Why does FM sound better than AM?
johndcook.com
johndcook.com
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.
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.
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.
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.
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.
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
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.
An image is quantized into pixels. A camera pixel is a receiver for a specific wavelength, subjected primarily to internal wideband thermal noise during the read-out process. Each final output pixel is averaged both in time (exposure and stacking) and space (debayer and noise reduction), with the final single being singular amplitudes per location.
An AM audio signal is a single wideband receiver subjected to wideband noise. Or, if viewed differently, a series of quantized frequency receivers each subject to a matching noise frequency. The sampling is in the frequency domain, but the final signal captured is is the amplitude variance over time for each frequency, responsible for a single audio frequency.
But yes, your underlying point stands: A theoretical AM receiver that demodulated repeated signals independently and correctly averaged their outputs might gain better wideband noise rejection. Better, but not good, and at a cost of complexity approaching that of better modulations.
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.
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.)
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?
The first step to improving AM (while making the receiver more complex) was removing the carrier wave, which is responsible for most of the transmitted energy (Double-sideband reduced carrier modulation, or DSB-SC). Then, to improve efficiency further without increasing receiver complexity too much, the second sideband is removed (Single-sideband suppressed carrier, SSB-SC - commonly just SSB).
The only benefit of traditional AM is transmitter and receiver simplicity. If you start increasing the complexity, there is no longer any point to using it.
I've spent a lot of time on the development of broadcast-quality FM exciters (as I've posted on HN in the past) but I'm not go to debate the disadvantages/advantages of FM versus AM in depth as most points have already been covered in other posts, I'll just add this:
The quality of AM can be remarkably good if it's engineered with care. The math and engineering tell us that, and excellent results can be and are achieved in practice (as a longtime FM-er I say that about AM as it's just fact)!
The reason why AM has a bad reputation is it's history and background: AM broadcast and other shortwave (3-30MHz) bands are much more prone to impulse and atmospheric noise than VHF and up. Also, the historical nature of RF amp design and detection never put its primary focus on the linearity of RF/IF amps, etc. (good linearity is easily achievable these days).
My primarily reason for responding to this part of your post is to point out that when receiving AM signals one can take excellent advantage of using a wider bandwidth than the actual spectrum occupied by the modulation products.
Unfortunately, most of us don't know or have forgotten about the Lamb Noise Silencer circuit which uses a wider bandwidth signal to gate out impulse noise.
Here, two IF amplifiers [or one especially modified] are employed. The main IF uses the required (narrow) channel bandwidth and the second IF uses a wider bandwidth. Shannon et al tell us the wider signal can arrive at the detector before the narrow BW signal and thus can be used gate out noise in the latter stages and or detector.
In operation a properly designed Lamb Noise Silencer is remarkably effective in reducing AM static etc.
Whilst never used as a broadcast service, I'd posit that if AM with say 20Hz-20kHz audio were put into the existing FM spectrum (88-108MHz) with its lower AM noise background together with receivers that used the Lamb circuit then AM would essentially be indistinguishable in quality from the existing FM service. In fact, it could be even better with audio reaching 20kHz [extra 5kHz] and this could be achieved with a much better utilization of the spectrum (with AM's inherently lower channel bandwidth)—many more stations could be added to the band.
Of course, that would have been impractical back 80 or so years ago when the FM band was conceived for reasons that in the days of tubes the required local oscillator stability would have been very difficult to achieve (but a non issue these days).
Sometimes, we overlook the fact that our predecessors have already been there, thought about and have done these things.
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.
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.
But hey, no static at all... https://www.youtube.com/watch?v=HV3zWSawJiw
In case of gaussian noise, double the bandwidth means 1.41x more noise. For signal, double the bandwidth double the signal.
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.
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.
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...
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.
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.
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.
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.
That is not a factor anymore. Capable wideband transcievers like the ones in Baofengs and similar supporting multiple types of modulation cost cents.
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.
AM is not providing any benefit of simplicity, but not changing standards avoids the transaction cost of change.
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 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.
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.
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.
My guess would be iHeartMedia
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.
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.
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.
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 AM you get sound effects such as chip monks
-for FM what do you get?
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.
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).
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.
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.
Going from red to orange is about 50 THz. Typical FM radio modulation width is 100 kHz.
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.
That's an oxymoron, really. Frequency and amplitude are closely interrelated concepts (e.g. see Heisenberg's uncertainty principle in the context of signal processing). Frequency-varying and amplitude-varying even more so!
It's like saying that the violins is merely an analogy for how a double base works.
Rubber ducks aren't battleships because they both float. 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".
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?
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.
> 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
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.
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.
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).
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.
IIRC, when Corning Co. first started working on optical fibers, the best available glass would be good for sending signals about ten meters. What was improved was not the total internal reflection; it was the purity of the glass.
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.
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.
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 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.
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.
That’s the hallmark of an analogy. It gives the general idea, but breaks down if we interrogate it in too much detail.
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 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.
Put visible light over a really long waveguide and modulate the colors, you invented fiber optic telecommunication.
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.
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.
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?
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.
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".
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".
It works, but it is a fairly expensive method to implement.
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.
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.
And even if leaves do sometimes block the entire signal, you're still going to do better with varying the color than the brightness.
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?
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.
of course, you shouldnt be listening to radio during a tornado, but...
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.
Correlated noise (e.g. multipath interference) and narrow-band noise (e.g. another FM transmitter) can both affect FM pretty badly.
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".
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).
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
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.
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)
As you're saying, it's about bandwidth and signal to noise. Not something inherent to modulation.
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.
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.
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.
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
As the OP has said, it cannot give louder bass, but simulates the bass harmonics.
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.
Now, from time to time I buy a cheap portable AM radio, mostly out of nostalgia, but with the excuse of being good emergency preparedness, and the sound is annoyingly bad even with decent headphones.
That music also sounds more era-appropriate coming from a vinyl record than a CD.
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.
The frequency range for AM radio is 540 to 1600 kHz
vs
30hz-15khz
Bass and fundamental frequencies really contribute to fidelity
said someone who didn't understand anything about signal processing.
Been debunked so many times: https://physics.stackexchange.com/questions/94198/why-does-n...
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.
« 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. »
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)
https://physics.stackexchange.com/questions/94198/why-does-n...
https://ham.stackexchange.com/questions/6312/why-are-fm-radi...
TL;DR: the information one can reliably send through a noisy channel (C) is proportional to the bandwidth of that channel.
- 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...)
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!
https://wiki.analog.com/university/courses/electronics/elect... has some of the analog approaches collected.
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.
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).
I've wondered if FM stereo drives pets nuts with its constant high-pitched tone.
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!
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.
The range does not change.
This technique is known as superheterodyne, and Technology Connections has a wonderful video explaining it better than I can.
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.
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…
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.
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.
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.
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.
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.
It's definitely easier to understand in the Fourier domain.
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.
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.
It’s tight.
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.
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?
> The idea of code switching between multiple traditions doesn’t seem to occur to a person who is fixated on The One True Aesthetic.
Loudness wars or other bad mastering? Broken (or low quality) equipment? Crowded spectrum? Neighboring HD signals? Poor line of sight? Echoes? Interference from other electronics? Low-quality receivers? Lossy compression? Weak signals? Bad/cheap DSP?
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.
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.
- 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.
Ps I don't think analogies are helpful.
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).
I’m a little surprised I’ve not seen audio equipment specifically for older people that just covers what they can hear.
1. Smaller market than more general market.
2. HiFi is a proven market.
3. “You are dying” is not a great marketing subtext.
4. Digital audio jellybeans produce 20khz and adding a low pass filter adds cost.
5. If a person only hears to 12KHz, there’s no advantage to a low pass filter.
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)
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.
I don't think that's "obvious" to most people.
Also, some of the comments did a better job than the article of explaining things.