New idea for a Shepard Function Generator....

[Chris Crosskey]

Hi Folks....
I've had a sort of idea, and I've talked it over with JH and he seems to think 
it's sane........

Basicaly the problem with Shep Functions is you've either got to build a true 
octature oscillator (pretty damn hard) or you've got to build seven 45 degree 
phase shifters that work over a decent frequency range (pretty damn hard as 
well)....the problem is the 45 degree shift.... well I can guarantee and 
absolutely locked 45 degree shift....#

Basically my idea consists of a dual pot of about a meg or so with one pot in 
the frequency control of a 555 timer circuit. This feeds a 4022 divide by 8 
counter, if the 555 is running at 8x the speed you want for the Shep function 
then you've got 8 outputs 45 degrees apart at the Shep speed......
These eight outputs act as resets on eight separate integrators being fed 
current from buffers running off the other part of the dual pot. If the pot is a 
Meg and you put 15V into one side and a 10K resistor between the other side and 
ground then the output (centre) sweeps between 0.15V and 15V in a basically 
loinear fashion. This ensures that as the clock speed of the 555 goes up 
(basically linear if all you want is 100:1) then the current being seen by each 
integrator goes up, therfore the reset voltage on the integrators stays 
constant......try to match the caps in the integrators and trim it in on the 
resistance. The voltage at output of the integrators will be pretty low so amp 
up to 8V P-P (probably best off trimming here rather than at the integrator 
iteself) .....Then you need to extract a 0-10V triangle wave from the sawteeth 
and you have a Shep generator...

OK bad points....no VC, good points no insanely matched components. I make it 
one 555 a 4022, eight TLO74's and a bunch of R's and C's.....
If you want VC then you'll need to give it an input for a VCO and one for the 
control voltage for the VCO, you'll need a clock divider for the VCO and an expo 
converter built into the integrator stage, however you'll only need one expo 
converter but with eight outputs, so that's nine matched transistors.....
Anybody willing to run with this, I'm busy ATM so don't hold your breath waiting 
for my version.....remember where you heard it first......

chrisc