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

[David G Dixon]

OK, but if you have, say, a triangle-wave VCO, and you give it a subtle
sinusoidal Linear FM modulation, it will just warble back and forth in time
a bit.  The amplitudes won't change.
 
The way you're describing it, phase modulation seems identical to frequency
modulation.  Hence, I don't see the need for another term -- FM is
sufficient.

  _____  

From: Donald Tillman [mailto:don at till.com] 
Sent: Saturday, November 26, 2022 12:03 PM
To: David G Dixon
Cc: synth-diy mailing list
Subject: Re: [sdiy] "FM" Synthesis (was Re: Buchla 295 10-band comb filter
topology)


[CAUTION: Non-UBC Email]	
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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