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Why does FM sound better than AM?

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259 points zdw | 2 comments | 13 Oct 24 22:32 UTC | HN request time: 0.41s | source
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massysett ◴[14 Oct 24 00:39 UTC] No.41833059[source]▶
What I’ve never understood is how the FM receiver can lock on to the signal if its frequency is always changing. Doesn’t the receiver need to lock on to something? If the answer is “it locks on to the amplitude, which doesn’t change,” well AM is bad because the amplitude is subject to interference, so wouldn’t FM have the same problem?
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jasonjayr ◴[14 Oct 24 01:36 UTC] No.41833388[source]▶
Having recently purchased a RTL-SDR and watched and learned about FM -- there is "pilot" frequency that doesn't change and is fixed relative to the tuner frequency. See this chart here:

https://en.wikipedia.org/wiki/FM_broadcasting#/media/File:RD...

Each radio station has 100khz of bandwidth centered on it's tuner frequency. in the, there are channel spacing rules that give some gaps +/- another 100khz of that. (That's why in the US, radio stations are typically on 'odd' decimals, ie 92.3 mhz, 94.1 mhz, etc) That chart does not show HD radio frequencies, which due to those spacing rules, and more accurate transmitters, are on the +/- 100khz spaces along side the original analog 100khz. You can "see" the audio modulating the frequency on the spectrogram. But the OFDM digital signal on either side looks like a band of more intense noise. It's mind blowing to realize there's a signal in that!

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lrasinen ◴[14 Oct 24 06:08 UTC] No.41834675[source]▶
The pilot is there for stereo decoding, it has nothing to do with the ability to tune to an FM station.

https://wiki.analog.com/university/courses/electronics/elect... has some of the analog approaches collected.

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1. jasonjayr ◴[14 Oct 24 10:09 UTC] No.41836017[source]▶
You're right -- after reading some of the peer responses, I realized that (I think...) my response is just how the Broadcast FM signal modulates the parts of the signal, and not how it actually 'locks on'. I'm still learning!
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2. lrasinen ◴[14 Oct 24 10:34 UTC] No.41836161[source]▶
I got an RTL-SDR this summer and brushed up my DSP skills playing with FM signals. PLLs are marvellous beasts; you can do a slapdash job in "designing" one and it'll still probably lock on just fine. Might not be optimal but will still lock.

Another fun one, when you have IQ samples, is the polar discriminator: calculate x[t] * x*[t-1] where x* is the complex conjugate, and take the angle with arctan. Feels a bit like magic ("is that all?") but is justified by the theory.