[sdiy] Stupid Sheppard Generator Question

[Magnus Danielson]

From: Thomas Hudson <thudson at tomy.net>
Subject: Re: [sdiy] Stupid Sheppard Generator Question
Date: 25 Sep 2002 16:53:44 -0500

> On Wed, 2002-09-25 at 10:04, Peter Grenader wrote:
> > Could one of you guys explain to me what exactly a Sheppard Generator does?
> > I know, I know...I'm the big know it all ....but this one I must of missed.
> > They SOUND great (samples on Grant's Page)....but I am not sure what I am
> > hearing.
> > 
> > 
> > Thanks in advance,
> > Peter
> > 
> Good description here:
> 
> http://machines.hyperreal.org/manufacturers/Paia/images/shepard.txt

The PAIA description and kit seems like a good way to be introduced to it and
its realization. It wasn't until I saw that description I really grasped it.

PAIA at least used to have it up on the web. I'm sure you could get it from
him/them if you ask.

In essence you have a number of oscillators one octave appart (1 V) in a
slowly rising (or falling) sawtooth. For each oscillator you have a VCA control
voltage wich rises and falls (triangle) such that you have full power half the
way up in pitch. The PAIA kit realises this by use of a counter, XOR gates to
create the phase variants for each output pair and triangle form for each, and
then D/A converts this.

Shockingly simple.

I once attempted to hack a barberpole in C on my SGI Indy workstation, but
there was some tiny flaw...

Cheers,
Magnus