Shepard Generator

[Terry Bowman]

>Just one simple question:
>What exactly is a Sheppard generator?
>I mean what does it do, and how is it usually done?
>
>Thanks for info.
>Bye
> René

René:
You're referring to the famous "sonic barberpole" illusion invented by
psychologist Roger Shepard at Bell Labs. The illusion consists of a
seemingly endlessly rising or falling set of tones. The trick is done by
simultaneously sweeping eight (or so) sinewaves tuned exactly one octave
apart. The human ear has a really hard time figuring out which pure tone is
the fundamental, so it "slips" periodically, just like an eye watching a
barberpole (or looking at an Escher staircase). To conceal the tones'
appearing and disappearing they fade in at one end of the cycle and fade
out at the other.

To create the Shepard Tones in the analog domain you need eight VCO's,
eight VCA's and an LFO with ramp and triangle outputs in octature (eight
outputs 40 degrees apart). Harold Bode used phasers instead of VCO's to
create his famous "Barberpole Phaser." PAiA used to have a Shepard Function
LFO kit, although I think it's long gone now.

I could post a sample if anyone's interested; it's too late for me to do
tonight.


The Science of Musical Sound--John R. Pierce, Scientific American Library
"Lab Notes: Shepard Functions"--John S. Simonton, Jr., Polyphony, 2/83


Terry Bowman, KA4HJH