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

[Richie Burnett]

Hi Tom,

Have a look at this video if you haven't seen it already...

http://www.youtube.com/watch?v=0oreYmOWgYE

Nigel Redmon does a good discussion of the whole expo vs linear ADSR thing. The intrinsic 1-e^(-t) exponential rise in the attack phase of the ADSR intuitively feels back-to-front in terms of linear volume perception. However, it does actually resemble how energy builds up to a steady-state equilibrium in real bowed or blown instruments. A real e^t exponential function gives perceptually linear rising volume in the attack phase but in practice it lingers too long at the low volumes to be useful on a synthesiser, and just plain sounds wrong. 

-Richie,

Sent from my Xperia SP on O2

---- Tom Wiltshire wrote ----

>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"?
>
>
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