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

[Donald Tillman]

Mmm, not really.

Wave shaping is along the Y axis only.  For each cycle, the phase angles remain as they were, but every given Y value gets warped to the same new Y value.

Phase modulation is shaping along the time axis only.  For each cycle, the Y values are unchanged, but their phase angles are warped to new values.

"Wave Morphing" might include both.

The subtle part is that certain kinds of waveshaping (again, Y axis only) can appear to warp the signal along the time axis, even though that didn't technically happen.  "PWM" is an extreme and familiar example.

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

> On Nov 26, 2022, at 11:25 AM, David G Dixon <dixon at mail.ubc.ca> wrote:
> 
> 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
> 

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