systems can be configured to mirror audio around so that if one person is transmitting on VHF they can be heard on the digital system (usually on a channel in the 800mhz or 900mhz band). It's basically a fancy version of a repeater.
Dispatches are then given out over the same old VHF channel AND the new digital channel. In theory you can eventually replace all the old pagers and radios and quit with the simulcast deal, but IME, sometimes things stay in "parallel" mode more or less indefinitely for whatever reason[1]. That said, to your original point, you typically do want to get at least radios standardized as much as possible, even if you maintain the split for (paging|operational communications).
To illustrate, two jurisdictions I'm familiar with: Orange County NC, and Brunswick County, NC. Both followed the path I talked about above: all VHF dispatch for fire/ems, then adopted the NC VIPER digital trunking system, but continue to page on VHF and simulcast the dispatch information over both channels. I'm not sure exactly when Orange County adopted VIPER but it's been quite some time and they're still doing both. FSM only knows if/when they'll ever completely abandon the old VHF system.
[1]: and that reason is often as simple as "money". Plenty of volunteer fire departments in rural areas are skating by with barely enough money to keep their apparatus road-worthy. Replacing every hand-held and mobile radio they own in one fell swoop is often out of reach.
[Source: was a firefighter and 911 dispatcher in a previous life]
You're writing about experience in a closed system - as far as I know all such dispatch systems for public safety etc are closed system where everyone who is ever going to be on the net is part of the system, and it might at most be a case of "we don't have money to replace every member's radio".
In comparison, aviation radio is an open system - not only you do not know who is going to communicate, the communication is also peer to peer, unlike many digital trunked systems which often depend at least on some level of cellular support system.
The only "access control" on the airband VHF and HF comms is of legal variety, with explicit carve out that the person actually flying the aircraft is way less bound by legalities in case of emergencies, and everyone has to be able to talk with everyone, especially on one of the standard common channels.
Examples from personal experience involved various combinations of small airfield ATZ, MiG-29, gliders, old ursus tractor (agricultural kind), busted up Opel Kadett, airliners, ultralights, small transport planes, private helicopters, and dunno who was responsible party but helicopter working as diplomatic flight.
All on one small airfield. And every one of those had to communicate independent of each other with everyone else on that list.
The only time we do "rebroadcast" is when we end up having to do a manual relay due to distance, which is also one of the rare cases where comms might switch over to a more modern system, because someone could ask ATC over VHF to pass something over CPDLC to airliner or using HF, and vice versa.
The poor A3E modulation on VHF airband is the lingua franca, the lowest common denominator, which allows random aircraft from anywhere in the world talk to another random aircraft, as well as ground.
There's cost in simultaneous replacement for huge portion of the fleet.
Don't devolve into simplism, consider that you need to replace the radio for everyone sharing the same space, and that there might be way more planes sharing that space than you think.
And who would pay for the Supplementary Type Certificate for every single aircraft model out there, including many that were built by manufacturers that no longer exist? I don't think you understand how this stuff actually works.
That article makes out as if transmission blocking leads to a safety problem if a transmission gets lost. It doesn't. What that article misses is that aviation radio communications require readback and verification of the readback, in safety critical instructions such as "cleared for take-off". Not just for radio transmission blocking reasons, but also to detect mistakes in mishearing instructions.
Since then, the word "take-off" is avoided in any other type of communication (eg. you might hear "report ready for departure" but never "report ready for take-off"), and every pilot knows never to assume that a clearance has been given unless they hear those exact words together with their callsign.
FM has 15kHz of bandwidth per stereo channel or an effective 30kHz sampling rate. The rest of the space is used for supplemental signals, including, the "pilot carrier" that is used to generate the "stereo image." There is space for three more full bandwidth mono channels on the end of an FM broadcast. One of them is often used for RBDS.
FM signals receive AM interference but heterodynes exclude them effectively. The cost is vulnerability to multipath reception in highly signal reflective environments and capture/wandering effects when two signals of similar strength are present.
AM _can_ sound pretty good. Most AM transmitter sites are poorly maintained, combined with other stations into one antenna system (something you can do on AM with a phasor), and are typically just simulcasts of FM content or satellite delivered content. There's no real care put into it. On a well maintained, tuned, and properly programmed station, mono content on AM sounds quite pleasant.
That's not even getting into "cost saving" measures that AM operators employ that completely compromise their signals. Or what Nielsen has convinced them to inject into their signals to register modern "ratings points" from the "portable people meter" system.
FM has 15 kHz of bandwidth available for the audio signal, which is much higher than what had been previously standardized for the AM channels and which is an important reason for the perceived high fidelity.
The modulated signal that is transmitted on the air has a much higher bandwidth. How much higher may differ between various broadcasting standards, but it can be e.g. 10 times or 20 times higher.
The ratio between the bandwidth of the transmitted radio signal and the bandwidth of the audio signal is what is relevant for the noise rejection properties of FM broadcasting.
When the bandwidth available for transmission is limited, FM is not an optimal kind of modulation from the point of view of resistance to noise, phase modulation (QPSK) is better (and optimum), so that is what is used for digital communications limited by noise.
> The ratio between the bandwidth of the transmitted radio signal and the bandwidth of the audio signal is what is relevant for the noise rejection properties of FM broadcasting.
Yes. FM radio is "narrow band" which gives it additional noise rejection properties; however, it's measured against the total available signal content not merely the audio portion of the content.
So, your pilot wave and RBDS and any additional carriers, if present, reduce this facility.
> FM is not an optimal kind of modulation from the point of view of resistance to noise, phase modulation (QPSK) is better
FM receivers often move and are often in highly reflective environments. FM is far better suited to this than plain PM.
Going back to first principles, modulating the frequency instead of the amplitude inherently makes the system less lossy. Imagine you were communicating with someone miles away on a hilltop, and they had a lot of data to convey. Would you find it easier to distinguish signal vs. noise if the light was increasing and decreasing rapidly in brightness (AM) or color (FM)?
If you AM modulate the carrier f0 by a single tone f1, you get a spectrum with only three tones: f0 - f1, f0, f0 + f1. If you FM modulate it, well the thing gets much more complex - depending on the depth of modulation you can get a much wider spectrum than from f0-f1 to f0+f1. So it is hard to compare. Your FM modulated signal may indeed be more resilient to noise but will require wider channel to be properly transmitted.
I haven’t seen a convincing explanation if FM would be really that better than AM if both were given exactly the same channel width.
Neither is true. 9kHz, with two sidebands, means that the transmitted audio is limited to 4.5kHz, which is way too low to sound good. It was this, and not the noise, that made it sound much worse.
While one reason for limiting the audio bandwidth to 4.5 kHz was to allow a great enough number of channels in the long wave and medium wave bands, the second reason was to be able to reject the high frequency noise by low-pass filtering.
So there were two reasons for the low audio fidelity of AM broadcasting, and noise was one of them, with the contention between multiple broadcasters for the narrow available bands being the other.
> with the contention between multiple broadcasters for the narrow available bands being the other.
A non-obvious aspect of medium and long wave AM broadcast is depending on weather/atmospheric conditions a signal can propagate much further than its output power would suggest. This means a distant station on the same channel as a near station may end up in contention at certain times of day or random conditions. Solar flare? Suddenly stations a hundred miles away are overpowering local stations or just adding a lot of noise.
Medium and long wave is also susceptible to local EM sources like switching power supplies and electric motors. So you can get the double whammy of local noice and distant stations adding additional interference to local stations.
There is no reason that the channel spacing need limit the sideband bandwidth.
The only downside to this is that listeners on adjacent stations hear a slight "monkey chatter" from the overlapping sidebands.
This is one of many reasons why station frequencies are never allocated close to stations which are physically close.
You only need glance at the waterfall display on a good SDR receiver to see that the actual audio bandwidth is often much wider than the channel spacing implies.
Someone gave me an analogy some time ago that made a lot of sense.
If you shine a flashlight through a tree blowing in the wind and vary the brightness to convey information, the signal can get distorted pretty easily.
However, if you have a constant brightness source and vary the color, it’s a lot easier to figure out what the source is trying to convey.
Except that color isn’t the same thing as wavelength when it comes to humans perceiving light, because our eyes only deal with the total energy within each of three overlapping bands. An FM receiver knows the difference between a single carrier varying in frequency, and two carriers of different frequencies varying in proportion. Our eyes don’t, hence the banner above appearing orange, even though it’s actually made of different proportions of red and green.
But that’s not how FM receivers generally work. They don’t just take a chunk of the spectrum and measure relative amplitudes within that window. Some very quick and dirty FM demodulators do something like that, but they have poor noise rejection, so the analogy fails.
Proper receivers use a phase-locked loop to “lock on” to a carrier, rejecting any weaker interference on nearby frequencies.
In the analogy, suppose you’re decoding a signal from a flashlight over the entire color spectrum, but sunlight shines through the leaves of the tree, adding a slight green noise component to what you see, while the flashlight is actually red. You’ll erroneously interpret the signal as slightly yellow.
We don’t have anything like the PLL in our eyes, so the analogy breaks down here. In the equivalent scenario with an actual FM signal, that slight “green” component would not affect the received signal (or it would affect it to a much lesser extent).
This makes a lot of sense so long as your source of noise is something like a tree swaying in the wind, ie something that interferes with the amplitude. If instead the source of noise is uhhh a piece of stained glass swaying in the wind then blinking the flashlight is the better bet. I guess it just turns out radio interference is more like the tree. But why?
Stained glass won’t (I think) shift any frequencies. It will attenuate different frequencies differently, but it won’t make up new ones.
So when the signal frequency changes, you’ll still see that change, but the light might get brighter or dimmer at the same time due to the stained glass. But you don’t care about the brightness to begin with.
In this analogy, the AM and FM signals you receive aren't usually experiencing interference, they are experiencing multipath effects which includes things like path loss, attenuation, reflections, and so on. This is driven by geometry. You also have gaussian noise that the receiver has to deal with.
You model this by taking your signal and convolving it with the channel vector. Usually the channel vector is a finite number of dirac deltas. Each delta is a different reflection. They are like echos. They can cause the signal to constructively and desconstructively interfere with itself.
I haven't seen the math, but I am guessing this doesn't do as much to the frequency of the signal compared to the amplitude.
a better analogy for frequency domain interference would be something like the spinning flashing lights on a fire engine or utility truck occasionally shining colored light on your detector.
I think this is reasonable analogy, for FM interference, and it points out why FM is resilient to noise: the flashing lights have to be relatively bright (high amplitude) to interfere with your color based scheme.
The stained glass would change the amplitude of some light selectively. But because the FM radio works at different distances, I wonder if it must have some way of adjusting for different amplitudes anyway?
Yes, stained glass is like band filter, they let through a particular frequency range, while reducing those outside the range. Your FM receiver will still lock on the desired frequency as long as their is enough signal strength. It's kind of the same as listening to an emitter that is very far while being very close to another. Of course, it'll stop to work at some point depending on minimum signal-to-noise ratio.
It _is_ the wrong feel for a concept. The analogy breaks down because the color changes are way too wide in frequency (and thus too robust to noise) compared to what happens in a radio broadcast. If you changed the color from RGB(127, 0, 0) to RGB(126.999999, 0.000001, 0), the movement of that tree would actually start to make your strategy difficult.
Going from red to orange is about 50 THz. Typical FM radio modulation width is 100 kHz.
The analogy isn't wrong, it's just that it is incomplete as given. You are bringing the sensitivity of the receiver into the equation. But it doesn't really break down the analogy, because the frequency shift in color is calibrated for the human eye's sensitivity. Calibrate it for a FM receiver and that tiny color shift becomes easily discernible. The tree leaves have no impact on frequency, just amplitude.
The reason the analogy is good is because it isn't even really an analogy, it is in fact a description of electromagnetic waves and a noise source.
The flashlight is the radio tower, the tree is the tree, and the radio in the car is your eyes. There is no analogy here, it is literally the same EM waves shifted up to where our eyes can see them.
It's like saying that the violins is merely an analogy for how a double base works.
a lot of people here have a lot of passions, but sometimes the passions overlap and we rub shoulders. if someone had made a pokemon playing card metaphor we might be in the same general condition - but i think we're better behaved showing each other how smart we must be with radio waves instead of greymon
Both are examples of communication by means of frequency modulated and amplitude modulated electromagnetic waves with distortion from a moving three. Also a good example that a large change in quantity is a change in kind. Probably a legit analogy imho.
It’s a terrific analogy. OP is arguing that it isn’t an analogy but an identity. For what should be obvious reasons, it isn’t. And in this case, the difference between analogy and our best model of reality is material.
But if you say that a battleship floats on the water in a similar way to a rubber duck floating in the water... it's actually not similar... they are the same. It's the same water and the same physics. The "only" appreciable difference is scale.
For me, the people saying they are the literal same thing are the same type of people that gave me that "aha" moment that really helped solidify my understanding of RF.
It was pretty mind blowing when I Understood that AM is a change in brightness and FM was a change in color. We just can't see RF, but if we could, that's what it would be.
> But if you say that a battleship floats on the water in a similar way to a rubber duck floating in the water... it's actually not similar... they are the same. It's the same water and the same physics. The "only" appreciable difference is scale.
But battleship doesn't equal floating in water, floating in water is a property it has.
If you're saying "the way a battleship floats in water is like how a rubber duck floats in water" then it's not an analogy, it's as you say just describing two versions of the same thing.
But it is an analogy to directly compare the two objects, because "floating on water" is a property of the objects it's not the object you are comparing.
Wikipedia begins its page on analogies with this, sourced from The Oxford Companion to the English Language: "Analogy is a comparison or correspondence between two things (or two groups of things) because of a third element that they are considered to share."
Or Marriam-Webster: "a comparison of two otherwise unlike things based on resemblance of a particular aspect"
Apart from rubber ducks and battleships both having the "third element", or aspect, of "primarily used for floating on water", they are definitely two
completely different things. Nobody could look at a rubber duck next to a warship and say "they seem to be the same thing".
The more closely related two things are the more useful and less stretched the analogy available, which is why the analogy about radio waves was so enlightening to so many people in this thread. But it's bang on as the definition of what an analogy is.
> Visible light and radio attenutate in meaningfully different ways. It's an analogy.
Lol news to me and my physics degree, Do tell because as far as I'm aware Maxwell's equations don't have an asterisk on them that say "doesn't work below 1 GHz".
To accurately model EM waves, you need more than just Maxwell's equations. You require material equations to model interactions of EM with media.
If you want to get really advanced, whereas Maxwell's equations are classical physics, there's Quantum electrodynamics (QED) which can model interactions of EM and matter.
Same with communications over coax. Obviously visible light doesn't transmit well over copper, but a spectrum of radio waves do, some better than others.
Fiber optics also uses _exceptionally_ clear glass.
If the ocean were as clear as your average long-distance fiber cable, you would see down to the bottom of the Mariana Trench (also in the range of visible light, AFAIK).
Silica glass behaves differently from ZBLAN (fluorozirconate glass).
Which goes to show how complicated EM interactions with media can be. It's generally easier to just empirically measure attenuation through some medium and use the empirical measurements as a model.
It's exceptionally clear compared to e.g. window glass even in the visible spectrum of light. You can shine a red light source into a 10 kilometer standard G.657 fiber (optimized for 1310/1550nm, i.e. deep infrared) and it will still be visible just fine on the other end. If you did that with regular glass, it would hardly go ten meters.
What are the relative contributions of the total internal reflection property of the fiber optic cable and the particular low-attenuation material it's made of?
Oh yeah. I'm not saying otherwise. Someone replied "Clear glass blocks about 5% visible light". I guess "clear glass" is pretty subjective. At what level of attenuation would someone consider glass not clear? xD
> It's amazing how condescending some people on here are
You literally started your comment with "Lol news to me", then you used your degree as if it made you more knowledgeable than anyone else here. Take a look in the mirror?
> ... Do tell
I did?
The extra information isn't to condescend. It's for other people that want to know more about the science.
It's not saying the violin IS "merely an analogy for how a double base works", just that a violin can be used as a simple analogy to somebody who understands how a violin works but doesn't know what a double bass is.
Comparing similar things is literally what an analogy is, the fact that in these two cases (radio/light and string instruments) the things being compared are very similar it doesn't make them the same thing, nor does it make it not an analogy.
It is not literally the same. The colors you perceive on the screen in front of you demonstrate why. That banner above is not orange; it’s red and green. You don’t actually have the ability to distinguish a varying frequency between red-orange and yellow-orange, and two amplitude modulated “carriers” at red and green.
That’s the hallmark of an analogy. It gives the general idea, but breaks down if we interrogate it in too much detail.
Essentially that is actually the case. Human-visible light and AM/FM radio waves are just different wavelengths along the EM spectrum.
A flashlight beams out waves that we can see; a radio transmitter beams out waves we can't. The brightness of the beam of light is related to its amplitude, just like the signal content in AM radio is related to its amplitude. And the color of the beam of light is related to its frequency, just like the signal content in FM radio is related to its frequency.
The explanation asks us to imagine shining a flashlight through a tree, first changing the flashlight brightness and then changing its color.
The flashlight is an analogy for a radio transmitter. We all get that they work on same principle but just on different wavelengths. But regardless I can't shine the flashlight in my kitchen drawer at my radio and pick up a signal.
Well... It kind of does. The source of the radio station is a kind of flashlight, just on a different frequency. The tree is still a tree (and all the other objects)
Good analogy, however if you move back and forth the transmitter or the receiver at enough speed, frequency (color) will vary as well, and that analogy could be used to explain Doppler effect, and why civilian airplanes use AM.
Civilian airplanes aren't using AM because of the Doppler effect. You're not accelerating that rapidly to make the Doppler effect that pronounced on the kind of radio being used in airplanes to the point they wouldn't be useful. Even if you're going hundreds of miles an hour the shift is going to be a few dozen Hz in drift. A cheap FM discriminator will be able to handle that without any problem. Doppler shift starts to matter when dealing with satellites, but not airplanes unless you're taking a SR-71 on a civilian stroll.
Doing the math, if you're going 200mph away from a station transmitting at say 121MHz, the drift frequency would be ~36Hz. Not going to be a problem.
And even then, your AM transmission still gets affected by Doppler shift as well.
Airplanes use AM because when two SSB transmissions happen at the same time you can actually hear both at the same time. If you're using FM it's either an incoherent mess or one transmitter drowns out the other.
It's not that simple, though. The only way you can detect frequency is by measuring the amplitude (and then differentiate; except of course in an analog circuit, you don't do that exactly, you have some mechanism that tries to track the carrier wave smoothly instead), so amplitude noise will necessarily also become frequency noise. But generally white AM noise will be pushed upwards in the spectrum after FM demodulation, away from the area where you care. (You can also add a hard limiter, which amplifies this effect; even more noise high up, even less noise further down.)
That's exactly what the parent comment described in the beautiful example where the AM noise is due to moving tree leafs affecting the intensity of transmitted light, and you can fix it by varying color, which means varying the frequency spectrum of the light.
In the example, the amplitude of the flashlight signal is distorted by the movement of the trees. The signal is never completely hidden. Not sure if that answers your question...
This seems great at first, but more so as an explanation of how AM and FM differ; one being by amplitude (brightness), and the other by frequency (color).
What I don't see is how it explains why one would work better than the other.
If the tree is blowing in the wind, and a leaf obstructs the entire signal, it doesn't matter whether it's a change in brightness, or a change in color. Either way, that information is lost by the blocked leaf. And if the entire signal is not lost, perhaps many leaves may have blocked the signal but some signal managed to get through, it doesn't matter whether the signal change was a change in brightness, or a change in color. Either way you're going to notice the change. So I don't see how this clarifies why FM is better. What am I missing?
I see from the article that "noise tends to be a an unwanted amplitude modulation, not a frequency modulation." In other words, the tree is providing an unwanted change in brightness. It never provides an unwanted change in color.
I guess the tree is able to dim the signal so much that it appears to be a deliberate signal change? Couldn't this be dealt with if you know the details of the tree's dimming ability?
The analogy is getting a bit tortured, so I'll try a more practical explanation.
An AM receiver is a machine that senses the amplitude at a specific em frequency. In this situation, noise and interference become random additions or subtractions to that amplitude. Draw a sine wave, then go over the line with vertical ticks or scribbles. Now imagine taking a random sampling of points and reconstructing the original wave perfectly (without a computer). Most of the information is just gone and you end up with a noisy output wave.
Now an FM receiver is one that measures frequency changes above and below a 'carrier' frequency. The amount of deviation away from center represents the amplitude of the sound signal being transmitted. In this setup, noise and interference are also random additions to the amplitude, but also at random frequencies. On average, interference happens evenly over the entire range of frequencies you're looking at. That means that the highest amplitude is still the same frequency away from center, it just has a slightly different amplitude.
Go back to that sine wave. You can't see the original signal behind all the noise, but you can still see how far apart the peaks are. You can still easily extract its frequency content.
FM uses the frequency dimension to transmit data because random noise can't really affect frequency. Noise mostly happens in the amplitude dimension across all frequencies at the same time.
FM is more robust because it uses two dimensions to encode information vs AM's single dimension. That's also why FM is in stereo!
Stereo FM is essentially two waves transmitted at the same time (it's common and difference instead of left and right, but that's math). Stereo AM would be possible, it was never done because two different AM transmissions have to be spaced further away than FM.
There were a number of successful AM stereo broadcasting methods proposed and trialed. These were completely compatible with conventional AM transmissions.
The conceptually simplest of course whas where the LSB and USB are used as separate channels.
Although most of the systems did work, they were not ultimately successful simply because insufficient stereo receivers reached the market.
Of the methods listed on Wikipedia only ISB is a true AM. All the others use phase modulation for the difference signal, as an easy way to achieve compatibility with mono AM receivers; and PM is basically the integral of FM.
Wikipedia says that there was basically one station doing ISB stereo; which I guess is close enough to "nobody did it", but not quite "it was never done".
FM works better because it is easier to detect the change in frequency independently of any change in the amplitude.
I'm unsure of what the correct terminology would be, but (for my linear algebra brain) you could say something like, for FM the noise dimension is orthogonal to the signal dimension, while for AM the noise and signal dimensions are the same. Therefore for FM any change in amplitude in the noise dimension should be mostly isolated from the signal dimension, while it is essentially impossible to tell what is noise and what is signal for AM - you could probably do some radio equivalent of a differential pair in order to detect noise and remove it, but then why would you bother when FM has improved noise rejection anyway.
The tree blowing in the wind will introduce its own amplitude (brightness) fluctuations. It will be hard for you to tell which amplitude changes are signal from the source and which are noise from the tree.
Edit: Looks like you answered yourself while I typed that, where you added:
> Couldn't this be dealt with if you know the details of the tree's dimming ability?
If the tree is moving, and you’re far enough away to resolve individual leaves (which is not unreasonable) then its “dimming ability” is constantly changing.
> leaf blocking some light doesn't change the color of the light that passes through
Of course it does. Real-life objects aren't perfectly opaque or transparent. Similarly, radio waves aren't blocked or received: they're mangled and self-interacted in complex ways.
Let's switch the analogy to sound. Amplitude is loudness. Frequency is pitch. You are trying to discern two sources of sound. One is a constant pitch but variable volume. The other can always blast at max volume with variable pitch.
Also harder to discern and then quantify the loudness of a sound or brightness of a light as a human modem but we are better and more certain of the color. We have different names for the ranges and everything.
> harder to discern and then quantify the loudness of a sound or brightness of a light as a human modem but we are better and more certain of the color
Fair enough, this might be a sensory artefact. In this case, however, nature had a point. Energy scales proportionally with frequency but exponentially with amplitude. Increasing amplitude delivers more bang than increasing frequency.
For this, it's better to stick to many leaves - the analogy holds up well here because when is the brightness change due to the number of leaves being in the way vs the source changing its brightness?
With AM, anything that causes a variation in the intensity of the signal will introduce noise.
With FM, anything that causes a variation in the timing of the signal will introduce noise.
Unless you’re traveling at relativistic speeds, operating a time dilation device, or colocated with a black hole, you usually aren’t going to see the rate that time flows at vary.
Thus if you can make the amplitude of your signal irrelevant past a certain threshold and embed all the information into the time domain, the only thing introducing interference should be other EM sources that happen to be on the same channel.
Speaking of multipath interference, why is it that we almost never hear the effect of that? Like, aren't these waves almost constantly bouncing off of other things and being reflected? How are we not always hearing echos all the time?
The other poster is correct but I feel there's still a simpler answer when you see the units at play. You don't constantly hear reflections as echos mostly because of the speed of light and the inverse square law.
Let's think about how far the echo has to come from to have even a half second delay. At the speed of light, that half second is 93,141mi. So it would have to reflect off something half that distance, ~46,500mi. Not a lot of good reflectors pointed at me 46,500mi away.
So then think of the inverse-square law on that. How weak of a signal is that going to be travelling those 93,141mi? Are you set up to even notice that from the noise? It's 11 billion times weaker than the original signal, assuming your reflector perfectly reflects the source signal and you're in outer space.
So obviously whatever "echo" we experience, it's not going to be something in the realm of humans directly detecting it. The shift that is possible to really mess with the signal at distance you'll actually receive reflections at are only going to shift the timing in a very small way, usually by being a slightly different phase. This means you'll get constructive and destructive interference from the same signal at slightly different phases, but not really a noticeable "echo".
The effect is easily noticeable only for high frequencies, where the wavelengths are no bigger than a few meters.
For the lower frequencies used by AM broadcasting, where the wavelengths are up to hundreds of meters or kilometers, and you use small antennas for reception, it is unlikely to have problems caused by multipath propagation (because the waves will go around obstacles instead of being reflected; only for the higher frequencies of the shortwave range you can have multipath reception of signals reflected by the ionosphere, but the objects that are around the receiver still do not cause problems).
When there is multipath propagation, you would not hear echos, because the time difference between the different paths is too small, due to the high speed of the radio waves. What you get is interference between the multiple signals, which can reduce too much the strength of the received signal. When the signal is reflected on some paths by mobile objects, or when the receiver itself is moving, the received combined signal will have an amplitude that varies in time, with intervals when the signal cannot be received (i.e. fading).
Other people answered your question, but if you ever saw shadowing artifacts on an analog UHF TV station, that was probably multipath. Note that lines on an NTSC TV are scanned at over 15kHz, so this is a very small time difference.
I was surprised this wasn't leaned on more explicitly for the explanation.
I think this is largely held in the assumptions that go into saying most noise will be amplitude modulation?
Edit: Reminds me of the banal but vital insight that digital uses repeaters to gain distance, whereas analog uses amplifiers. Makes it very easy to consider why/how digital took over.
It sure is, why would everyone or anyone be aware of AM/FM differences? Even if one is tech savvy it doesn't mean this would be something trivial to understand at glance
Right. Isn’t FM just flat out higher bandwidth? So unless it’s wasting that somehow it’s just going to carry more information. And for audio, that means sounding better.
The use of FM doesn’t inherently imply higher bandwidth. For example, those consumer-grade FRS/GMRS radios you get from Costco use narrowband FM, which typically occupies about 12.5 kHz per channel. This is much narrower compared to the 200 kHz bandwidth used by FM broadcast radio.
FM is simply a method of modulating the carrier signal by varying its frequency. The actual bandwidth depends on factors like frequency deviation and modulation, so FM can range from narrow to wide bandwidth depending on the application.
FM sounds better than AM partly because frequency is more durable than amplitude, but it's not the whole story.
Frequency does not diminish with the inverse square law, as does the amplitude of a wave that is broadcast in all directions. This is because frequency is related to a count of events over time.
Frequency from a source light years away is intact; we can look at frequency bands from a radiating celestial body and know which chemical elements there are, and also tell exactly how fast it is moving away from us from the red shift in that spectral pattern.
Be all that as it may, AM should sound great when you are close to the radio tower, and have ideal reception with no multi-path reflections, and good signal/noise ratio.
It still doesn't sound good, and that simply because of the bandwidth allocated to it is low. Furthermore, AM Stereo is a retrofit and crams two channels into one via phase modulation.
AM stations are separated only by 10 kHz, as you can see on your AM tuner (which you likely have only in your car, if that).
The bandwidth is directly related to the audio bandwidth because modulation produces side bands.
For instance, if we modulate the amplitude of a 650 kHz carrier with a 1 kHz audio tone, we get side bands of 651 kHz and 649 kHz. You see where this is going? We can only go up to 5 kHz before we bump into the next station, which also needs +/- 5 kHz for its side bands.
This 5 kHz limitation is why AM radio sounds like your speakers have a heavy woolen blanket over them. It's almost as bad as the bandwidth limitation as narrow band phone calls. Listening to AM music is almost as bad as listening to on-hold music over a narrow band codec like G.711.
The kicker is that only one side band is needed to reconstruct the signal, so in theory AM stations could have 10 kHz bandwith. Unfortunately, SSB was not deployed for broadcast AM, even though it was already known at the dawn of radio.
This is a myth. There is no reason that channel spacing need limit the modulation bandwidth. The only downside is that listeners to adjacent stations will hear a slight "monkey chatter" from the overlapping sidebands. In reality stations are never allocated adjacent frequencies within the same coverage area so this usually doesn't happen.
Be that as it may, AM radio is obviously low-pass filtered. It might not be a brick wall at 5 kHz, but it sounds obviously muffled to someone who can't hear anywhere near up to 20 kHz. If I were to guess, based on years of experience of playing with EQs, I would say that it has next to no content beyond somewhere around 8 kHz.
It would also be harder to tune into a station that is SSB because there's no carrier to detect. If you're slightly too high or low, the audio will have a slightly higher or lower pitch. I'm just guessing but with modern radios that wouldn't be a problem, but when AM was still used a lot I think (analog) oscillators tended to drift a bit, and you would have to adjust your radio often to correct for the changing pitch.
Even though the frequency survives the long trip, any changes in that frequency are observed only after a delay corresponding to the speed of light.
Someone once pointed out that shadows (which aren't objects with a mass and position) can move fast than light, at a sufficiently large distance from their origin. That is, the location of the border between the shadowed and unshadowed region can be changing faster than light speed. But that fact can't be used to communicate faster than light, because the changes in the location of the shadow's edge still take a comparatively enormous amount of time to propagate from their source to their destination. If you're creating the shadow, you can know that one galaxy will observe the shadow long before another galaxy does, but you can't use that knowledge to signal something to one galaxy or the other without waiting for the light (or lack of light) to travel all the way to that galaxy.
As a non radio engineer, reading this thread on HN is so fascinating because there are so many heated conflicting explanations for a common phenomenon in a well-established technology. I thought this simple question would be settled already. If such disagreement is possible even in an established "hard" science like this, then no wonder some people think everything is subjective.
This question is of course settled. However, it has more than one aspect and I am pretty sure there are also a lot of rather amateur people chiming in here ...
I am an EE, but in power electronics and not an RF engineer so I am a little bit hesitant to comment too much on it, but my understanding is that it mostly breaks down to two aspects:
1) noise interferes more with amplitude
2) the fidelity of the modulating signal in FM is higher (more bandwidth)
Also the audio frequency bandwidth is narrower on AM, so fewer treble frequencies.
TBH I think music from up to the late '60s (especially if originally released in mono) sounds really good, or at least more "era-appropriate" on AM radio. I remember my grandparents tuning in to easy-listening AM stations as I grew up in the '70s and '80s, to my ear Tennessee Ernie Ford's "16 Tons" or a classic Phil Spector "Wall of Sound" production sounds more "right" coming through the AM bands.
And, in the age of cellphone speakers and compressed MP3/Bluetooth codecs - I'm not sure how much people actually care about audio quality.
Those codecs got better with time. Also notebook and little portable speakers, while they are unable to physically reproduce low frequencies are getting better at emulating those. Somebody cares.
And here's (dunno if true as they write in the description - probably the very first stereo) studio turntable from 1958 playing a record from 1988 through Youtube's compression. I did have a lousy vinyl deck with so so speakers when growing up and this impresses me a lot:
https://youtu.be/PRty-_eBEpg?si=GsrctxRbkvT3xRAV
It may be impossible for a little speaker to produce any sound at some low frequency, so manufacturers use "virtual pitch" psychoacoustic phenomenon by introducing harmonics above that frequency. There is no low bass, but there will be added harmonics that will be perceived by the listeners as low bass: https://sound.stackexchange.com/questions/37755/how-do-psych...
Here's also a project and some info from Asahi devs on Macbook audio:
https://github.com/AsahiLinux/asahi-audio
The "Woolyness" of AM broadcast (at least in America) is due to the stations purposefully tailoring their audio processing to suit typical cheap AM receivers. And this in turn is because designers of cheap AM receivers fit narrow filters instead of using noise reduction techniques, eg a good outside antenna.
There was a period (in the rest of the world) where high quality AM receivers had a narrow/wide switch to give better audio response to stronger signals.
The good news is that modern SDR receivers usually have selectable bandwidth on AM so as to derive the full transmitted audio. And many of these have AM stereo decoders as well.
If you listen to a good quality AM broadcast (eg Gov AM stations in Australia) you will hear audio which are very hard to tell from FM audio.
Go back and read the many high-quality AM tuner articles in the electronic hobby magazines from the past.
TBH I think music from up to the late '60s (especially if originally released in mono) sounds really good, or at least more "era-appropriate" on AM radio.
That music also sounds more era-appropriate coming from a vinyl record than a CD.
Look, I'm an audio snob and will talk shit about terrible design of BT headsets that halve bandwidth in duplex until the cows come home.
But the reason that codecs have survived this long without substantial changes is because they're far and away good enough (*) for the vast majority of listeners. To the point where today, even trained listeners can't perceive a difference in audio quality between lossless and lossy encoded audio at high enough bit rates (which is 320kbps MP3, or comparable AAC which can be as low as 50% of that).
(*) what we don't talk about is the latency of the codec itself, where regardless of available compute resources is still atrocious outside of proprietary codecs. While a listener cannot perceive noticeable differences in fidelity, they can perceive the delay, and this is a problem that doesn't have good solutions outside of specialized equipment today, although OPUS (as a descendant of CELT) is pretty darn good for the cases that consumers care about. Professionals still spend oodles of money on the proprietary gear that have codecs that not even ffmpeg supports.
I would go so far as to say there is no practical benefit to uncompressed audio today at all. Lossy is fine for all consumers, and lossless encoding is faster to decode and playback (as well as encode and write) while using less disk/bandwidth than uncompressed for archival purposes.
FM is not very practical on frequencies that bounce off the atmosphere with any regularity as they have a lot less bandwidth to offer.
The reason this conversation is so foreign to most of us is because back in the day of shortwave radio our suburban and urban environments weren’t soaked in RF interference.
Go out to the middle of nowhere and try a shortwave radio or an SDR with an extremely long wire antenna strung up in trees. You’ll receive the world.
One thing this seems to let out is that in FM, the signal is being broadcast with constant power. A 1MW FM station is always sending 1MW of signal; with AM the signal power varies; if you broadcast in AM powerful enough to mask the noise during periods of typical signal level, then you would still hear noise during quieter than typical periods.
« FM signals were much more immune to interference than AM due to its “capture effect” – an interfering signal needed to be more than 50% the strength of the desired signal to cause audible interference, compared to 5% or less with AM. This characteristic would considerably reduce the required separation between stations occupying the same channel and allow more channel re-use, which compensated for its greater occupied bandwidth. And most importantly, because all natural and man-made static is amplitude modulated, FM proved to be amazingly noise-free. Armstrong improved its resistance to noise still further by incorporating a new receiver component – a limiter that stripped off the amplitude variations in the received signal before it reached the detector. He had finally solved the problem of static interference that had confounded radio experts since the beginnings of the art. »
It's quite possible to have wideband AM radio. Some radio stations did it in the US before the FCC standardized bandwidth and started checking envelopes. Radio Caroline, the UK offshore pirate station (1964-1968), was wideband AM.
Noise on AM can to some extent be overcome with power and a low modulation percentage. That's how analog broadcast TV worked. (Broadcast TV was AM video, FM audio.) The black level for the video signal was well above zero. A high black level allowed showing black areas without excessive noise. About 80% of the RF power went into the carrier because of that. Simple, but inefficient. The same trick can be done with AM audio radio, although it seems that's not done much.
Radio Caroline would be such a good HN topic in its own right. Peter Chicago's name in particular should be up there in hacker lore for some of the things he did to keep Caroline on the air.
Lightning is a great example of noise causing amplitude changes and not frequency changes. That’s why during a thunderstorm am radio plays each strike between the station and you. The is usually not any indication of lightning strikes on FM.
There are many differences that explain why one can be better. All else being equal:
- bandwidth of modulated signal: it is better to spread the signal over a large bandwidth => Nlog(1+snr) > log(1+Nsnr). the bandwidth used by the FM signal is larger
- wasted energy on the DC signal: AM signal is A + s(t) where A > abs(s(t)) to make sure the sent signal is always positive. A (DC) does not carry information so the effective signal to noise ratio of a DC-less signal should be higher (phase/frequency modulation, signalling that can detect the negative part...)
- filtering of baseband signal => if you filter too much the original signal, you lose information even before transmission. Voice is usually filtered and 4KHz, but music needs more. FM has more margin (more allocated bandwidth) so can have less stringent filters
- tolerance to fading: the wireless channel is not AWGN, it is frequency dependent due to multipath. While radio signals are relatively narrow, signal modulated in frequency are more robust to fading (OFDM...)
I always assumed it was because FM station bandwidths (200kHz) are much wider than AM (10kHz). AM's 10 kHz chops off a lot of human-hearable frequencies.
When you amplitude-modulate a carrier wave with an audio signal, you spread it out into a bunch of sum and difference frequencies, as you can see if you use the trigonometric angle-sum formula to factor cos(85000·2πt) · (2 + cos(440·2πt)), a 440-hertz flute being transmitted on 85-kilohertz AM. These so-called "sidebands" mean that the bandwidth of AM does matter, and consequently, using a too-narrow bandpass filter on your AM radio station will result in low-pass filtering your demodulated audio signal.
AM does use the frequency, it just doesn't need as much and uses it differently than FM. If it was all at a single frequency, there just be a single tone getting louder and softer.
Once you change the amplitude of a sine wave (modulate it) it's no longer a side wave. It spreads in the frequency domain. Take the fourier transform of that and you can see the frequency components.
The article kinda sucks as it does not really answer the question it poses. Why ”noise tends to be a an unwanted amplitude modulation, not a frequency modulation”?
Is it due to naturally occurring background noise being low frequency high amplitude, showing up as AM? Could the situation change if humans keep generating more high-frequency noise? Or is it just that high frequencies do not travel as far so there will always be relatively little?
What I’ve never understood is how the FM receiver can lock on to the signal if its frequency is always changing. Doesn’t the receiver need to lock on to something? If the answer is “it locks on to the amplitude, which doesn’t change,” well AM is bad because the amplitude is subject to interference, so wouldn’t FM have the same problem?
Having recently purchased a RTL-SDR and watched and learned about FM -- there is "pilot" frequency that doesn't change and is fixed relative to the tuner frequency. See this chart here:
Each radio station has 100khz of bandwidth centered on it's tuner frequency. in the, there are channel spacing rules that give some gaps +/- another 100khz of that. (That's why in the US, radio stations are typically on 'odd' decimals, ie 92.3 mhz, 94.1 mhz, etc) That chart does not show HD radio frequencies, which due to those spacing rules, and more accurate transmitters, are on the +/- 100khz spaces along side the original analog 100khz. You can "see" the audio modulating the frequency on the spectrogram. But the OFDM digital signal on either side looks like a band of more intense noise. It's mind blowing to realize there's a signal in that!
You're right -- after reading some of the peer responses, I realized that (I think...) my response is just how the Broadcast FM signal modulates the parts of the signal, and not how it actually 'locks on'. I'm still learning!
I got an RTL-SDR this summer and brushed up my DSP skills playing with FM signals. PLLs are marvellous beasts; you can do a slapdash job in "designing" one and it'll still probably lock on just fine. Might not be optimal but will still lock.
Another fun one, when you have IQ samples, is the polar discriminator: calculate x[t] * x*[t-1] where x* is the complex conjugate, and take the angle with arctan. Feels a bit like magic ("is that all?") but is justified by the theory.
One possibility is a phase-locked loop. I don't know if there's anything better. It matches the frequency of a voltage controlled oscillator to the frequency of the incoming signal by detecting the phase mismatch. Then, the control voltage for the VCO becomes the audio signal.
I don't think radios use a PLL to demodulate the FM audio: the signal has a huge "pilot" tone at 19kHz that you can match to get the first part of the spectrum, mono audio (L+R channels), and at double that frequency you know you'll find the stereo part (L-R channels). Precise phase estimation is only necessary to decode the RDS digital data (station name, datetime, etc.).
First of all, the pilot is only required for decoding stereo and RDS. Mono FM does not use a pilot, so obviously there had to be a way to detect FM before stereo came along. I linked to a few of the approaches in a sibling (cousin?) comment.
Second, the pilot is embedded in the decoded FM audio. You need to demodulate FM to get to it in the first place. If you look at the waterfall display in an SDR receiver, it might seem like the signal is already present in the original radio frequencies (especially during silent periods), but it's there only indirectly.
If you have silence in an FM transmission (say 96.6 MHz), the only audio component present is the 19 kHz pilot signal, which causes the FM radio signal frequency to vary between 96.6 MHz ± k*19 kHz (not sure what's the value for k, but it's not 1). The sine likes to spend most of the time near the extreme values of its range; plot a histogram of a sine wave and you'll see peaks on either end.
The waterfall is basically a histogram over frequencies so it gets those peaks as streaks on both sides of the main carrier frequency (plus smaller ones for other components in the signal).
The 19 KHz pilot tone is interesting: it's not filtered out and is often reproduced by the speakers if you have good ones. You can verify this by using an audio spectrum analyzer application on your phone.
I've wondered if FM stereo drives pets nuts with its constant high-pitched tone.
Do you mean AFT (automatic fine tuning)? This has nothing to do with a PLL-based demodulator. Older receivers with ratio detectors or discriminators also had AFT- there is a low frequency feedback path from the demodulator to the local oscillator to make this work.
Disclaimer: I don't really know any of this stuff, and I've never built a radio. I'm just repeating what I've read, or in some cases, simulated in software.
The simplest answer is that you use a narrowband bandpass filter around the transmitting station's center frequency to eliminate the signals from other radio stations, just as you do for AM radio, and then you measure the frequency of the remaining signal instead of its amplitude. This works because the frequency deviations are small compared to the spacing between the frequencies on which different stations are transmitting. Downconverting to an intermediate frequency by mixing with a local oscillator, as CodeBeater correctly said most FM receivers do, doesn't really alter this fundamental principle, although it does alter the details. (Most current AM radios are also superheterodyne designs.)
Most current FM radios use a phase-locked loop, as analog31 correctly said, which is sort of the same but sort of different; it gives better results. A PLL uses a much narrower bandpass filter which is centered on, not the nominal center frequency of the radio station, but the instantaneous, modulated frequency, which makes it much better at rejecting interference than the simpler approach. So the frequency band you're filtering down to gets swept back and forth in real time, thousands of times a second, to follow the FM signal.
There's the question of how your PLL can initially achieve its lock if its passband is so narrow, of course. I don't know how mainstream FM radio does this, but it's not as hard a problem as you might think; because broadcast FM radio's frequency is always oscillating back and forth around its nominal center frequency, you can just wait for the audio signal to cross zero. Alternatively, you can sweep the PLL's local oscillator frequency over the band until you achieve a lock.
I'm actually studying for my general ham radio license right now! Most FM receivers use something called a "mixer" to modulate the frequency to a known constant, then they use a circuit called a "discriminator" or "quadrature", both of which are "detectors".
Typically they're not measuring the frequency or phase itself, but rather the change in frequency or phase.
Edit: I should note that's only for analog circuits. DSP is also common.
Most FM receivers nowadays rely on creating a signal of a specific frequency that interferes with the desired on-dial frequency, this is called an intermediate frequency. Then the actual audio signal is analogous to the changes on that IF.
This technique is known as superheterodyne, and Technology Connections has a wonderful video explaining it better than I can.
Because noise is in line with AM and perpendicular to FM? Let’s read if that’s still so.
Armstrong reasoned that the effect of random noise is primarily to amplitude-modulate the carrier without consistently producing frequency derivations.
…It doesn’t talk much about the noise physics, but basically yes.
Better is in the ears of the beholder. Personally I prefer AM because I can hear multiple stations at once, and hear sources of wideband EM interference in my environment.
I think this article is focussed on broadcast radio, where crosstalk and em interference are both considered negative properties (not that they're not useful in other applications)
It's certainly perceived that way. "Diff'rent Strokes" "Baseball Blues", 1985... Willis does mention stereo is part of the appeal.
- Now, Dad, you gotta picture me cruising along in my Mercedes. Head held high. Rocking to the FM stereo. Waving to the chicks. Hey there, mama, looking good. Catch you later, baby.
(making engine noises)
- You can do all of that in a $4,000 car.
(imitating brakes squealing)
- Dad, for $4,000 I'll have to slouch way down in my seat so no one can see me. And turn on my AM radio. Wave at the chicks. Hi there, mama, you're looking quite adequate. Chug, chug, chug.
- That's just fine, son, chug chug chug means that you won't be spending any of your days in traffic court.
- Or any of my nights at a drive-in movie.
- Willis, you don't want to date a girl who only likes you for your car.
I grew up listening to the radio and was always curious why FM indeed sounds cleaner than AM. My assumption before is that it's setup that way since FM is intended for music stations.
Define "sound better", please. As we all know this is something subjective - mostly everyone has individual preferences. I would have found it better if the title was "Why is frequency stable in spacetime and amplitude not (which can be verified so easily by listening to audio radio)".
The most familiar definition, though not spelled out in the article, is audio fidelity which is the degree to which the output reproduces the input. It's fair to take this definition as a default, or implied by the article. In this specific case, frequency response and signal-to-noise are both fidelity measures.
Also, some of the comments did a better job than the article of explaining things.
And it's not correct. Resulting frequency due to random noise is also changed, but in FM, noise is less perceivable. There is no such thing as "affecting amplitude" and "affecting frequency" - they are not separate concepts.
This is 100% nonsense. Phase noise exists too, not just amplitude noise.
The answer is actually rather simple. AM stations are limited to 10KHz band width. FM gets 200KHz. More bandwidth allows representing a higher fidelity signal…
Yes, this is the right answer, although I would correct the numbers a bit.
If we look only at the audio bandwidth, AM stations are limited to 5 kHz of audio spectrum. The 10 kHz figure comes from the fact that AM is double sideband modulation (as opposed to single sideband as used in ham radio and other radio services). So the broadcast signal uses twice the bandwidth of the audio.
FM stations have 15 kHz of audio bandwidth, three times that of AM. They are able to do this because they transmit at a much higher frequency.
The 200 kHz figure includes other things like stereo (two channels of audio), subcarriers for RDS data and such, and the "Carson bandwidth rule" that 'basementcat' mentioned.
I am surprised that the article overlooked this simple and obvious explanation.
You provided a citation, but it doesn't prove your point. In general frequency modulation is more efficient than amplitude modulation (but requires more complicated receivers). For example GMSK in the 2G standard GSM replaced the less efficient 1G AMPS system which used amplitude modulation.
Yes, phase noise exists, but I would think that in practice amplitude noise is greater.
In physics, when a wave passes from one medium to another, its frequency is supposed to stay the same. Even if this isn't perfectly true in the real world, I would think amplitude is more likely to decrease due to obstacles, distance, and the medium absorbing some energy.
But the two are the same thing. You took the fourier transform of white noise and find white noise, both real (amplitude noise) as complex (phase noise).
You can think of it like this: the noise is not about the phase changing, it is about your ability to tell what the phase is. The noisier the signal gets, the harder time you will have to tell what the amplitude is, as well as what the phase is.
If the noise is white gaussian noise (AWGN) then the phase noise is essentially the same as the amplitude noise (by the properties of the Fourier Transform).
Also, the information in AM is carried by the relative amplitude of the signal. Flat attenuation like you're describing doesn't really distort the AM signal. What does impact both AM and FM is frequency selectivity. Imagine light traveling through a prism and being split by frequency. If there are obstacles in the way, some colors won't pass through as well. The is can cause distortions in FM as the receiver loses lock on the signal. Am suffers from this too, but people are less likely to notice because they're used to these distortions -- these kind of effects happen with sound too.
As other posters have mentioned, the reason FM sounds better is that it has more bandwidth for the signal.
Very interesting, I'll have to look into AWGN and the Fourier transform. I guess in the trees blocking the flashlight example that's not at all AWGN.
Although while we care about the relative amplitudes in AM, AWGN would make this harder to pick out if the signal is attenuated. Is the same idea true for frequencies? I don't see a direct parallel here.
You do get some frequency deviation from AWGN. The sum of equal-amplitude 100Hz and 120Hz sine waves is a 110Hz sine wave that "beats", which is to say, is amplitude-modulated, at 10 Hz (or 20Hz from a certain point of view). So, if you have a 120Hz signal and you add a 100Hz signal to it, you should expect that to deviate the frequency of the detected signal downwards. AWGN will have varying, random amounts of all frequencies in it, which will cause varying, random amounts of frequency deviation as they add to your signal.
It's definitely easier to understand in the Fourier domain.
It's not 100% nonsense, though it's true that phase noise does exist. FM radio can transmit silence, which gives it a better dynamic range, which is important for music. If your AM radio signal is 10dB stronger than the radio noise in the band, you'll get noise in the demodulated signal only 10dB quieter than the signal. Due to the so-called "capture effect" https://en.wikipedia.org/wiki/Capture_effect the effect on an FM-demodulated radio signal is potentially much less—though it's true that, with narrowband FM, it won't be.
That's why commercial FM broadcasting uses a ±75kHz deviation even though it was originally only transmitting audio of ≤20kHz. Adding all this extra bandwidth to an AM station wouldn't actually help, because beyond ±20kHz, you're only improving your radio station's ability to reproduce ultrasound. But it does help FM; it greatly reduces the amplitude of demodulated noise, because, even without a PLL, the frequency deviation caused by additive white noise increases much more slowly with bandwidth than the frequency deviation you can use for your signal. With a PLL, I think the frequency deviation caused by additive white noise basically doesn't increase at all with bandwidth. (I guess I should simulate this; it should be pretty easy.)
Unfortunately neither Cook's article nor the flashlight analogy explains any of this.
Back in 2005-ish a Clearchannel enginer, Littlejohn proposed narrowing bandwidth of Am stations to improve the signal to noise ratio. They implemented a 5 Khz bandwidth. Back in the 70s we were all blasting 12 kHz. While that's probably OK-ish for talk, it's dreadful for music.
https://www.radioworld.com/columns-and-views/the-5-khz-am-re...
12 kHz AM on a good receiver sounds absolutely fantastic.
I have a Royal 51/810 (one of each) that I use as a travel/bathroom radio, ironically, both have fantastic AM performance, and.. lacking FM - the IF/Audio bandwidth appears twice as wide on AM, and FM just sounds like crunchy - probably needs caps in the audio section, but its so tightly packed, and has a PCB with the heaviest plating I've ever seen - which means it needs work I cannot easily do.
AM reception is essentially the direct conversion of the strength (amplitude) of a given radio frequency into an audio signal. Any other noise present at the same frequency is added to the signal (superposition/interference) and therefore impacts the strength of that frequency at the receiver. Therefore it is impossible for the receiver to know whether the amplitude it received is just signal or is signal plus noise.
The claim 'the effect of random noise is to amplitude modulate' is probably not 100% correct, because to my understanding it's not actually performing modulation (the modulation happens at the transmitter but the noise happens between the transmitter and receiver), but it is impacting the amplitude at a given frequency and to a receiver this is impossible to know whether said change in amplitude happened before modulation (signal) or after modulation (noise).
There's actually one important factor that's missing:
AM radio is limited in bandwidth. The audio is cutting off around 10kHz or such (that's why it kinda sounds like a telephone)
> To allow room for more stations on the mediumwave broadcast band in the United States, in June 1989 the FCC adopted a National Radio Systems Committee (NRSC) standard that limited maximum transmitted audio bandwidth to 10.2 kHz, limiting occupied bandwidth to 20.4 kHz
That's because AM has a bearer around 800 kHz, while FM a bearer around 100 MHz. I guess if doing AM at 100 MHz it wouldn't be a problem granting wider channels, too. But the problem that amplitude is more sensitive to noise wouldn't go away.
I don’t buy this explanation. The FM modulation uses a much higher bandwidth than AM. The distance between channels on FM radio is 200 kHz compared to only 9 kHz on AM. That’s more than 20x more bandwidth for FM. On AM, no matter how deeply you modulate the carrier, the bandwidth will not exceed twice the bandwidth of the input signal. On FM, the deeper you modulate it, the wider the output spectrum will be, and it can easily exceed the bandwidth of the input signal.
In addition to that, the whole FM band is much higher frequency, while I guess quite a lot of noise, especially burst noise caused by eg thunderstorms is relatively low frequency. So it’s not picked up because it’s out of band.
Any noise that falls inside the channel does get picked up by the receiver regardless of modulation. However because the available bandwidth is so much higher than the real bandwidth of the useful signal, there is actually way more information redundancy in FM encoding, so this allows to remove random noise as it will likely cancel out.
If I encoded the same signal onto 20 separate AM channels and then averaged the output from all of them (or better - use median filter) that would cancel most of random noise just as well.
Also another thing with modulation might be that if there is any narrow-band non-white noise happening to fall inside the channel (eg a distant sender on colliding frequency), on AM it will be translated as-is to the audible band and you’ll hear it as a single tone. On FM demodulation it will be spread across the whole output signal spectrum, so it will be perceived quieter and nicer by human ear, even if its total energy is the same. That’s why AM does those funny sounds when tuning, but FM does not.
The wider channels is the source of the available audio fidelity, but wider channels make you more exposed to noise, not less. A wider channel means listening to more noise sources, and having transmitter power stretched thinner for a much lower SNR.
In other words, the noise rejection of FM is what enabled the use of wider channels and therefore better audio quality. An analog answer before digital error correction.
In FM, the rejection is so strong that if you have two overlapping transmissions, you will only hear the stronger one assuming it is notably stronger. This in turn is why air traffic still use AM where you can hear both overlapping transmissions at once (possibly garbled if carrier wave was off), and react accordingly rather than being unaware that it happened.
Technology moved on from both plain AM and plain FM a long time ago, and modern “digital” modulation schemes have different approach to interference rejection.
Shannon theorem disagrees with you. The wider the channel, the MORE noise you can tolerate when transmitting signal at a given data rate.
In audio, the amount of information you need to transmit is naturally limited by the audio bandwidth (for FM truncated at about 15 kHz), so the useful signal bandwidth is fixed. Hence, if you transmit the same audio band over a broader channel of frequencies, you can tolerate more noise; or, for the same density of noise in the channel, you can get better SNR at the output. This is exactly what FM does. It uses the information multiplied in the most of that 200 kHz channel and projects it on 0-15 kHz band.
While you are right that a wider channel captures more noise in total, noise does not add up the same way as useful signal, because it’s random. Doubling the channel width only increases the amplitude of noise by sqrt(2).
There is no “magic noise rejection” coming from different ways of modulating the signal if all other things are the same. You can’t remove noise; you can’t magically increase SNR. If anything, FM makes the noise more pleasant to listen to and perceivably quieter by spreading non random, irregular noise over the whole band so it sounds more like white noise.
But it also allows to use wider channels, and increase the fidelity of the signal, including increasing SNR. But that’s thanks to using significantly wider channels than audio.
Also, it’s not like FM can use wider channels because of better SNR. FM can use wider channels because of how this modulation works - the spectrum of FM signal can be arbitrarily wide, depending on the depth of modulation. AM cannot do that. It only shifts the audio band up (and mirrors on both sides of the carrier). It can’t “spread it”.
Btw: this is a very similar phenomenon as when you average multiple shots of the same thing in photography, eg when photographing at night. By adding more frames (or using very long exposures) you obviously capture more total noise, but the amount of useful signal grows much faster because signal is correlated in time, but noise is not.
It’s not immediately clear that Shannon’s theorem is a good point of comparison here, since it’s only recently that coding schemes have really approached the Shannon limits, and FM and AM do not use these.
Even if one does assume a Shannon-perfect coding scheme, as the noise ratio gets greater the benefits of spreading a signal across a higher bandwidth fades. Furthermore, most coding schemes hit their maximum inefficiency as the signal to noise ratio decreases and messages start to be too garbled to be well decoded.
I’d additionally note that folks get near the Shannon noise limit _through_ ‘magic noise rejection’ (aka turbo and ldpc codes). It’s therefore not obvious that FM isn’t gaining clarity due to a noise rejection mechanic. The ‘capture effect’ is well described as an interference reducing mechanism.
Empirically, radio manufacturers who do produce sophisticated long range radio usually advertise a longer range when spreading available power across a narrower rather than wider bandwidth.
Well, yes and no. It's a bit more complex than just taking the numbers from Shannon-Hartley, but I admit that my original description was at best lacking, so thank you for pointing that out.
Shannon-Hartley describes that the theoretical information capacity of a signal given a bandwidth and an SNR. Doubling bandwidth halves your SNR (received noise increases, received signal does not), in turn reducing the bits gained per unit of bandwidth. At very high SNR, doubling bandwidth almost doubles capacity, but as SNR goes down, the benefit of additional bandwidth levels off until bandwidth no longer has any effect.
However, this provides the number achievable by a perfect modulation scheme using all available bandwidth and signal strength. AM and FM are both incredibly inefficient, and more importantly have very different reactions to noise - something Shannon-Hartley does not concern itself with.
With truly random noise, FM and AM noise both scale based on noise amplitude as you say. In AM, all noise overlapping with the band is played back verbatim, whereas in FM only the noise causing frequency variations in the carrier wave have any effect on the signal, and end up with a non-linear response to noise.
However, we do not deal with purely white noise, and FM has far superior handling of non-random noise. In order to have any effect, it need to either induce frequency shifts to the carrier wave, or have enough power to cause the interference to be captured instead. There's also the far higher power efficiency, as FM puts all its power into the signal, whereas traditional AM puts most of it into a useless carrier and wastes half the remaining power on the redundant sideband (yes, SSB is a thing). These were certainly also factors in FMs demise.
A simpler means to remove bandwidth from the equation would be to compare with a narrow-band FM transmission, or by multiplying the input waveform for an AM transmitter by some factor to fill the bandwidth. I believe FM should still handily beat it at least above its threshold. I don't see anyone giving exact numbers of this though, so I guess it could be a fun SDR project for someone wanting to prove either point. :)
(Neither AM nor FM is of anything but historic value at this point - their only redeeming quality is discrete circuit simplicity if you need to MacGyver one out of shoelace and bubblegum, but that's it.)
> …traditional AM puts most of it into a useless carrier and wastes half the remaining power on the redundant sideband.
I had not thought about this before, and I have no intuition as to what the answer is, but does the redundant sideband have any effect, positive or negative, on noise rejection?
You are applying Shannon theorem incorrectly. Both AM and FM modulations are nowhere even remotely close to using their bandwidth with 100% efficiency, due to technology costs, and the difference in modulation is crucial. The article is correct and the mathematical models of AM and FM are well understood since decades.
Where did I say AM and FM are close to 100% efficiency?
I was only replying to an obviously incorrect statement that by using more bandwidth you decrease SNR. If it were the case, Shannon theorem would not work.
It doesn’t matter how close to the limit your encoding is, whether it is 20% or 99% the relationship between the bandwidth, noise floor and how much data you can send stays the same - by increasing bandwidth you can usually send considerably more information even if your encoding is poor. Which in translates to either a wider useful bandwidth or lower noise floor or any combination of both.
A trivial thought experiment to illustrate this: For any analog encoding, if I double the transmission bandwidth by encoding the same signal over 2 channels instead of one, I can average the output signal coming out the receivers and get better SNR than using one channel and one receiver. That works regardless of AM, FM or whatever fancy encoding you could use.
> A trivial thought experiment..
That's not how this works. That's not how any of this works. Averaging a high SNR channel with a low SNR channel is likely to produce something less good than the high SNR channel. Could you get an improvement over the high SNR channel? Yes, and the limit of the improvement is related to the SNR of each and averaging the signals won't get you anywhere near that.
Averaging two noisy signals increases SNR. That’s not even a thought experiment, that’s a reality. This is a technique used by probably all modern smartphone cameras to do night photos, as well as a common technique used by astrophotographers. Instead of taking one picture, you take a series of pictures and then align them and average. This improves SNR dramatically. A very long time ago we used this technique to get razor sharp, low noise pictures of the Moon at 3k x 3k resolution using… a cheap VGA internet camera: https://astronet.pl/wydarzenia/n2309/ Note that cameras at those times were barely capable of doing videoconferencing in artificial evening light - what you saw was mostly noise. Those sensors were really, really terrible.
What you seem to be missing is the fact we’re talking here about transferring the same fixed bandwidth signal over a wider channel, not transferring a wider bandwidth signal over a wider channel.
// edit: just noticed someone else gave another nice application of this phenomenon: GPS
> This is a technique used by probably all modern smartphone cameras to do night photos, as well as a common technique used by astrophotographers...
I think this is a lot simpler because each of your pixels is assumed to have a single, correct DC value. This doesn't hold for a time varying signal like AM/FM.
Let's take it to the limit:
Signal0: infinite SNR. Signal1: anything less.
I just don't see how the output of averaging these would improve over Signal0. I don't think it can.
They're thinking about when you sample from the same noise distribution, averaging gives an unbiased estimator of the mean. But when you know one SNR is higher than the other, maybe this doesn't hold? But maybe if you transform the distributions to look the same, thus taking a weighted average? I'm not sure.
No, that's one time varying signal.
What you "more bandwidth more noise" people miss is the difference in randomness: the noise is random while the signal is not.
In case of gaussian noise, double the bandwidth means 1.41x more noise. For signal, double the bandwidth double the signal.
Where are you getting 1.41x? What you'd really like to increase is the SNR. As you open up the bandwidth, the amount of energy you can collect in your band increases, but there's no way to collect the energy from only the signal and not collect the energy from noise. So as you increase your bandwidth, your SNR stays the same.
Not all noise is gaussian. And the fact that the noise is random while the signal is not, is useful when you can average and drop your noise floor. But you need multiple measurements to do that.
1.41x is sqrt(2), which suggests that they meant noise amplitude rather than noise power.
Noise power increases twice but signal power increases 4x. Noise amplitude increases sqrt(2) times, signal amplitude increases 2x.
Noise is not gaussian.
> (...) use AM where you can hear both overlapping transmissions at once
Yes. Assuming signal strengths for both are comparable. Say, within 20 dB of each other.
> (possibly garbled if carrier wave was off)
Nah. If both stations have sufficient energy fall into receiver's bandwidth window (IF filter for analog receiver), no garbling. If one of stations has carrier sufficiently off to fall entirely outside IF, only other will be audible.
You are probably thinking about SSB, where two stations with carrier offset indeed produce weird sounding interference.
https://en.wikipedia.org/wiki/Single-sideband_modulation
In SSB there is no carrier transmitted. Two SSB stations on top of each other sounds exactly like two microphones mixed.
Interesting. However, if one of those stations runs on a slightly different frequency, I guess its output would be garbled, correct? Like I guess SSB receiver just shifts down the band by a constant?
If it's kind of close it just sounds like someone talking in a slightly lower or higher pitch. It can still be pretty intelligible with the frequency slightly off. Eventually it gets very distorted though and you start losing a big part of the waveform entirely. Try listening in to some websdrs and you'll see what it's like.
The wider channels is the source of the available audio fidelity, but wider channels make you more exposed to noise, not less.
From a signal:noise perspective, what matters is the ratio of bandwidth available in the transmission channel to the bandwidth of the content you are trying to send. Consider GPS, for instance, where the use of a 2 MHz channel to send 50 bps data provides an SNR advantage that would otherwise be achievable only through witchcraft.
FM has strong noise immunity advantages -- notably AM rejection and the capture effect -- but they don't provide additional sound quality by themselves. That's where the bandwidth helps. An FM channel that is only as wide as an AM channel would sound pretty awful.
> FM has strong noise immunity advantages -- notably AM rejection and the capture effect -- but they don't provide additional sound quality by themselves. That's where the bandwidth helps. An FM channel that is only as wide as an AM channel would sound pretty awful.
Comparing such low-modulation factor FM with traditional AM would be an interesting experiment.
It certainly wouldn't sound good, but I'm not sure it would sound worse than traditional AM at the same SNR. The NFM use-cases I'm familiar with tend to cap audio bandwidth, so they're not fair comparisons.
I'm mostly imagining what music would sound like via NBFM on a VHF amateur or public-safety radio channel. It's not an appealing thought... the words "toll quality" come to mind.
But hey, no static at all... https://www.youtube.com/watch?v=HV3zWSawJiw
> This in turn is why air traffic still use AM where you can hear both overlapping transmissions at once (possibly garbled if carrier wave was off), and react accordingly rather than being unaware that it happened.
I’m not convinced this is the reason. The carrier wave is always off by a little. While you’re transmitting you hear nothing anyway. And when two parties are transmitting simultaneously, any third parties just hear very loud screeching. A 0.001% difference in carrier frequency would be more than enough to cause this effect in a VHF radio. Notably, this exact problem was a major contributing cause to the worst accident in aviation history. Using FM would have prevented it.
https://archive.ph/2013.02.01-162840/http://www.salon.com/20...
AM is used for two reasons - simplicity of transceivers
AND the fact that two simultaneous transmissions result in buzz instead of locking onto stronger signal. We WANT to know that there's a collision in transmission so that we know we need to retransmit. What would be the expected effect if two FM transmission on same channel were sent?
Fixing the "glitch" would result in way more problems than it solves. Interestingly, aviation authorities do not blame collission behaviour of AM radio for Tenerife, but instead corrected crew management procedures and pushed greater radio phraseology standardisation.
>We WANT to know that there's a collision in transmission so that we know we need to retransmit
Digital trunked public safety systems solved this problem decades ago. If you key up when the frequency is in use you get a distinct rejected tone. I'd think prevention is far preferable to sorting it out once everyone's finished walking on each other.
It also means you need to replace everyones radio at the same time because everyone needs to hear everyone on the channel.
Where new additional technologies are possible, they have been applied (digital packet networks, like with CPLDC - Controller-Pilot Data Link Communications).
Replacing A3E modulated VHF radio requires you replace it for literally everyone, because there are way more users at airport than you think.
> It also means you need to replace everyones radio at the same time because everyone needs to hear everyone on the channel.
In the public safety context it's not uncommon to phase in new systems (like digital trunked systems) incrementally. You accomplish that by simulcasting the dispatch audio over both systems, and monitoring incoming audio from both systems.
A common pattern for how this plays out would be something like this: all the fire departments and ems agencies in a given jurisdiction are dispatched using two-tone (eg, motorola) paging over a VHF frequency. New digital radios are introduced, and all the fire/ems personnel keep their existing pagers, and (some|most|all) are given the new digital radios. People without the new radios can still talk to dispatch using VHF. And of course systems can be configured to mirror audio around so that if one person is transmitting on VHF they can be heard on the digital system (usually on a channel in the 800mhz or 900mhz band). It's basically a fancy version of a repeater.
Dispatches are then given out over the same old VHF channel AND the new digital channel. In theory you can eventually replace all the old pagers and radios and quit with the simulcast deal, but IME, sometimes things stay in "parallel" mode more or less indefinitely for whatever reason[1]. That said, to your original point, you typically do want to get at least radios standardized as much as possible, even if you maintain the split for (paging|operational communications).
To illustrate, two jurisdictions I'm familiar with: Orange County NC, and Brunswick County, NC. Both followed the path I talked about above: all VHF dispatch for fire/ems, then adopted the NC VIPER digital trunking system, but continue to page on VHF and simulcast the dispatch information over both channels. I'm not sure exactly when Orange County adopted VIPER but it's been quite some time and they're still doing both. FSM only knows if/when they'll ever completely abandon the old VHF system.
[1]: and that reason is often as simple as "money". Plenty of volunteer fire departments in rural areas are skating by with barely enough money to keep their apparatus road-worthy. Replacing every hand-held and mobile radio they own in one fell swoop is often out of reach.
[Source: was a firefighter and 911 dispatcher in a previous life]
You're perfectly correct except one small thing.
You're writing about experience in a closed system - as far as I know all such dispatch systems for public safety etc are closed system where everyone who is ever going to be on the net is part of the system, and it might at most be a case of "we don't have money to replace every member's radio".
In comparison, aviation radio is an open system - not only you do not know who is going to communicate, the communication is also peer to peer, unlike many digital trunked systems which often depend at least on some level of cellular support system.
The only "access control" on the airband VHF and HF comms is of legal variety, with explicit carve out that the person actually flying the aircraft is way less bound by legalities in case of emergencies, and everyone has to be able to talk with everyone, especially on one of the standard common channels.
Examples from personal experience involved various combinations of small airfield ATZ, MiG-29, gliders, old ursus tractor (agricultural kind), busted up Opel Kadett, airliners, ultralights, small transport planes, private helicopters, and dunno who was responsible party but helicopter working as diplomatic flight.
All on one small airfield. And every one of those had to communicate independent of each other with everyone else on that list.
The only time we do "rebroadcast" is when we end up having to do a manual relay due to distance, which is also one of the rare cases where comms might switch over to a more modern system, because someone could ask ATC over VHF to pass something over CPDLC to airliner or using HF, and vice versa.
The poor A3E modulation on VHF airband is the lingua franca, the lowest common denominator, which allows random aircraft from anywhere in the world talk to another random aircraft, as well as ground.
> AM is used for two reasons - simplicity of transceivers
That is not a factor anymore. Capable wideband transcievers like the ones in Baofengs and similar supporting multiple types of modulation cost cents.
There's cost in simultaneous replacement for huge portion of the fleet.
Don't devolve into simplism, consider that you need to replace the radio for everyone sharing the same space, and that there might be way more planes sharing that space than you think.
And who would pay for the Supplementary Type Certificate for every single aircraft model out there, including many that were built by manufacturers that no longer exist? I don't think you understand how this stuff actually works.
That article makes out as if transmission blocking leads to a safety problem if a transmission gets lost. It doesn't. What that article misses is that aviation radio communications require readback and verification of the readback, in safety critical instructions such as "cleared for take-off". Not just for radio transmission blocking reasons, but also to detect mistakes in mishearing instructions.
https://en.wikipedia.org/wiki/Tenerife_airport_disaster#Comm... tells a more accurate story: the root cause was that the captain assumed they were cleared for take-off without actually hearing their own callsign and the word "cleared".
Since then, the word "take-off" is avoided in any other type of communication (eg. you might hear "report ready for departure" but never "report ready for take-off"), and every pilot knows never to assume that a clearance has been given unless they hear those exact words together with their callsign.
FM has 15kHz of bandwidth per stereo channel or an effective 30kHz sampling rate. The rest of the space is used for supplemental signals, including, the "pilot carrier" that is used to generate the "stereo image." There is space for three more full bandwidth mono channels on the end of an FM broadcast. One of them is often used for RBDS.
FM signals receive AM interference but heterodynes exclude them effectively. The cost is vulnerability to multipath reception in highly signal reflective environments and capture/wandering effects when two signals of similar strength are present.
AM _can_ sound pretty good. Most AM transmitter sites are poorly maintained, combined with other stations into one antenna system (something you can do on AM with a phasor), and are typically just simulcasts of FM content or satellite delivered content. There's no real care put into it. On a well maintained, tuned, and properly programmed station, mono content on AM sounds quite pleasant.
That's not even getting into "cost saving" measures that AM operators employ that completely compromise their signals. Or what Nielsen has convinced them to inject into their signals to register modern "ratings points" from the "portable people meter" system.
Guess where I used to work.
FM has 15 kHz of bandwidth available for the audio signal, which is much higher than what had been previously standardized for the AM channels and which is an important reason for the perceived high fidelity.
The modulated signal that is transmitted on the air has a much higher bandwidth. How much higher may differ between various broadcasting standards, but it can be e.g. 10 times or 20 times higher.
The ratio between the bandwidth of the transmitted radio signal and the bandwidth of the audio signal is what is relevant for the noise rejection properties of FM broadcasting.
When the bandwidth available for transmission is limited, FM is not an optimal kind of modulation from the point of view of resistance to noise, phase modulation (QPSK) is better (and optimum), so that is what is used for digital communications limited by noise.
> The ratio between the bandwidth of the transmitted radio signal and the bandwidth of the audio signal is what is relevant for the noise rejection properties of FM broadcasting.
Yes. FM radio is "narrow band" which gives it additional noise rejection properties; however, it's measured against the total available signal content not merely the audio portion of the content.
So, your pilot wave and RBDS and any additional carriers, if present, reduce this facility.
> FM is not an optimal kind of modulation from the point of view of resistance to noise, phase modulation (QPSK) is better
FM receivers often move and are often in highly reflective environments. FM is far better suited to this than plain PM.
Thanks for the interesting info!
My guess would be iHeartMedia
Going back to first principles, modulating the frequency instead of the amplitude inherently makes the system less lossy. Imagine you were communicating with someone miles away on a hilltop, and they had a lot of data to convey. Would you find it easier to distinguish signal vs. noise if the light was increasing and decreasing rapidly in brightness (AM) or color (FM)?
If you AM modulate the carrier f0 by a single tone f1, you get a spectrum with only three tones: f0 - f1, f0, f0 + f1. If you FM modulate it, well the thing gets much more complex - depending on the depth of modulation you can get a much wider spectrum than from f0-f1 to f0+f1. So it is hard to compare. Your FM modulated signal may indeed be more resilient to noise but will require wider channel to be properly transmitted.
I haven’t seen a convincing explanation if FM would be really that better than AM if both were given exactly the same channel width.
Neither is true. 9kHz, with two sidebands, means that the transmitted audio is limited to 4.5kHz, which is way too low to sound good. It was this, and not the noise, that made it sound much worse.
While one reason for limiting the audio bandwidth to 4.5 kHz was to allow a great enough number of channels in the long wave and medium wave bands, the second reason was to be able to reject the high frequency noise by low-pass filtering.
So there were two reasons for the low audio fidelity of AM broadcasting, and noise was one of them, with the contention between multiple broadcasters for the narrow available bands being the other.
> with the contention between multiple broadcasters for the narrow available bands being the other.
A non-obvious aspect of medium and long wave AM broadcast is depending on weather/atmospheric conditions a signal can propagate much further than its output power would suggest. This means a distant station on the same channel as a near station may end up in contention at certain times of day or random conditions. Solar flare? Suddenly stations a hundred miles away are overpowering local stations or just adding a lot of noise.
Medium and long wave is also susceptible to local EM sources like switching power supplies and electric motors. So you can get the double whammy of local noice and distant stations adding additional interference to local stations.
There is no reason that the channel spacing need limit the sideband bandwidth.
The only downside to this is that listeners on adjacent stations hear a slight "monkey chatter" from the overlapping sidebands.
This is one of many reasons why station frequencies are never allocated close to stations which are physically close.
You only need glance at the waterfall display on a good SDR receiver to see that the actual audio bandwidth is often much wider than the channel spacing implies.
for fun, try using a square wave amplifier to shift the wave:
-for AM you get sound effects such as chip monks
-for FM what do you get?
Someone gave me an analogy some time ago that made a lot of sense.
If you shine a flashlight through a tree blowing in the wind and vary the brightness to convey information, the signal can get distorted pretty easily.
However, if you have a constant brightness source and vary the color, it’s a lot easier to figure out what the source is trying to convey.
It's not merely an analogy, just the same EM waves scaled up in frequency by a few orders of magnitude.
Except that color isn’t the same thing as wavelength when it comes to humans perceiving light, because our eyes only deal with the total energy within each of three overlapping bands. An FM receiver knows the difference between a single carrier varying in frequency, and two carriers of different frequencies varying in proportion. Our eyes don’t, hence the banner above appearing orange, even though it’s actually made of different proportions of red and green.
Right, but let’s just consider the visual spectrum a single carrier/station
But that’s not how FM receivers generally work. They don’t just take a chunk of the spectrum and measure relative amplitudes within that window. Some very quick and dirty FM demodulators do something like that, but they have poor noise rejection, so the analogy fails.
Proper receivers use a phase-locked loop to “lock on” to a carrier, rejecting any weaker interference on nearby frequencies.
In the analogy, suppose you’re decoding a signal from a flashlight over the entire color spectrum, but sunlight shines through the leaves of the tree, adding a slight green noise component to what you see, while the flashlight is actually red. You’ll erroneously interpret the signal as slightly yellow.
We don’t have anything like the PLL in our eyes, so the analogy breaks down here. In the equivalent scenario with an actual FM signal, that slight “green” component would not affect the received signal (or it would affect it to a much lesser extent).
This makes a lot of sense so long as your source of noise is something like a tree swaying in the wind, ie something that interferes with the amplitude. If instead the source of noise is uhhh a piece of stained glass swaying in the wind then blinking the flashlight is the better bet. I guess it just turns out radio interference is more like the tree. But why?
Stained glass won’t (I think) shift any frequencies. It will attenuate different frequencies differently, but it won’t make up new ones.
So when the signal frequency changes, you’ll still see that change, but the light might get brighter or dimmer at the same time due to the stained glass. But you don’t care about the brightness to begin with.
In the stained glass case, maybe you need to go digital where brightness and color don't matter, but only on-off state.
You'd be surprised by the amount of brightness and color produced if you are turning things on-off sufficiently fast.
Related: lots of optical illusions.
In this analogy, the AM and FM signals you receive aren't usually experiencing interference, they are experiencing multipath effects which includes things like path loss, attenuation, reflections, and so on. This is driven by geometry. You also have gaussian noise that the receiver has to deal with.
You model this by taking your signal and convolving it with the channel vector. Usually the channel vector is a finite number of dirac deltas. Each delta is a different reflection. They are like echos. They can cause the signal to constructively and desconstructively interfere with itself.
I haven't seen the math, but I am guessing this doesn't do as much to the frequency of the signal compared to the amplitude.
a better analogy for frequency domain interference would be something like the spinning flashing lights on a fire engine or utility truck occasionally shining colored light on your detector.
I think this is reasonable analogy, for FM interference, and it points out why FM is resilient to noise: the flashing lights have to be relatively bright (high amplitude) to interfere with your color based scheme.
The stained glass would change the amplitude of some light selectively. But because the FM radio works at different distances, I wonder if it must have some way of adjusting for different amplitudes anyway?
Yes, stained glass is like band filter, they let through a particular frequency range, while reducing those outside the range. Your FM receiver will still lock on the desired frequency as long as their is enough signal strength. It's kind of the same as listening to an emitter that is very far while being very close to another. Of course, it'll stop to work at some point depending on minimum signal-to-noise ratio.
I always shy away from analogies because more often than not they give the wrong "feel" for a concept. But this is one of those rare exceptions.
It _is_ the wrong feel for a concept. The analogy breaks down because the color changes are way too wide in frequency (and thus too robust to noise) compared to what happens in a radio broadcast. If you changed the color from RGB(127, 0, 0) to RGB(126.999999, 0.000001, 0), the movement of that tree would actually start to make your strategy difficult.
Going from red to orange is about 50 THz. Typical FM radio modulation width is 100 kHz.
The analogy isn't wrong, it's just that it is incomplete as given. You are bringing the sensitivity of the receiver into the equation. But it doesn't really break down the analogy, because the frequency shift in color is calibrated for the human eye's sensitivity. Calibrate it for a FM receiver and that tiny color shift becomes easily discernible. The tree leaves have no impact on frequency, just amplitude.
The reason the analogy is good is because it isn't even really an analogy, it is in fact a description of electromagnetic waves and a noise source.
It's not an analogy. This is precisely how it works.
Unless your car radio consists of a flashlight and a tree, this is an analogy.
The flashlight is the radio tower, the tree is the tree, and the radio in the car is your eyes. There is no analogy here, it is literally the same EM waves shifted up to where our eyes can see them.
It's like saying that the violins is merely an analogy for how a double base works.
> it is literally the same EM waves shifted up to where our eyes can see them
Rubber ducks aren't battleships because they both float. Visible light and radio attenutate in meaningfully-different ways. It's an analogy.
You could make Bob Ross a new wig from all the hairs split in this subthread.
a lot of people here have a lot of passions, but sometimes the passions overlap and we rub shoulders. if someone had made a pokemon playing card metaphor we might be in the same general condition - but i think we're better behaved showing each other how smart we must be with radio waves instead of greymon
Both are examples of communication by means of frequency modulated and amplitude modulated electromagnetic waves with distortion from a moving three. Also a good example that a large change in quantity is a change in kind. Probably a legit analogy imho.
> Probably a legit analogy imho
It’s a terrific analogy. OP is arguing that it isn’t an analogy but an identity. For what should be obvious reasons, it isn’t. And in this case, the difference between analogy and our best model of reality is material.
Rubber ducks and battleships both displace water in the same way.
> Rubber ducks and battleships both displace water in the same way
Yes. Just like light and radio waves are both EM. A rubber duck remains an analogy for the buoyancy of a battleship. Not "literally the same" thing.
But if you say that a battleship floats on the water in a similar way to a rubber duck floating in the water... it's actually not similar... they are the same. It's the same water and the same physics. The "only" appreciable difference is scale.
For me, the people saying they are the literal same thing are the same type of people that gave me that "aha" moment that really helped solidify my understanding of RF.
It was pretty mind blowing when I Understood that AM is a change in brightness and FM was a change in color. We just can't see RF, but if we could, that's what it would be.
> But if you say that a battleship floats on the water in a similar way to a rubber duck floating in the water... it's actually not similar... they are the same. It's the same water and the same physics. The "only" appreciable difference is scale.
But battleship doesn't equal floating in water, floating in water is a property it has.
If you're saying "the way a battleship floats in water is like how a rubber duck floats in water" then it's not an analogy, it's as you say just describing two versions of the same thing.
But it is an analogy to directly compare the two objects, because "floating on water" is a property of the objects it's not the object you are comparing.
Wikipedia begins its page on analogies with this, sourced from The Oxford Companion to the English Language: "Analogy is a comparison or correspondence between two things (or two groups of things) because of a third element that they are considered to share."
Or Marriam-Webster: "a comparison of two otherwise unlike things based on resemblance of a particular aspect"
Apart from rubber ducks and battleships both having the "third element", or aspect, of "primarily used for floating on water", they are definitely two completely different things. Nobody could look at a rubber duck next to a warship and say "they seem to be the same thing".
The more closely related two things are the more useful and less stretched the analogy available, which is why the analogy about radio waves was so enlightening to so many people in this thread. But it's bang on as the definition of what an analogy is.
Ducks and witches, on the other hand. . .
But RC boats and battleships both have propellers and rudders.
> Visible light and radio attenutate in meaningfully different ways. It's an analogy.
Lol news to me and my physics degree, Do tell because as far as I'm aware Maxwell's equations don't have an asterisk on them that say "doesn't work below 1 GHz".
> as I'm aware Maxwell's equations don't have an asterisk on them that say "doesn't work below 1 GHz"
You don’t see how one being able to attenuate around a hill while another needs line of sight isn’t material to the way we use light and radio waves?
> Do tell because as far as I'm aware Maxwell's equations don't have an asterisk on them that say "doesn't work below 1 GHz".
Did you really just pull out Maxwell's equations?
EM interacts with matter in different ways. Glass hardly attenuates visible light, but wood does. 2.4 Ghz can pass through walls better than 5Ghz.
There's the concept of permittivity wherein Maxwell's equations are defined in free space with vacuum permittivity.
https://en.wikipedia.org/wiki/Vacuum_permittivity#Permittivi...
To accurately model EM waves, you need more than just Maxwell's equations. You require material equations to model interactions of EM with media.
If you want to get really advanced, whereas Maxwell's equations are classical physics, there's Quantum electrodynamics (QED) which can model interactions of EM and matter.
https://en.wikipedia.org/wiki/Quantum_electrodynamics
RE "....Glass hardly attenuates visible light...." Clear glass blocks about 5% visible light
Depends on the frequency of EM. Fiber optic communications use specific frequencies to minimize attenuation in cables.
https://en.wikipedia.org/wiki/Optical_fiber#Mechanisms_of_at...
Same with communications over coax. Obviously visible light doesn't transmit well over copper, but a spectrum of radio waves do, some better than others.
Fiber optics also uses _exceptionally_ clear glass.
If the ocean were as clear as your average long-distance fiber cable, you would see down to the bottom of the Mariana Trench (also in the range of visible light, AFAIK).
> Fiber optics also uses _exceptionally_ clear glass.
Clear in certain wavelengths. Depends on the composition of the glass.
https://en.wikipedia.org/wiki/Optical_fiber#/media/File:Si_Z...
Silica glass behaves differently from ZBLAN (fluorozirconate glass).
Which goes to show how complicated EM interactions with media can be. It's generally easier to just empirically measure attenuation through some medium and use the empirical measurements as a model.
It's exceptionally clear compared to e.g. window glass even in the visible spectrum of light. You can shine a red light source into a 10 kilometer standard G.657 fiber (optimized for 1310/1550nm, i.e. deep infrared) and it will still be visible just fine on the other end. If you did that with regular glass, it would hardly go ten meters.
What are the relative contributions of the total internal reflection property of the fiber optic cable and the particular low-attenuation material it's made of?
Oh yeah. I'm not saying otherwise. Someone replied "Clear glass blocks about 5% visible light". I guess "clear glass" is pretty subjective. At what level of attenuation would someone consider glass not clear? xD
> There's the concept of permittivity
> You require material equations to model interactions of EM with media
> Quantum electrodynamics (QED) which can model interactions of EM and matter.
It's amazing how condescending some people on here are; how could you possibly have missed literally in the first sentence of my response
> ... my physics degree
> It's amazing how condescending some people on here are
You literally started your comment with "Lol news to me", then you used your degree as if it made you more knowledgeable than anyone else here. Take a look in the mirror?
> ... Do tell
I did?
The extra information isn't to condescend. It's for other people that want to know more about the science.
It's not saying the violin IS "merely an analogy for how a double base works", just that a violin can be used as a simple analogy to somebody who understands how a violin works but doesn't know what a double bass is.
Comparing similar things is literally what an analogy is, the fact that in these two cases (radio/light and string instruments) the things being compared are very similar it doesn't make them the same thing, nor does it make it not an analogy.
It is not literally the same. The colors you perceive on the screen in front of you demonstrate why. That banner above is not orange; it’s red and green. You don’t actually have the ability to distinguish a varying frequency between red-orange and yellow-orange, and two amplitude modulated “carriers” at red and green.
That’s the hallmark of an analogy. It gives the general idea, but breaks down if we interrogate it in too much detail.
Essentially that is actually the case. Human-visible light and AM/FM radio waves are just different wavelengths along the EM spectrum.
A flashlight beams out waves that we can see; a radio transmitter beams out waves we can't. The brightness of the beam of light is related to its amplitude, just like the signal content in AM radio is related to its amplitude. And the color of the beam of light is related to its frequency, just like the signal content in FM radio is related to its frequency.
The explanation asks us to imagine shining a flashlight through a tree, first changing the flashlight brightness and then changing its color.
The flashlight is an analogy for a radio transmitter. We all get that they work on same principle but just on different wavelengths. But regardless I can't shine the flashlight in my kitchen drawer at my radio and pick up a signal.
Remove cover, locate LNA, modulate light correctly and voila... :-)
Well... It kind of does. The source of the radio station is a kind of flashlight, just on a different frequency. The tree is still a tree (and all the other objects)
More accurately a giant lightbulb, but emitting at 102.7 MHz (my favorite local radio station) rather than ~450 THz (my favorite color).
Put visible light over a really long waveguide and modulate the colors, you invented fiber optic telecommunication.
Both concepts are based on frequency and amplitude of waves (radio vs light).
That's what makes the analogy so clear.
Good analogy, however if you move back and forth the transmitter or the receiver at enough speed, frequency (color) will vary as well, and that analogy could be used to explain Doppler effect, and why civilian airplanes use AM.
Civilian airplanes aren't using AM because of the Doppler effect. You're not accelerating that rapidly to make the Doppler effect that pronounced on the kind of radio being used in airplanes to the point they wouldn't be useful. Even if you're going hundreds of miles an hour the shift is going to be a few dozen Hz in drift. A cheap FM discriminator will be able to handle that without any problem. Doppler shift starts to matter when dealing with satellites, but not airplanes unless you're taking a SR-71 on a civilian stroll.
Doing the math, if you're going 200mph away from a station transmitting at say 121MHz, the drift frequency would be ~36Hz. Not going to be a problem.
And even then, your AM transmission still gets affected by Doppler shift as well.
Airplanes use AM because when two SSB transmissions happen at the same time you can actually hear both at the same time. If you're using FM it's either an incoherent mess or one transmitter drowns out the other.
That's not the real story. The RF environment is noisy, with naturally occuring static "sparks", but also with manmade RF noise.
This static and RF noise is AM. It's impossible to filter it out from an AM signal, and so the background noise gets amplified with the signal.
Encoding the signal in a modulated frequency (FM) means we don't need to amplify the detected AM signal and it's associated background noise.
It's not that simple, though. The only way you can detect frequency is by measuring the amplitude (and then differentiate; except of course in an analog circuit, you don't do that exactly, you have some mechanism that tries to track the carrier wave smoothly instead), so amplitude noise will necessarily also become frequency noise. But generally white AM noise will be pushed upwards in the spectrum after FM demodulation, away from the area where you care. (You can also add a hard limiter, which amplifies this effect; even more noise high up, even less noise further down.)
That's exactly what the parent comment described in the beautiful example where the AM noise is due to moving tree leafs affecting the intensity of transmitted light, and you can fix it by varying color, which means varying the frequency spectrum of the light.
How does the radio follow the frequency modulations if the radio cannot "see" at a specific direction?
In the example, the amplitude of the flashlight signal is distorted by the movement of the trees. The signal is never completely hidden. Not sure if that answers your question...
I read a similar explanation on slashdot a few decades ago that's stuck with me.
This seems great at first, but more so as an explanation of how AM and FM differ; one being by amplitude (brightness), and the other by frequency (color).
What I don't see is how it explains why one would work better than the other.
If the tree is blowing in the wind, and a leaf obstructs the entire signal, it doesn't matter whether it's a change in brightness, or a change in color. Either way, that information is lost by the blocked leaf. And if the entire signal is not lost, perhaps many leaves may have blocked the signal but some signal managed to get through, it doesn't matter whether the signal change was a change in brightness, or a change in color. Either way you're going to notice the change. So I don't see how this clarifies why FM is better. What am I missing?
I see from the article that "noise tends to be a an unwanted amplitude modulation, not a frequency modulation." In other words, the tree is providing an unwanted change in brightness. It never provides an unwanted change in color.
I guess the tree is able to dim the signal so much that it appears to be a deliberate signal change? Couldn't this be dealt with if you know the details of the tree's dimming ability?
The analogy is getting a bit tortured, so I'll try a more practical explanation.
An AM receiver is a machine that senses the amplitude at a specific em frequency. In this situation, noise and interference become random additions or subtractions to that amplitude. Draw a sine wave, then go over the line with vertical ticks or scribbles. Now imagine taking a random sampling of points and reconstructing the original wave perfectly (without a computer). Most of the information is just gone and you end up with a noisy output wave.
Now an FM receiver is one that measures frequency changes above and below a 'carrier' frequency. The amount of deviation away from center represents the amplitude of the sound signal being transmitted. In this setup, noise and interference are also random additions to the amplitude, but also at random frequencies. On average, interference happens evenly over the entire range of frequencies you're looking at. That means that the highest amplitude is still the same frequency away from center, it just has a slightly different amplitude.
Go back to that sine wave. You can't see the original signal behind all the noise, but you can still see how far apart the peaks are. You can still easily extract its frequency content.
FM uses the frequency dimension to transmit data because random noise can't really affect frequency. Noise mostly happens in the amplitude dimension across all frequencies at the same time.
FM is more robust because it uses two dimensions to encode information vs AM's single dimension. That's also why FM is in stereo!
> That's also why FM is in stereo!
Stereo FM is essentially two waves transmitted at the same time (it's common and difference instead of left and right, but that's math). Stereo AM would be possible, it was never done because two different AM transmissions have to be spaced further away than FM.
AM stereo does exist, however.
https://en.wikipedia.org/wiki/AM_stereo
There were a number of successful AM stereo broadcasting methods proposed and trialed. These were completely compatible with conventional AM transmissions.
The conceptually simplest of course whas where the LSB and USB are used as separate channels.
Although most of the systems did work, they were not ultimately successful simply because insufficient stereo receivers reached the market.
Go search in Wikipedia on "AM Stereo".
Of the methods listed on Wikipedia only ISB is a true AM. All the others use phase modulation for the difference signal, as an easy way to achieve compatibility with mono AM receivers; and PM is basically the integral of FM.
Wikipedia says that there was basically one station doing ISB stereo; which I guess is close enough to "nobody did it", but not quite "it was never done".
Could you make AM stereo by somehow using the two sidebands (on each side of the carrier) for left and right?
Yes, this is one of the proposed methods. It's known as "Independent Sideband".
It works, but it is a fairly expensive method to implement.
FM works better because it is easier to detect the change in frequency independently of any change in the amplitude.
I'm unsure of what the correct terminology would be, but (for my linear algebra brain) you could say something like, for FM the noise dimension is orthogonal to the signal dimension, while for AM the noise and signal dimensions are the same. Therefore for FM any change in amplitude in the noise dimension should be mostly isolated from the signal dimension, while it is essentially impossible to tell what is noise and what is signal for AM - you could probably do some radio equivalent of a differential pair in order to detect noise and remove it, but then why would you bother when FM has improved noise rejection anyway.
> What am I missing?
The tree blowing in the wind will introduce its own amplitude (brightness) fluctuations. It will be hard for you to tell which amplitude changes are signal from the source and which are noise from the tree.
Edit: Looks like you answered yourself while I typed that, where you added:
> Couldn't this be dealt with if you know the details of the tree's dimming ability?
If the tree is moving, and you’re far enough away to resolve individual leaves (which is not unreasonable) then its “dimming ability” is constantly changing.
A leaf blocking some light doesn't change the color of the light that passes through.
> leaf blocking some light doesn't change the color of the light that passes through
Of course it does. Real-life objects aren't perfectly opaque or transparent. Similarly, radio waves aren't blocked or received: they're mangled and self-interacted in complex ways.
I think the idea is that the leaves don't block the entire signal. They just partially obscure it sometimes.
And even if leaves do sometimes block the entire signal, you're still going to do better with varying the color than the brightness.
Let's switch the analogy to sound. Amplitude is loudness. Frequency is pitch. You are trying to discern two sources of sound. One is a constant pitch but variable volume. The other can always blast at max volume with variable pitch.
Also harder to discern and then quantify the loudness of a sound or brightness of a light as a human modem but we are better and more certain of the color. We have different names for the ranges and everything.
> harder to discern and then quantify the loudness of a sound or brightness of a light as a human modem but we are better and more certain of the color
Fair enough, this might be a sensory artefact. In this case, however, nature had a point. Energy scales proportionally with frequency but exponentially with amplitude. Increasing amplitude delivers more bang than increasing frequency.
> Either way you're going to notice the change.
For this, it's better to stick to many leaves - the analogy holds up well here because when is the brightness change due to the number of leaves being in the way vs the source changing its brightness?
if the leaves are blowing back and forth between the transmitter and the receiver, they would introduce a doppler shift into the signal
of course, you shouldnt be listening to radio during a tornado, but...
This is the best explanation I have ever heard.
Wow, that is pretty clever.
I'm stealing this.
I’m confused how this is even a question.
With AM, anything that causes a variation in the intensity of the signal will introduce noise.
With FM, anything that causes a variation in the timing of the signal will introduce noise.
Unless you’re traveling at relativistic speeds, operating a time dilation device, or colocated with a black hole, you usually aren’t going to see the rate that time flows at vary.
Thus if you can make the amplitude of your signal irrelevant past a certain threshold and embed all the information into the time domain, the only thing introducing interference should be other EM sources that happen to be on the same channel.
That is true for uncorrelated broad-band noise.
Correlated noise (e.g. multipath interference) and narrow-band noise (e.g. another FM transmitter) can both affect FM pretty badly.
Speaking of multipath interference, why is it that we almost never hear the effect of that? Like, aren't these waves almost constantly bouncing off of other things and being reflected? How are we not always hearing echos all the time?
The other poster is correct but I feel there's still a simpler answer when you see the units at play. You don't constantly hear reflections as echos mostly because of the speed of light and the inverse square law.
Let's think about how far the echo has to come from to have even a half second delay. At the speed of light, that half second is 93,141mi. So it would have to reflect off something half that distance, ~46,500mi. Not a lot of good reflectors pointed at me 46,500mi away.
So then think of the inverse-square law on that. How weak of a signal is that going to be travelling those 93,141mi? Are you set up to even notice that from the noise? It's 11 billion times weaker than the original signal, assuming your reflector perfectly reflects the source signal and you're in outer space.
So obviously whatever "echo" we experience, it's not going to be something in the realm of humans directly detecting it. The shift that is possible to really mess with the signal at distance you'll actually receive reflections at are only going to shift the timing in a very small way, usually by being a slightly different phase. This means you'll get constructive and destructive interference from the same signal at slightly different phases, but not really a noticeable "echo".
The effect is easily noticeable only for high frequencies, where the wavelengths are no bigger than a few meters.
For the lower frequencies used by AM broadcasting, where the wavelengths are up to hundreds of meters or kilometers, and you use small antennas for reception, it is unlikely to have problems caused by multipath propagation (because the waves will go around obstacles instead of being reflected; only for the higher frequencies of the shortwave range you can have multipath reception of signals reflected by the ionosphere, but the objects that are around the receiver still do not cause problems).
When there is multipath propagation, you would not hear echos, because the time difference between the different paths is too small, due to the high speed of the radio waves. What you get is interference between the multiple signals, which can reduce too much the strength of the received signal. When the signal is reflected on some paths by mobile objects, or when the receiver itself is moving, the received combined signal will have an amplitude that varies in time, with intervals when the signal cannot be received (i.e. fading).
Other people answered your question, but if you ever saw shadowing artifacts on an analog UHF TV station, that was probably multipath. Note that lines on an NTSC TV are scanned at over 15kHz, so this is a very small time difference.
I was surprised this wasn't leaned on more explicitly for the explanation.
I think this is largely held in the assumptions that go into saying most noise will be amplitude modulation?
Edit: Reminds me of the banal but vital insight that digital uses repeaters to gain distance, whereas analog uses amplifiers. Makes it very easy to consider why/how digital took over.
> I’m confused how this is even a question.
It sure is, why would everyone or anyone be aware of AM/FM differences? Even if one is tech savvy it doesn't mean this would be something trivial to understand at glance
FM usually has higher fidelity than AM even if no noise is introduced.
Right. Isn’t FM just flat out higher bandwidth? So unless it’s wasting that somehow it’s just going to carry more information. And for audio, that means sounding better.
The use of FM doesn’t inherently imply higher bandwidth. For example, those consumer-grade FRS/GMRS radios you get from Costco use narrowband FM, which typically occupies about 12.5 kHz per channel. This is much narrower compared to the 200 kHz bandwidth used by FM broadcast radio.
FM is simply a method of modulating the carrier signal by varying its frequency. The actual bandwidth depends on factors like frequency deviation and modulation, so FM can range from narrow to wide bandwidth depending on the application.
Good point. I was referring to the public airwaves with the layman’s definition of “a radio”. But that was how it was chosen there.
https://www.pbs.org/wgbh/aso/tryit/radio/radiorelayer.html
FM sounds better than AM partly because frequency is more durable than amplitude, but it's not the whole story.
Frequency does not diminish with the inverse square law, as does the amplitude of a wave that is broadcast in all directions. This is because frequency is related to a count of events over time.
Frequency from a source light years away is intact; we can look at frequency bands from a radiating celestial body and know which chemical elements there are, and also tell exactly how fast it is moving away from us from the red shift in that spectral pattern.
Be all that as it may, AM should sound great when you are close to the radio tower, and have ideal reception with no multi-path reflections, and good signal/noise ratio.
It still doesn't sound good, and that simply because of the bandwidth allocated to it is low. Furthermore, AM Stereo is a retrofit and crams two channels into one via phase modulation.
AM stations are separated only by 10 kHz, as you can see on your AM tuner (which you likely have only in your car, if that). The bandwidth is directly related to the audio bandwidth because modulation produces side bands.
For instance, if we modulate the amplitude of a 650 kHz carrier with a 1 kHz audio tone, we get side bands of 651 kHz and 649 kHz. You see where this is going? We can only go up to 5 kHz before we bump into the next station, which also needs +/- 5 kHz for its side bands.
This 5 kHz limitation is why AM radio sounds like your speakers have a heavy woolen blanket over them. It's almost as bad as the bandwidth limitation as narrow band phone calls. Listening to AM music is almost as bad as listening to on-hold music over a narrow band codec like G.711.
The kicker is that only one side band is needed to reconstruct the signal, so in theory AM stations could have 10 kHz bandwith. Unfortunately, SSB was not deployed for broadcast AM, even though it was already known at the dawn of radio.
(https://en.wikipedia.org/wiki/Single-sideband_modulation has a note about why)
This is a myth. There is no reason that channel spacing need limit the modulation bandwidth. The only downside is that listeners to adjacent stations will hear a slight "monkey chatter" from the overlapping sidebands. In reality stations are never allocated adjacent frequencies within the same coverage area so this usually doesn't happen.
Be that as it may, AM radio is obviously low-pass filtered. It might not be a brick wall at 5 kHz, but it sounds obviously muffled to someone who can't hear anywhere near up to 20 kHz. If I were to guess, based on years of experience of playing with EQs, I would say that it has next to no content beyond somewhere around 8 kHz.
It would also be harder to tune into a station that is SSB because there's no carrier to detect. If you're slightly too high or low, the audio will have a slightly higher or lower pitch. I'm just guessing but with modern radios that wouldn't be a problem, but when AM was still used a lot I think (analog) oscillators tended to drift a bit, and you would have to adjust your radio often to correct for the changing pitch.
I think we had some fairly recent discussion on HN since I remember commenting.
As you're saying, it's about bandwidth and signal to noise. Not something inherent to modulation.
The modulation is important. FM is more robust against external noise than AM.
Only while signals are strong. On weak signals however, AM has a considerable benefit in intelligibility over FM.
FM stays good as the signal weakens, and then kind of drops off a cliff almost.
Can we use this fact to enable faster than light communication?
Even though the frequency survives the long trip, any changes in that frequency are observed only after a delay corresponding to the speed of light.
Someone once pointed out that shadows (which aren't objects with a mass and position) can move fast than light, at a sufficiently large distance from their origin. That is, the location of the border between the shadowed and unshadowed region can be changing faster than light speed. But that fact can't be used to communicate faster than light, because the changes in the location of the shadow's edge still take a comparatively enormous amount of time to propagate from their source to their destination. If you're creating the shadow, you can know that one galaxy will observe the shadow long before another galaxy does, but you can't use that knowledge to signal something to one galaxy or the other without waiting for the light (or lack of light) to travel all the way to that galaxy.
Not if measuring by relative speed.
As a non radio engineer, reading this thread on HN is so fascinating because there are so many heated conflicting explanations for a common phenomenon in a well-established technology. I thought this simple question would be settled already. If such disagreement is possible even in an established "hard" science like this, then no wonder some people think everything is subjective.
This question is of course settled. However, it has more than one aspect and I am pretty sure there are also a lot of rather amateur people chiming in here ...
I am an EE, but in power electronics and not an RF engineer so I am a little bit hesitant to comment too much on it, but my understanding is that it mostly breaks down to two aspects:
1) noise interferes more with amplitude 2) the fidelity of the modulating signal in FM is higher (more bandwidth)
HN is not a meritocracy, anyone with an opinion can just chime in and then get upvoted by people who don't really understand what's going on.
Also the audio frequency bandwidth is narrower on AM, so fewer treble frequencies.
TBH I think music from up to the late '60s (especially if originally released in mono) sounds really good, or at least more "era-appropriate" on AM radio. I remember my grandparents tuning in to easy-listening AM stations as I grew up in the '70s and '80s, to my ear Tennessee Ernie Ford's "16 Tons" or a classic Phil Spector "Wall of Sound" production sounds more "right" coming through the AM bands.
And, in the age of cellphone speakers and compressed MP3/Bluetooth codecs - I'm not sure how much people actually care about audio quality.
> And, in the age of cellphone speakers and compressed MP3/Bluetooth codecs - I'm not sure how much people actually care about audio quality
Bizarre thing to say after waxing nostalgic about incredibly lo-fi bandwidth limited AM.
This is also the age of $9 per month unlimited lossless 24/96 streaming and $1000+ headphone amps.
they have to justify their non newtonian vibration dampeners (blutak) and custom AC power filter used to play noisy vinyls
You can use literally any bandwidth with literally any form of radio-wave modulation.
Those codecs got better with time. Also notebook and little portable speakers, while they are unable to physically reproduce low frequencies are getting better at emulating those. Somebody cares.
And here's (dunno if true as they write in the description - probably the very first stereo) studio turntable from 1958 playing a record from 1988 through Youtube's compression. I did have a lousy vinyl deck with so so speakers when growing up and this impresses me a lot: https://youtu.be/PRty-_eBEpg?si=GsrctxRbkvT3xRAV
What does "emulating low frequencies" mean?
It may be impossible for a little speaker to produce any sound at some low frequency, so manufacturers use "virtual pitch" psychoacoustic phenomenon by introducing harmonics above that frequency. There is no low bass, but there will be added harmonics that will be perceived by the listeners as low bass: https://sound.stackexchange.com/questions/37755/how-do-psych... Here's also a project and some info from Asahi devs on Macbook audio: https://github.com/AsahiLinux/asahi-audio
Is commonly known as "Bass Boost".
As the OP has said, it cannot give louder bass, but simulates the bass harmonics.
This is yet another myth:
The "Woolyness" of AM broadcast (at least in America) is due to the stations purposefully tailoring their audio processing to suit typical cheap AM receivers. And this in turn is because designers of cheap AM receivers fit narrow filters instead of using noise reduction techniques, eg a good outside antenna.
There was a period (in the rest of the world) where high quality AM receivers had a narrow/wide switch to give better audio response to stronger signals.
The good news is that modern SDR receivers usually have selectable bandwidth on AM so as to derive the full transmitted audio. And many of these have AM stereo decoders as well.
If you listen to a good quality AM broadcast (eg Gov AM stations in Australia) you will hear audio which are very hard to tell from FM audio.
Go back and read the many high-quality AM tuner articles in the electronic hobby magazines from the past.
TBH I think music from up to the late '60s (especially if originally released in mono) sounds really good, or at least more "era-appropriate" on AM radio.
That music also sounds more era-appropriate coming from a vinyl record than a CD.
Look, I'm an audio snob and will talk shit about terrible design of BT headsets that halve bandwidth in duplex until the cows come home.
But the reason that codecs have survived this long without substantial changes is because they're far and away good enough (*) for the vast majority of listeners. To the point where today, even trained listeners can't perceive a difference in audio quality between lossless and lossy encoded audio at high enough bit rates (which is 320kbps MP3, or comparable AAC which can be as low as 50% of that).
(*) what we don't talk about is the latency of the codec itself, where regardless of available compute resources is still atrocious outside of proprietary codecs. While a listener cannot perceive noticeable differences in fidelity, they can perceive the delay, and this is a problem that doesn't have good solutions outside of specialized equipment today, although OPUS (as a descendant of CELT) is pretty darn good for the cases that consumers care about. Professionals still spend oodles of money on the proprietary gear that have codecs that not even ffmpeg supports.
I would go so far as to say there is no practical benefit to uncompressed audio today at all. Lossy is fine for all consumers, and lossless encoding is faster to decode and playback (as well as encode and write) while using less disk/bandwidth than uncompressed for archival purposes.
its a big difference.
The frequency range for AM radio is 540 to 1600 kHz
vs
30hz-15khz
Bass and fundamental frequencies really contribute to fidelity
FM is not very practical on frequencies that bounce off the atmosphere with any regularity as they have a lot less bandwidth to offer.
The reason this conversation is so foreign to most of us is because back in the day of shortwave radio our suburban and urban environments weren’t soaked in RF interference.
Go out to the middle of nowhere and try a shortwave radio or an SDR with an extremely long wire antenna strung up in trees. You’ll receive the world.
> noise tends to be a an unwanted amplitude modulation, not a frequency modulation
said someone who didn't understand anything about signal processing.
Been debunked so many times: https://physics.stackexchange.com/questions/94198/why-does-n...
One thing this seems to let out is that in FM, the signal is being broadcast with constant power. A 1MW FM station is always sending 1MW of signal; with AM the signal power varies; if you broadcast in AM powerful enough to mask the noise during periods of typical signal level, then you would still hear noise during quieter than typical periods.
Deeper explanations for those who aren't satisfied:
https://physics.stackexchange.com/questions/94198/why-does-n...
https://ham.stackexchange.com/questions/6312/why-are-fm-radi...
Some more about FM at http://www.theradiohistorian.org/fm/fm.html — https://news.ycombinator.com/item?id=41471355
« FM signals were much more immune to interference than AM due to its “capture effect” – an interfering signal needed to be more than 50% the strength of the desired signal to cause audible interference, compared to 5% or less with AM. This characteristic would considerably reduce the required separation between stations occupying the same channel and allow more channel re-use, which compensated for its greater occupied bandwidth. And most importantly, because all natural and man-made static is amplitude modulated, FM proved to be amazingly noise-free. Armstrong improved its resistance to noise still further by incorporating a new receiver component – a limiter that stripped off the amplitude variations in the received signal before it reached the detector. He had finally solved the problem of static interference that had confounded radio experts since the beginnings of the art. »
It's quite possible to have wideband AM radio. Some radio stations did it in the US before the FCC standardized bandwidth and started checking envelopes. Radio Caroline, the UK offshore pirate station (1964-1968), was wideband AM.
Noise on AM can to some extent be overcome with power and a low modulation percentage. That's how analog broadcast TV worked. (Broadcast TV was AM video, FM audio.) The black level for the video signal was well above zero. A high black level allowed showing black areas without excessive noise. About 80% of the RF power went into the carrier because of that. Simple, but inefficient. The same trick can be done with AM audio radio, although it seems that's not done much.
Radio Caroline would be such a good HN topic in its own right. Peter Chicago's name in particular should be up there in hacker lore for some of the things he did to keep Caroline on the air.
Lightning is a great example of noise causing amplitude changes and not frequency changes. That’s why during a thunderstorm am radio plays each strike between the station and you. The is usually not any indication of lightning strikes on FM.
Here's some SDR generated AM with 15 kHz audio bandwidth. Also shows why SSB isn't used for music broadcast.
https://www.w6rz.net/am.mp4
Needed an emphasis curve applied to it.
There are many differences that explain why one can be better. All else being equal:
- bandwidth of modulated signal: it is better to spread the signal over a large bandwidth => Nlog(1+snr) > log(1+Nsnr). the bandwidth used by the FM signal is larger
- wasted energy on the DC signal: AM signal is A + s(t) where A > abs(s(t)) to make sure the sent signal is always positive. A (DC) does not carry information so the effective signal to noise ratio of a DC-less signal should be higher (phase/frequency modulation, signalling that can detect the negative part...)
- filtering of baseband signal => if you filter too much the original signal, you lose information even before transmission. Voice is usually filtered and 4KHz, but music needs more. FM has more margin (more allocated bandwidth) so can have less stringent filters
- tolerance to fading: the wireless channel is not AWGN, it is frequency dependent due to multipath. While radio signals are relatively narrow, signal modulated in frequency are more robust to fading (OFDM...)
The way I had this explained to me when I was in highschool was something like:
Imagine someone is shining a flashlight at you through some trees. It's a lot easier to tell what color it is, than how bright it is.
I always assumed it was because FM station bandwidths (200kHz) are much wider than AM (10kHz). AM's 10 kHz chops off a lot of human-hearable frequencies.
AM doesn't use the frequency for modulation though so it shouldn't matter.
When you amplitude-modulate a carrier wave with an audio signal, you spread it out into a bunch of sum and difference frequencies, as you can see if you use the trigonometric angle-sum formula to factor cos(85000·2πt) · (2 + cos(440·2πt)), a 440-hertz flute being transmitted on 85-kilohertz AM. These so-called "sidebands" mean that the bandwidth of AM does matter, and consequently, using a too-narrow bandpass filter on your AM radio station will result in low-pass filtering your demodulated audio signal.
AM does use the frequency, it just doesn't need as much and uses it differently than FM. If it was all at a single frequency, there just be a single tone getting louder and softer.
Thanks. I just learned that doing a rabbit hole about sidebands! Still getting my head around it.
Once you change the amplitude of a sine wave (modulate it) it's no longer a side wave. It spreads in the frequency domain. Take the fourier transform of that and you can see the frequency components.
Other comments gave a nice explanation of why AM does need a bandwidth, but here's the information theory explanation: https://en.m.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theo...
TL;DR: the information one can reliably send through a noisy channel (C) is proportional to the bandwidth of that channel.
The article kinda sucks as it does not really answer the question it poses. Why ”noise tends to be a an unwanted amplitude modulation, not a frequency modulation”?
Is it due to naturally occurring background noise being low frequency high amplitude, showing up as AM? Could the situation change if humans keep generating more high-frequency noise? Or is it just that high frequencies do not travel as far so there will always be relatively little?
What I’ve never understood is how the FM receiver can lock on to the signal if its frequency is always changing. Doesn’t the receiver need to lock on to something? If the answer is “it locks on to the amplitude, which doesn’t change,” well AM is bad because the amplitude is subject to interference, so wouldn’t FM have the same problem?
Having recently purchased a RTL-SDR and watched and learned about FM -- there is "pilot" frequency that doesn't change and is fixed relative to the tuner frequency. See this chart here:
https://en.wikipedia.org/wiki/FM_broadcasting#/media/File:RD...
Each radio station has 100khz of bandwidth centered on it's tuner frequency. in the, there are channel spacing rules that give some gaps +/- another 100khz of that. (That's why in the US, radio stations are typically on 'odd' decimals, ie 92.3 mhz, 94.1 mhz, etc) That chart does not show HD radio frequencies, which due to those spacing rules, and more accurate transmitters, are on the +/- 100khz spaces along side the original analog 100khz. You can "see" the audio modulating the frequency on the spectrogram. But the OFDM digital signal on either side looks like a band of more intense noise. It's mind blowing to realize there's a signal in that!
The pilot is there for stereo decoding, it has nothing to do with the ability to tune to an FM station.
https://wiki.analog.com/university/courses/electronics/elect... has some of the analog approaches collected.
You're right -- after reading some of the peer responses, I realized that (I think...) my response is just how the Broadcast FM signal modulates the parts of the signal, and not how it actually 'locks on'. I'm still learning!
I got an RTL-SDR this summer and brushed up my DSP skills playing with FM signals. PLLs are marvellous beasts; you can do a slapdash job in "designing" one and it'll still probably lock on just fine. Might not be optimal but will still lock.
Another fun one, when you have IQ samples, is the polar discriminator: calculate x[t] * x*[t-1] where x* is the complex conjugate, and take the angle with arctan. Feels a bit like magic ("is that all?") but is justified by the theory.
One possibility is a phase-locked loop. I don't know if there's anything better. It matches the frequency of a voltage controlled oscillator to the frequency of the incoming signal by detecting the phase mismatch. Then, the control voltage for the VCO becomes the audio signal.
I don't think radios use a PLL to demodulate the FM audio: the signal has a huge "pilot" tone at 19kHz that you can match to get the first part of the spectrum, mono audio (L+R channels), and at double that frequency you know you'll find the stereo part (L-R channels). Precise phase estimation is only necessary to decode the RDS digital data (station name, datetime, etc.).
They do.
First of all, the pilot is only required for decoding stereo and RDS. Mono FM does not use a pilot, so obviously there had to be a way to detect FM before stereo came along. I linked to a few of the approaches in a sibling (cousin?) comment.
Second, the pilot is embedded in the decoded FM audio. You need to demodulate FM to get to it in the first place. If you look at the waterfall display in an SDR receiver, it might seem like the signal is already present in the original radio frequencies (especially during silent periods), but it's there only indirectly.
If you have silence in an FM transmission (say 96.6 MHz), the only audio component present is the 19 kHz pilot signal, which causes the FM radio signal frequency to vary between 96.6 MHz ± k*19 kHz (not sure what's the value for k, but it's not 1). The sine likes to spend most of the time near the extreme values of its range; plot a histogram of a sine wave and you'll see peaks on either end.
The waterfall is basically a histogram over frequencies so it gets those peaks as streaks on both sides of the main carrier frequency (plus smaller ones for other components in the signal).
The 19 KHz pilot tone is interesting: it's not filtered out and is often reproduced by the speakers if you have good ones. You can verify this by using an audio spectrum analyzer application on your phone.
I've wondered if FM stereo drives pets nuts with its constant high-pitched tone.
Most cheap FM receivers definitely use a PLL to detect FM. Giving automatic tuning is just a side benifit.
Do you mean AFT (automatic fine tuning)? This has nothing to do with a PLL-based demodulator. Older receivers with ratio detectors or discriminators also had AFT- there is a low frequency feedback path from the demodulator to the local oscillator to make this work.
It is not “huge”, it is no more than 10 percent and no less than 8 percent of the total modulation.
Disclaimer: I don't really know any of this stuff, and I've never built a radio. I'm just repeating what I've read, or in some cases, simulated in software.
The simplest answer is that you use a narrowband bandpass filter around the transmitting station's center frequency to eliminate the signals from other radio stations, just as you do for AM radio, and then you measure the frequency of the remaining signal instead of its amplitude. This works because the frequency deviations are small compared to the spacing between the frequencies on which different stations are transmitting. Downconverting to an intermediate frequency by mixing with a local oscillator, as CodeBeater correctly said most FM receivers do, doesn't really alter this fundamental principle, although it does alter the details. (Most current AM radios are also superheterodyne designs.)
Most current FM radios use a phase-locked loop, as analog31 correctly said, which is sort of the same but sort of different; it gives better results. A PLL uses a much narrower bandpass filter which is centered on, not the nominal center frequency of the radio station, but the instantaneous, modulated frequency, which makes it much better at rejecting interference than the simpler approach. So the frequency band you're filtering down to gets swept back and forth in real time, thousands of times a second, to follow the FM signal.
There's the question of how your PLL can initially achieve its lock if its passband is so narrow, of course. I don't know how mainstream FM radio does this, but it's not as hard a problem as you might think; because broadcast FM radio's frequency is always oscillating back and forth around its nominal center frequency, you can just wait for the audio signal to cross zero. Alternatively, you can sweep the PLL's local oscillator frequency over the band until you achieve a lock.
I hope this is helpful!
I'm actually studying for my general ham radio license right now! Most FM receivers use something called a "mixer" to modulate the frequency to a known constant, then they use a circuit called a "discriminator" or "quadrature", both of which are "detectors".
Typically they're not measuring the frequency or phase itself, but rather the change in frequency or phase.
Edit: I should note that's only for analog circuits. DSP is also common.
Most FM receivers can lock on the carrier because they have a "Phase locked loop" to cancel any tuning errors.
Many good AM receivers do exactly the same thing, especially those receivers which have "Synchronous Detectors" for AM.
It's just that the circuitry involved is simple for FM, but rather more complex for AM.
Most FM receivers nowadays rely on creating a signal of a specific frequency that interferes with the desired on-dial frequency, this is called an intermediate frequency. Then the actual audio signal is analogous to the changes on that IF.
This technique is known as superheterodyne, and Technology Connections has a wonderful video explaining it better than I can.
It locks into the range where the number on your dial is the center of the range, then listens over the whole range.
The range does not change.
Because noise is in line with AM and perpendicular to FM? Let’s read if that’s still so.
Armstrong reasoned that the effect of random noise is primarily to amplitude-modulate the carrier without consistently producing frequency derivations.
…It doesn’t talk much about the noise physics, but basically yes.
This short piece reminds me of a thread about the The Hedgehog and the Fox essay (1953) https://twitter.com/strangeattracto/status/13506001425970544...
> The idea of code switching between multiple traditions doesn’t seem to occur to a person who is fixated on The One True Aesthetic.
Better is in the ears of the beholder. Personally I prefer AM because I can hear multiple stations at once, and hear sources of wideband EM interference in my environment.
I think this article is focussed on broadcast radio, where crosstalk and em interference are both considered negative properties (not that they're not useful in other applications)
because the FM system provides a wider audio bandwidth signal than the audio signal bandwidth provided by AM
It doesn't. In both it depends on what the stations engineer chooses to transmit.
Yet many (if not all?) digital wireless protocols use some form QAM.
Presumably because there's very little additive white noise inside copper wires
It's certainly perceived that way. "Diff'rent Strokes" "Baseball Blues", 1985... Willis does mention stereo is part of the appeal.
- Now, Dad, you gotta picture me cruising along in my Mercedes. Head held high. Rocking to the FM stereo. Waving to the chicks. Hey there, mama, looking good. Catch you later, baby.
(making engine noises)
- You can do all of that in a $4,000 car.
(imitating brakes squealing)
- Dad, for $4,000 I'll have to slouch way down in my seat so no one can see me. And turn on my AM radio. Wave at the chicks. Hi there, mama, you're looking quite adequate. Chug, chug, chug.
- That's just fine, son, chug chug chug means that you won't be spending any of your days in traffic court.
- Or any of my nights at a drive-in movie.
- Willis, you don't want to date a girl who only likes you for your car.
- Sure I do.
I grew up listening to the radio and was always curious why FM indeed sounds cleaner than AM. My assumption before is that it's setup that way since FM is intended for music stations.
Before FM, all music stations were on AM.
I had AM associated with international broadcast while FM with local one.
And because FM broadcast radio caps audio at 15KHz, a CD “sounds better” than FM…yes, back in the day am SM57 was often good enough.
…if you are young enough. By somewhere around 25 to 30 most people won’t be able to hear above 15 KHz.
I’m a little surprised I’ve not seen audio equipment specifically for older people that just covers what they can hear.
I always thought about it as I can arbitrarily amplify and saturate the FM signal without changing it.
Define "sound better", please. As we all know this is something subjective - mostly everyone has individual preferences. I would have found it better if the title was "Why is frequency stable in spacetime and amplitude not (which can be verified so easily by listening to audio radio)".
The most familiar definition, though not spelled out in the article, is audio fidelity which is the degree to which the output reproduces the input. It's fair to take this definition as a default, or implied by the article. In this specific case, frequency response and signal-to-noise are both fidelity measures.
Also, some of the comments did a better job than the article of explaining things.
No need to read the article. It’s literally in the name.
AM = Amplitude Modulation FM = Frequency Modulation
Obviously environmental factors can affect the amplitude of a radio signal. But environmental factors are less likely to affect the frequency.
> Obviously environmental factors can affect the amplitude of a radio signal. But environmental factors are less likely to affect the frequency.
I don't think that's "obvious" to most people.
And it's not correct. Resulting frequency due to random noise is also changed, but in FM, noise is less perceivable. There is no such thing as "affecting amplitude" and "affecting frequency" - they are not separate concepts.
FM spends bandwidth to reduce noise.
Bandwidth.
This is 100% nonsense. Phase noise exists too, not just amplitude noise.
The answer is actually rather simple. AM stations are limited to 10KHz band width. FM gets 200KHz. More bandwidth allows representing a higher fidelity signal…
Yes, this is the right answer, although I would correct the numbers a bit.
If we look only at the audio bandwidth, AM stations are limited to 5 kHz of audio spectrum. The 10 kHz figure comes from the fact that AM is double sideband modulation (as opposed to single sideband as used in ham radio and other radio services). So the broadcast signal uses twice the bandwidth of the audio.
FM stations have 15 kHz of audio bandwidth, three times that of AM. They are able to do this because they transmit at a much higher frequency.
The 200 kHz figure includes other things like stereo (two channels of audio), subcarriers for RDS data and such, and the "Carson bandwidth rule" that 'basementcat' mentioned.
I am surprised that the article overlooked this simple and obvious explanation.
WFM in mono without RDS is still ~200kHz wide, the width isn't primarily a product of the extra signals, it's a product of the modulation index.
It’s more than that. FM is several times less spectrum efficient than AM and needs more bandwidth to transmit the same information.
https://en.m.wikipedia.org/wiki/Carson_bandwidth_rule
You provided a citation, but it doesn't prove your point. In general frequency modulation is more efficient than amplitude modulation (but requires more complicated receivers). For example GMSK in the 2G standard GSM replaced the less efficient 1G AMPS system which used amplitude modulation.
Yes, phase noise exists, but I would think that in practice amplitude noise is greater.
In physics, when a wave passes from one medium to another, its frequency is supposed to stay the same. Even if this isn't perfectly true in the real world, I would think amplitude is more likely to decrease due to obstacles, distance, and the medium absorbing some energy.
But the two are the same thing. You took the fourier transform of white noise and find white noise, both real (amplitude noise) as complex (phase noise).
You can think of it like this: the noise is not about the phase changing, it is about your ability to tell what the phase is. The noisier the signal gets, the harder time you will have to tell what the amplitude is, as well as what the phase is.
If the noise is white gaussian noise (AWGN) then the phase noise is essentially the same as the amplitude noise (by the properties of the Fourier Transform).
Also, the information in AM is carried by the relative amplitude of the signal. Flat attenuation like you're describing doesn't really distort the AM signal. What does impact both AM and FM is frequency selectivity. Imagine light traveling through a prism and being split by frequency. If there are obstacles in the way, some colors won't pass through as well. The is can cause distortions in FM as the receiver loses lock on the signal. Am suffers from this too, but people are less likely to notice because they're used to these distortions -- these kind of effects happen with sound too.
As other posters have mentioned, the reason FM sounds better is that it has more bandwidth for the signal.
Very interesting, I'll have to look into AWGN and the Fourier transform. I guess in the trees blocking the flashlight example that's not at all AWGN.
Although while we care about the relative amplitudes in AM, AWGN would make this harder to pick out if the signal is attenuated. Is the same idea true for frequencies? I don't see a direct parallel here.
You do get some frequency deviation from AWGN. The sum of equal-amplitude 100Hz and 120Hz sine waves is a 110Hz sine wave that "beats", which is to say, is amplitude-modulated, at 10 Hz (or 20Hz from a certain point of view). So, if you have a 120Hz signal and you add a 100Hz signal to it, you should expect that to deviate the frequency of the detected signal downwards. AWGN will have varying, random amounts of all frequencies in it, which will cause varying, random amounts of frequency deviation as they add to your signal.
It's definitely easier to understand in the Fourier domain.
It's not 100% nonsense, though it's true that phase noise does exist. FM radio can transmit silence, which gives it a better dynamic range, which is important for music. If your AM radio signal is 10dB stronger than the radio noise in the band, you'll get noise in the demodulated signal only 10dB quieter than the signal. Due to the so-called "capture effect" https://en.wikipedia.org/wiki/Capture_effect the effect on an FM-demodulated radio signal is potentially much less—though it's true that, with narrowband FM, it won't be.
That's why commercial FM broadcasting uses a ±75kHz deviation even though it was originally only transmitting audio of ≤20kHz. Adding all this extra bandwidth to an AM station wouldn't actually help, because beyond ±20kHz, you're only improving your radio station's ability to reproduce ultrasound. But it does help FM; it greatly reduces the amplitude of demodulated noise, because, even without a PLL, the frequency deviation caused by additive white noise increases much more slowly with bandwidth than the frequency deviation you can use for your signal. With a PLL, I think the frequency deviation caused by additive white noise basically doesn't increase at all with bandwidth. (I guess I should simulate this; it should be pretty easy.)
Unfortunately neither Cook's article nor the flashlight analogy explains any of this.
Kinda funny to see people argue AM vs FM when much of the world has already switched to DAB.
Back in 2005-ish a Clearchannel enginer, Littlejohn proposed narrowing bandwidth of Am stations to improve the signal to noise ratio. They implemented a 5 Khz bandwidth. Back in the 70s we were all blasting 12 kHz. While that's probably OK-ish for talk, it's dreadful for music. https://www.radioworld.com/columns-and-views/the-5-khz-am-re...
12 kHz AM on a good receiver sounds absolutely fantastic.
I have a Royal 51/810 (one of each) that I use as a travel/bathroom radio, ironically, both have fantastic AM performance, and.. lacking FM - the IF/Audio bandwidth appears twice as wide on AM, and FM just sounds like crunchy - probably needs caps in the audio section, but its so tightly packed, and has a PCB with the heaviest plating I've ever seen - which means it needs work I cannot easily do.
Is it packed too tight for a solder sucker to get in there?
I guess the crux here is the claim that "the effect of random noise is to amplitude modulate". Does anyone here understand why?
Ps I don't think analogies are helpful.
AM reception is essentially the direct conversion of the strength (amplitude) of a given radio frequency into an audio signal. Any other noise present at the same frequency is added to the signal (superposition/interference) and therefore impacts the strength of that frequency at the receiver. Therefore it is impossible for the receiver to know whether the amplitude it received is just signal or is signal plus noise.
The claim 'the effect of random noise is to amplitude modulate' is probably not 100% correct, because to my understanding it's not actually performing modulation (the modulation happens at the transmitter but the noise happens between the transmitter and receiver), but it is impacting the amplitude at a given frequency and to a receiver this is impossible to know whether said change in amplitude happened before modulation (signal) or after modulation (noise).
There's actually one important factor that's missing:
AM radio is limited in bandwidth. The audio is cutting off around 10kHz or such (that's why it kinda sounds like a telephone)
> To allow room for more stations on the mediumwave broadcast band in the United States, in June 1989 the FCC adopted a National Radio Systems Committee (NRSC) standard that limited maximum transmitted audio bandwidth to 10.2 kHz, limiting occupied bandwidth to 20.4 kHz
(from Wikipedia)
That's because AM has a bearer around 800 kHz, while FM a bearer around 100 MHz. I guess if doing AM at 100 MHz it wouldn't be a problem granting wider channels, too. But the problem that amplitude is more sensitive to noise wouldn't go away.
TL/DR: because it's frequency modulated and not amplitude modulated, which makes it less susceptible to perturbation.