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

[Tom Wiltshire]

Hi all,

I've been thinking about linear and logarithmic VCAs and ADSR envelopes. We're usually told that the standard exponential cap-charging/discharging ADSR curve into a linear VCA produces a "natural" sound, because our ears understand volume logarithmically - e.g. we hear in dB. This is why we use log pots for volume controls, after all. Ok, so what does an ADSR actually look like to your ear? Here's my results.

Sustain set at 1/16th (-24dB roughly), first the typical linear view, then the log:

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

With sustain at 1/8th (-18dB)

http://www.tomwiltshire.co.uk/images/8ADSROutput.png
http://www.tomwiltshire.co.uk/images/8LogADSROutput.png

You're getting the idea by now, but here's 1/4 and 1/2. The last of these is interesting because the "typical ADSR" diagrams always have sustain set close to a half, which is only -6dB.

http://www.tomwiltshire.co.uk/images/4ADSROutput.png
http://www.tomwiltshire.co.uk/images/4LogADSROutput.png
http://www.tomwiltshire.co.uk/images/2ADSROutput.png
http://www.tomwiltshire.co.uk/images/2LogADSROutput.png

To be honest, this doesn't look exceptionally "natural" to me. But I don't think I'm going to argue with 50 years of synth history. If that's what we like, that's what we like.

Now, what I'm thinking is that I could easily tweak the tables in one of my PIC envelope generators to give these "log" curves. I could then feed the envelope to a log VCA and get the same sound you get with a typical ADSR into a OTA/linear VCA. It's a "log-compensated" envelope. Being able to switch between the two curves would enable people to use either a log or linear VCA and still get the same effect.

What do you all think?

Thanks,
Tom

PS: Secondary issue: Nomenclature - do you think of the VCAs as "exponential" or "log"? What about the ADSRs? Are they "log envelopes" or "exponential envelopes"?