Sun Jan 9 09:55:45 EST 2011

Envelopes: problem is offset

So, sticking to the full DC 1-pole LP, the problem is offset voltage.

Can this somehow be incorporated into the design, knowing that
envelopes will always add to another control signal.  Since the base
control level will always come from a PWM signal, all compensation can
be moved to there.

Problem is the lin/log thing though: offset is more of a linear,
constant compensation, instead of a log one.  I.e. for very small
control voltages the offset's relative component would be large,
killing the relative error property.

One thing that does work for the circuit is that the current goes to
zero perfectly.  What about using a current mirror on the cap output?
Problem with that is the current is not unipolar, and the current is
highpass (integral of 1st order lowpass).

So, how good does the offset need to be?  

  - VCA: it should be possible to turn an oscillator off.

  - VCF: not so critical as there is usually some programmed offset
         anyway: env is only used on top of base line filter setting.