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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matrix2003 ◴[14 Oct 24 00:15 UTC] No.41832921[source]▶

Someone gave me an analogy some time ago that made a lot of sense.

If you shine a flashlight through a tree blowing in the wind and vary the brightness to convey information, the signal can get distorted pretty easily.

However, if you have a constant brightness source and vary the color, it’s a lot easier to figure out what the source is trying to convey.

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beala ◴[14 Oct 24 01:10 UTC] No.41833220[source]▶

>>41832921 #

This makes a lot of sense so long as your source of noise is something like a tree swaying in the wind, ie something that interferes with the amplitude. If instead the source of noise is uhhh a piece of stained glass swaying in the wind then blinking the flashlight is the better bet. I guess it just turns out radio interference is more like the tree. But why?

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1. abnry ◴[14 Oct 24 01:20 UTC] No.41833281[source]▶

>>41833220 #

In this analogy, the AM and FM signals you receive aren't usually experiencing interference, they are experiencing multipath effects which includes things like path loss, attenuation, reflections, and so on. This is driven by geometry. You also have gaussian noise that the receiver has to deal with.

You model this by taking your signal and convolving it with the channel vector. Usually the channel vector is a finite number of dirac deltas. Each delta is a different reflection. They are like echos. They can cause the signal to constructively and desconstructively interfere with itself.

I haven't seen the math, but I am guessing this doesn't do as much to the frequency of the signal compared to the amplitude.

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