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

[Vinicius Brazil]

Hi Tom,
Good questions.
I've been thinking quite the same way to a digital ADSR but I have my
criticisms for various "natural" said topologies.

 I´d test many combinations, and i´m not so sure the "best sounding".

Vinicius Brazil


On Sat, Feb 6, 2016 at 5:08 PM, Tom Wiltshire <tom at electricdruid.net> 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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