VCOs & Tempco

[Fraser, Colin J]

> -----Original Message-----
> From: Ingo Debus [mailto:debus at cww.de]
> Sent: 13 May 1998 17:30
> To: synth-diy at mailhost.bpa.nl
> Subject: Re: VCOs & Tempco
> 
> If you're using a microprocessor anyway, what about "hybrid" envelope
> generators? The analog hardware is just a 
> processor-controlled resistor
> (a Xicor chip maybe), a capacitor and two comparators. One processor
> generated control voltage (being the "asymptote" level) 
> charges the cap
> via the resistor. The comparators are fed with another 
> control voltage,
> the threshold level. They detect if the voltage across the cap is
> higher/lower than the threshold, and if it is, a processor 
> interrupt is
> generated. Then the processor sets new values for resistance, 
> asymptote
> and threshold.

Yes, I've thought about this as well. 
The voltage controlled envelope generators in my modular use an analogue
switch (4053 or 4052 - can't remember! the dual 4 into 1 type), a
flip-flop and a comparator to switch the various rate and level voltages
to a LM13700 slew generator with has an expo convertor on the control
input to give a useful control law. It would be simple to adapt this
circuit for processor control in the way you describe.
The only doubts I have are where a large number of envelope generators
are required  - what effect on the sound might there be if the processor
is under heavy load.
One solution to this might be to use a separate processor on each voice.
Then it would only be driving 3 or 4 envelopes at most. Another control
line could be added to switch the slew generator between linear and
exponential curves.

This definitely needs to be investigated.
Have any commercial synths used this technique ?

Colin f