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

[David G Dixon]

OK, I get it.  It's just a subtle form of waveshaping.  It's basically
linear FM where the modulating wave is at the same frequency as the
modulated wave, so that when the waveforms return to the same position they
are at the same frequency that they started at.

-----Original Message-----
From: Donald Tillman [mailto:don at till.com] 
Sent: Saturday, November 26, 2022 10:05 AM
To: David G Dixon
Cc: synth-diy mailing list
Subject: Re: [sdiy] "FM" Synthesis (was Re: Buchla 295 10-band comb filter
topology)

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