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

[Neil Johnson]

Hi all,

David G Dixon wrote:
> 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.

Not quite.  In your case of a sinusoidal modulator then FM will sound
identical to PM.
But here's a very simple thought experiment you can try, and indeed
try out on a real VCO:

Using a low frequency square wave modulation signal, an FM oscillator
will give you two tones (nee-nah-nee-nah).  A PM oscillator will give
you a more musical p-ying-p-yang sound.

Because frequency is the derivative of phase (frequency is the rate of
change of phase) you can hack PM onto an FM synth with an input
capacitor:
https://www.electronics-tutorials.ws/opamp/opamp_7.html
As the derivative of a sine is a cosine, it sounds the same (same
shape anyways).  But the derivative of a square wave is spiky.  A
triangle modulator into a PM oscillator would give you the
nee-nah-nee-nah sound.

At high modulation frequencies the difference between FM and PM is
often subtle, but for low modulation frequencies the difference is
night-and-day.

Cheers,
Neil