EXPLAIN LIKE I’M MODERATELY INTELLIGENT: - You tune FM radio to a dead channel, it’s loud static. - You turn FM radio to an active channel playing silence, it’s quiet. Is this ONLY because of automatic gain control, or is there a more complicated reason?
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Math/SDR version: radio is made of complex numbers (I/Q data). FM demodulation means taking the derivative of the *angle* in polar coords. When the signal is strong, the *distance* is large. When the signal is weak, the distance is near 0 and the angle becomes garbage.
So silence is a non-rotating vector with some magnitude; that keeps the angle constant. No signal is just nothing, zero, and then the angle is meaningless (in practice, just dominated by noise). Effectively FM has its own "gain control" of sorts built right into the modulation.
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Yep, this is why you will very rarely hear two stations on one frequency; the stronger one wins and the weaker one effectively goes away.
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So this shouldn't work for AM then...
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AM has a different behavior, yes. AM demodulators need explicit AGC or else the output volume would vary to an extreme extent depending on signal strength.
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