6 ms·

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 t

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.
- hilbert42 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." 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.
zb 2y ago
> 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/2002/03/28/heterodyne/singleton/ https://archive.ph/2013.02.01-162840/http://www.salon.com/20...
- p_l 2y ago
  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.
  - arghwhat 2y ago
    > 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.
    - p_l 2y ago
      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.
zsellera 2y ago
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.
- kabouseng 2y ago
  Noise is not gaussian.
- analogwzrd 2y ago
  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.
  - arghwhat 2y ago
    1.41x is sqrt(2), which suggests that they meant noise amplitude rather than noise power.
    - pkolaczk 2y ago
      Noise power increases twice but signal power increases 4x. Noise amplitude increases sqrt(2) times, signal amplitude increases 2x.
CamperBob2 2y ago
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.
- arghwhat 2y ago
  > 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.
  - CamperBob2 2y ago
    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 https://www.youtube.com/watch?v=HV3zWSawJiw

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.

hilbert42 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." 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.

zb 2y ago

> 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/2002/03/28/heterodyne/singleton/ https://archive.ph/2013.02.01-162840/http://www.salon.com/20...

p_l 2y ago
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.
- arghwhat 2y ago
  > 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.
  - p_l 2y ago
    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.

zsellera 2y ago

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.

kabouseng 2y ago
Noise is not gaussian.
analogwzrd 2y ago
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.
- arghwhat 2y ago
  1.41x is sqrt(2), which suggests that they meant noise amplitude rather than noise power.
  - pkolaczk 2y ago
    Noise power increases twice but signal power increases 4x. Noise amplitude increases sqrt(2) times, signal amplitude increases 2x.

CamperBob2 2y ago

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.

arghwhat 2y ago
> 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.
- CamperBob2 2y ago
  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 https://www.youtube.com/watch?v=HV3zWSawJiw