[sdiy] Stupid Sheppard Generator Question

[Gene Stopp]

When I implemented John Simonton's single-DAC multiplexed-analog 8-phase
saw/triangle generator, I put a reversable attenuator between the DAC and
the sawtooth mux so I'd have a one-knob control over the amplitude of all
sawtooths, from full down-ramp thru zero to full up-ranp. This of course
changes the "spread" of the VCO pitch intervals. One regret that I had is
that this should have been a 4-quadrant multiplier so the effect could be
voltage controlled!

Anyway the Shepard effect is still interesting at intervals other than an
octave, with waveforms other than sinewave. I used 4 ASM-1's for the 8
VCO/VCA pairs.

- Gene


-----Original Message-----
From: Jim Johnson [mailto:jamos at technotoys.com]
Sent: Thursday, September 26, 2002 11:33 AM
To: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Stupid Sheppard Generator Question


>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 dictum to use sines 1 octave apart is directed at a "classic" Shepard
effect, which has a simple timbre; but variants are interesting too. I used
to use the Xpander's 6 voices to do a Shepard-like tone, by using identical
voices with an LFO sawtooth controlling VC pitch, and offsetting the phase
angle of each LFO by 60 degrees. The tracking generator is used to derive
an properly aligned triangle wave from the sawtooth, which drives the VCA.
The sound does infinitely rise or fall in pitch, but the timbre varies
somewhat in time with the sweep. If the sweep is really slow, this is
acceptable, but this slows the rate of pitch change as well, so very deep
pitch modulation may be required (which the Xpander could not handle well,
IIRC) to get the desired effect.




Jim Johnson 
jamos at technotoys.com