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Why does FM sound better than AM?

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259 points zdw | 1 comments | 13 Oct 24 22:32 UTC | HN request time: 0.206s | source
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spease ◴[14 Oct 24 15:29 UTC] No.41838425[source]▶
I’m confused how this is even a question.

With AM, anything that causes a variation in the intensity of the signal will introduce noise.

With FM, anything that causes a variation in the timing of the signal will introduce noise.

Unless you’re traveling at relativistic speeds, operating a time dilation device, or colocated with a black hole, you usually aren’t going to see the rate that time flows at vary.

Thus if you can make the amplitude of your signal irrelevant past a certain threshold and embed all the information into the time domain, the only thing introducing interference should be other EM sources that happen to be on the same channel.

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aidenn0 ◴[14 Oct 24 16:22 UTC] No.41839019[source]▶
That is true for uncorrelated broad-band noise.

Correlated noise (e.g. multipath interference) and narrow-band noise (e.g. another FM transmitter) can both affect FM pretty badly.

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polishdude20 ◴[14 Oct 24 16:56 UTC] No.41839373[source]▶
Speaking of multipath interference, why is it that we almost never hear the effect of that? Like, aren't these waves almost constantly bouncing off of other things and being reflected? How are we not always hearing echos all the time?
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1. vel0city ◴[14 Oct 24 18:48 UTC] No.41840589[source]▶
The other poster is correct but I feel there's still a simpler answer when you see the units at play. You don't constantly hear reflections as echos mostly because of the speed of light and the inverse square law.

Let's think about how far the echo has to come from to have even a half second delay. At the speed of light, that half second is 93,141mi. So it would have to reflect off something half that distance, ~46,500mi. Not a lot of good reflectors pointed at me 46,500mi away.

So then think of the inverse-square law on that. How weak of a signal is that going to be travelling those 93,141mi? Are you set up to even notice that from the noise? It's 11 billion times weaker than the original signal, assuming your reflector perfectly reflects the source signal and you're in outer space.

So obviously whatever "echo" we experience, it's not going to be something in the realm of humans directly detecting it. The shift that is possible to really mess with the signal at distance you'll actually receive reflections at are only going to shift the timing in a very small way, usually by being a slightly different phase. This means you'll get constructive and destructive interference from the same signal at slightly different phases, but not really a noticeable "echo".