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259 points zdw | 1 comments | | HN request time: 0.201s | source
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pkolaczk ◴[] No.41835074[source]
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.

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arghwhat ◴[] No.41835320[source]
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.

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pkolaczk ◴[] No.41835752[source]
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.

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1. CHY872 ◴[] No.41836495[source]
It’s not immediately clear that Shannon’s theorem is a good point of comparison here, since it’s only recently that coding schemes have really approached the Shannon limits, and FM and AM do not use these.

Even if one does assume a Shannon-perfect coding scheme, as the noise ratio gets greater the benefits of spreading a signal across a higher bandwidth fades. Furthermore, most coding schemes hit their maximum inefficiency as the signal to noise ratio decreases and messages start to be too garbled to be well decoded.

I’d additionally note that folks get near the Shannon noise limit _through_ ‘magic noise rejection’ (aka turbo and ldpc codes). It’s therefore not obvious that FM isn’t gaining clarity due to a noise rejection mechanic. The ‘capture effect’ is well described as an interference reducing mechanism.

Empirically, radio manufacturers who do produce sophisticated long range radio usually advertise a longer range when spreading available power across a narrower rather than wider bandwidth.