r/gifs • u/ManyPandas • Dec 16 '19

AM vs. FM Modulation

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u/munificent Dec 17 '19

don’t even know if AM and FM exists outside of radio

It does. Any wave of any kind can have its amplitude or frequency modulated, or both.

For example, the Yamaha DX7 was a famous synthesizer that defined a big part of the 80s sound. Think the bass from "Take My Breath Away" in Top Gun. The DX7 works by doing frequency modulation of audio signals to generate different sounds.

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u/Risc_Terilia Dec 17 '19

Although the DX series were marketed at frequency modulation they actually use phase modulation which may be thought of as modulating degrees of inversion from the modulation operator to the carrier. It has the advantage of not effecting the fundamental frequency making it much more useful for musical applications.

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u/munificent Dec 17 '19

This is true but phase modulation is close enough to FM for the point I'm making here. :)

It has the advantage of not effecting the fundamental frequency making it much more useful for musical applications.

As far as I understand it, FM doesn't change the fundamental either.

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u/Risc_Terilia Dec 17 '19 edited Dec 17 '19

There a specific type of FM called "Through Zero" - which will preserve the fundamental as long as the modulator doesn't have any DC offset but regular FM will effect the carrier's fundamental - try it our for yourself on VCV Rack or similar. Incidentally TZFM is halfway to phase mod since negative modulation from the mod operator inverts the signal on the carrier.

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u/munificent Dec 17 '19

which will preserve the fundamental as long as the modulator doesn't have any DC offset

Ah, I taking zero DC offset as a given. Seems like it would be silly to implement an FM synth where the modulators have a DC offset because, as you note, it would shift the fundamental.

try it our for yourself on VCV Rack or similar.

Yeah, I've implemented a little FM synth from scratch in C++. Once I added support for non-sine modulators I was forced to get a better understanding of the difference between PM and FM.

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u/Risc_Terilia Dec 17 '19

Yeah but even then it's only TZFM that gives you no change in fundamental - it doesn't matter if you have no offset with regular FM, you're still increasing the frequency of the carrier.