[sdiy] Lin/Log VCAs and envelopes - compensating for log VCAs

[Tom Wiltshire]

This discussion has thrown up several important issues and problems with the way I was doing things in my PIC digital envelopes. These are:

1) Cropping the tail of the decay/release curve too harshly leads to an obvious "nose dive" in the output. This is visible here:

	http://www.tomwiltshire.co.uk/images/16LogADSROutput.png

(Exponential ADSR plotted on a log scale, Sustain at 1/16th)
Release is pretty much a straight line until -45dB, then nosedives to -100dB. The stored curve needs enough data to ensure this nosedive is at a low enough level to not be audible.

2) Using a linear decay into a log VCA doesn't give the same effect as an exponential decay into a linear VCA. This is visible at larger sustain settings:

	http://www.tomwiltshire.co.uk/images/2LogADSROutput.png

(Exponential ADSR plotted on a log scale, Sustain at 1/2)
Note that on the first graph with sustain at 1/16th, the decay does appear to be a straight line. This effect becomes more and more noted as the sustain gets smaller and smaller.

3) The usual "semi-linear" attack curve of ADSR generators that charge to x1.3 of the max value (this is all the CEM and SSM designs, as far as I can tell) doesn't seem to actually do a lot below -50dB. This is visible on either of the graphs above.


I'm going to do some more work on this and I'll get back to you all when I've got some more data. Until then, I feel like I'm either just spouting opinions or speculating.

Thanks for the discussion so far,
Tom