[sdiy] ADSR Sustain pot law?

[Tom May]

Neil Johnson <nej22 at hermes.cam.ac.uk> writes:

> All,
> 
> While putting together a list of replacement pots for my "Jenny" project,
> I was looking at the envelope generators and noticed that the two sustain
> pots are both linear.  Now, since in this synth everything is hardwired
> (rather than patchable) it strikes me that it would make better sense to
> replace them with log-law pots, which would lead to a more natural
> behaviour:
> 
> - VCA: sustain sets steady-state volume, so log-law would lead to natural
> "linear" setting (same as volume control);
> 
> - VCF: sustain sets steady-state cutoff frequency: this is linear w.r.t.
> the control voltage, so for a "musical" control reponse would need a
> log-law curve, producing a 1-2-5-10-20-50-100, not 1-2-3-4-5-6-7,
> behaviour, so that the same rotation angle produces the same octave shift
> irrespective of where on the scale you are.

I've always thought that the interactivity between the filter cutoff,
envelope sustain, and modulation amount on a typical synthesizer was a
poor design.  If you tweak any one of them, the sustain level of the
cutoff changes.  So on my asm-1 I added outputs to the envelopes which
sustain at 0V, i.e., they are the standard envelope voltage with the
sustain voltage subtracted out.  They will attack above 0V, sustain at
0V, and release below 0V.  Now the cutoff control changes the cutoff,
the sustain control changes the relative sizes of the attack peak and
release voltages, and the modulation amount changes the modulation
amount (and I allow both + and - modulation).  The latter two controls
interact a bit but don't change the sustain cutoff.

Tom.