[sdiy] Soft ADSR ways?

[David G. Dixon]

> I'm suprised nobody is using a hybid approach to this..
> after all, the micro has output pins that are begging to be toggled to
> charge & discharge caps -
> and one could monitor these with an AtoD input.
> I think you could keep the digital in & out, but pass the heavy
> calculating
> to the 'analog' side.
> Depending how you want to throw it into the rest of the system.

I was thinking exactly the same thing.  Use an RC network to make the
envelope sections (with optional switching between linear and exponential).
One could use digital PWM to determine the effective resistance of the RC
resistor (see pg. 178 et seq. and Fig. 4-23 of the TTL Cookbook by Don
Lancaster for inspiration).  If the resistor is switched in and out of the
circuit by a clock, and the time it spends in the circuit is dictated by the
variable PW of the clock signal, then the effective resistance (and hence
the delay time) will be proportional to the PW of the gate.  Eight bits of
PWM would give 256 different delay times, and the exponential character of
the delay would be achieved naturally, in analog.