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

[Donald Tillman]

> 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.

Groovy:



And this works across the keyboard.

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


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20221201/2654bc96/attachment.htm>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: IMG_0666.png
Type: image/png
Size: 118978 bytes
Desc: not available
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20221201/2654bc96/attachment.png>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: IMG_0665.png
Type: image/png
Size: 135657 bytes
Desc: not available
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20221201/2654bc96/attachment-0001.png>