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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: 0.199s | 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. adrian_b ◴[14 Oct 24 18:22 UTC] No.41840326[source]▶
The effect is easily noticeable only for high frequencies, where the wavelengths are no bigger than a few meters.

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).