[sdiy] PM vs FM was: Re: Buchla 295 10-band comb filter topology

Donald Tillman don at till.com
Thu Dec 1 20:37:59 CET 2022

> On Nov 29, 2022, at 5:29 PM, Ian Fritz <ijfritz at comcast.net> wrote:
> Oh, you should look at the Chowning/Bristol book again.  They go step-by-step teaching how to understand FM.  And as mentioned before they treat the DX7 as an FM synth.  Their DX7 exercises and examples would not work for a PM synth. 

Okay, let's check it out...

The book is called "FM Theory & Applications, By Musicians for Musicians" by Dr. John Chowning and David Bristow.  A pdf scan is easy to find on the interwebs.

Oh dear, this is very badly written.  No matter...

So let's try an experiment.  I've got a DX7II-FD here.  It's been a long time; dusty, presets rotted away, 'forgot how to use the thing...

Okay, Alg 3.
Op2, 2x frequency, level 70
Op1, 1x frequency.

And this is how you make a DX-7 square wave:

One of the cool things you can do with PM is to use an audio signal to modulate a low frequency oscillator.  (Huh?  That sounds so backwards.)  This creates a phase shifty chorus sound, so it's a sweet trick for Hammond patches.

If the DX-7 used frequency modulation, that would integrate the above square wave into a triangle, while phase modulation would preserve the square wave.

Move Ops 1,2 up to 2,3 and set Op1 to 1Hz.


And this works across the keyboard.

  -- Don
Donald Tillman, Palo Alto, California

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