6 ms·

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 k

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.

mannykannot 2y ago
The context here is the technology available at the time of the introduction of FM in the mid-1930s. In that context, schemes for reduplicating AM over a much wider bandwidth than the ~40 kHz needed for high-fidelity audio may be interesting in their own right, but they have no bearing on the article we are discussing. AM, as it was construed and implemented at the time, was inherently limited to a bandwidth just twice the signal frequency.
arghwhat 2y ago
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.
- kees99 2y ago
  > (...) 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 https://en.wikipedia.org/wiki/Single-sideband_modulation
  - tomfanning 2y ago
    In SSB there is no carrier transmitted. Two SSB stations on top of each other sounds exactly like two microphones mixed.
    - pkolaczk 2y ago
      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?
      - vel0city 2y ago
        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.
- pkolaczk 2y ago
  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.
akira2501 2y ago
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.
- adrian_b 2y ago
  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.
  - akira2501 2y ago
    > 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.
Anotheroneagain 2y ago
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.
- adrian_b 2y ago
  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.
  - giantrobot 2y ago
    > 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.
- Johnythree 2y ago
  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.
fredgrott 2y ago
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?
- deleted 2y ago
  [deleted]
xd1936 2y ago
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)?
- pkolaczk 2y ago
  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.

mannykannot 2y ago

The context here is the technology available at the time of the introduction of FM in the mid-1930s. In that context, schemes for reduplicating AM over a much wider bandwidth than the ~40 kHz needed for high-fidelity audio may be interesting in their own right, but they have no bearing on the article we are discussing. AM, as it was construed and implemented at the time, was inherently limited to a bandwidth just twice the signal frequency.

arghwhat 2y ago

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.

kees99 2y ago
> (...) 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 https://en.wikipedia.org/wiki/Single-sideband_modulation
- tomfanning 2y ago
  In SSB there is no carrier transmitted. Two SSB stations on top of each other sounds exactly like two microphones mixed.
  - pkolaczk 2y ago
    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?
    - vel0city 2y ago
      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.
pkolaczk 2y ago
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.

akira2501 2y ago

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.

adrian_b 2y ago
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.
- akira2501 2y ago
  > 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.

Anotheroneagain 2y ago

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.

adrian_b 2y ago
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.
- giantrobot 2y ago
  > 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.
Johnythree 2y ago
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.

fredgrott 2y ago

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?

deleted 2y ago
[deleted]

xd1936 2y ago

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)?

pkolaczk 2y ago
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.