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

[Andrew Simper]

In a synth I think you really need both linear and exponential control
of the main amp.

If you take the exp decay of an envelope to exponentially control the
amp and scale the amplitude, offset, and time factor they come out to
be pretty close to using an exp decay directly as linear control to an
amp, so the decay part I don't think is the biggest concern.

https://www.desmos.com/calculator/4hvlzeu0sy

For the attack portion there aren't any hard and fast rules. For quick
transient type attacks < 1 second then either a linear or slightly exp
attack charging towards x1.5 of the max works fine into a linear
control of the amp, but for swell type attacks of > 1 second where you
have a choir or the string section of an orchestra slowly getting
louder then you need a linear (-ish) control of an exponential amp
otherwise the sound gets too loud too quickly.

An ADSR with linear (-ish) attack, and exponential decay / release
into the linear control of the main amp I think is a good start for
normal duties, then having a linear A(H)D into the exponential control
of the amp helps for swells. LFOs and other slow modulators are also
good into the exponential control of the main amp.

Andy

On 12 February 2016 at 21:29, Tom Wiltshire <tom at electricdruid.net> wrote:
> 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
>
>
>
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