[sdiy] all analog FM synthesis?

[Mattias Rickardsson]

Johannes:

>You know all available "FM"-synths (like the DX-series) uses Phase
>Modulation and not Frequency Modulation, right? I know little about
>VCO construction, but Phase Modulation the way it's done in the DX
>synths (which is really a modulated sinetable scanning routine) must
>be terribly difficult to implement in analog technology!

No. Actually it's easier than FM. (If you want to do it correctly and with 
a deep modulation.) Maybe even this easy:
         http://home.swipnet.se/cfmd/synths/schematics/

>They sound quite different; FM tends to sound like disharmonic and
>"bad" wierd FX sounds, while PM sounds like electric pianos and
>metal-hit-with-a-stick-harps.

But they are essentially almost the same:

Modulating the frequency of signal A with signal B is equivalent to
modulating the phase of signal A with the DERIVATIVE of signal B.
(Because the derivative of phase is frequency.)

So phase modulation should sound like FM with a constant-slope HPF:ed 
modulator (i.e., derivative of the modulator), which means there is much 
more treble content in the modulator.

To some extent, I think it should be possible to use a constant-slope LPF 
on the phase modulator in order to make it sound like FM (and vice versa).


/mr