[sdiy] "FM" Synthesis (was Re: Buchla 295 10-band comb filter topology)

[Donald Tillman]

On Nov 25, 2022, at 5:58 PM, David G Dixon via Synth-diy <synth-diy at synth-diy.org> wrote:
> 
> I've struggled with this idea of "phase modulation" -- I don't really get
> it.
> 
> [...]
> 
> Is any of that correct?  If not, then I'd very much appreciate an
> explanation of phase modulation -- specifically, what the hell is it?

You're way off.  Not your fault; the goofy terminology has made this very difficult to understand.

Here's my zen explanation:

You know that "subtractive synthesis" starts with a harmonically rich waveform (ie., painfully bright) and runs it through low pass filters to shape the spectrum dynamically.

And "additive synthesis" starts with component waveforms and mixes them dynamically.

And "waveshaping synthesis" dynamically distorts the waveform, or more specifically warping it along the Y axis.

Well, "phase modulation" starts with a sine wave and warps the hell out of it along the *time* axis, dynamically, with the help of a second waveform at the same frequency (or simple ratio).

Warping a waveform along the time axis means stretching this part of the waveform, and squishing the rest.  If the effect is subtle it ends up resembling a PWM wave.

If the effect is more than subtle, the stretches and squishes can go over multiple cycles.

  -- Don
--
Donald Tillman, Palo Alto, California
https://www.till.com