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

Thu Feb 11 23:52:47 CET 2016

Hi Tom,

When you combine a linear EG with an exponential VCA you will certainly not get
a picture as in http://www.tomwiltshire.co.uk/images/2LogADSROutput.png

Instead, the decay will start out as in
http://www.tomwiltshire.co.uk/images/16LogADSROutput.png, but as soon as the
curve reaches -6dB it will abruptly change into a straight line.

You are correct in observing that, at low sustain levels, this EG stops to
"ease" into the sustain. However, note that the envelope is apparently clipped
at a certain level; below that level the EG is not exponential anymore. This is
obvious from the log diagrams: if the release phase were a true exponentially
decaying curve, it would show as a straight line. Instead its slope is not
constant, but towards the end it starts falling steeper and steeper.

Best regards
Simon

> Op 11 februari 2016 om 22:16 schreef Tom Wiltshire <tom at electricdruid.net>:
>
>
> Hi Simon,
>
> The data I gathered suggests that the "gradual evolution" you talk about with
> a standard ADSR into a linear VCA only really holds up at higher sustain
> levels. Note that I'm *only* talking about envelope-to-VCA here. In other
> situations, this doesn't apply, but I'm specifically interested in that
> particular situation.
>
> Here's -6dB sustain:
>
> http://www.tomwiltshire.co.uk/images/2LogADSROutput.png
>
> It has some curve on the decay.
>
> By the time we're at -12dB it's straightening out:
>
> http://www.tomwiltshire.co.uk/images/4LogADSROutput.png
>
> And -18dB and -24dB are basically straight:
>
> http://www.tomwiltshire.co.uk/images/8LogADSROutput.png
> http://www.tomwiltshire.co.uk/images/16LogADSROutput.png
>
> So although the classic ADSR might *look* like it "eases into" the sustain, it
> only really does for fairly loud sustains where the volume drop is relatively
> small.
>
> That said, you're quite right that there is a difference here between a
> log-compensated envelope into a log VCA and a standard ADSR into a lin VCA.
> It's just that I suspect it isn't half as big a difference as the typical
> linear ADSR plot makes it look like. If you start plotting ADSRs on a log
> scale, the differences look a lot less significant. The next question is "how
> much can we hear it?". The human ear is pretty poor on volume, so my bet is
> "not much, bordering on not at all", but I need some more experiments to prove
> it.
>
> Tom
>
>
> On 11 Feb 2016, at 20:37, Simon Brouwer <simon.o at brousant.nl> wrote:
>
> > Hi Tom,
> >
> > A linear decay will result in an exponentially decreasing gain if the decay
> > phase continues until the gain is practically zero (say -100 dB).
> >
> > But if the sustain level of the linear EG is set to correspond with, for
> > example, -6dB (or 0.5) then until this level is reached the curve will be
> > the same, but after that, all of a sudden, the gain stop decreasing.
> >
> > In other words, in this approach the asymptote of the exponential curve
> > remains the 0 gain, while for a gradual evolution into the sustain level we
> > want the asymptote to be the sustain level gain.
> >
> > Best regards
> > Simon
> >
> > > Op 11 februari 2016 om 20:14 schreef Tom Wiltshire
> > > <tom at electricdruid.net>:
> > >
> > >
> > > Hi Simon,
> > >
> > > Sorry, I don't understand why a linear decay into a exponential VCA only
> > > gives an exponential decay when sustain is zero. If the VCA is set up to
> > > drop by 6dB for 30 mV, then it drops by 6dB when the linear control
> > > voltage drops by 30mV. I don't see how the sustain level can affect that.
> > >
> > > Am I missing something?
> > >
> > > Thanks,
> > > Tom
> > >
> > > On 11 Feb 2016, at 15:59, Simon Brouwer <simon.o at brousant.nl> wrote:
> > >
> > > > Hi Tom,
> > > >
> > > > To my ears, linear slope envelope generators (as found in many digital
> > > > synthesizers, as a linear slope is so much easier to calculate) are
> > > > vastly inferior to exponential slope ones, as at the end of the decay
> > > > phase the envelope does not gradually evolve into the sustain level, but
> > > > switches from changing into static all at once. This often results in a
> > > > sound that is interesting in the beginning, but loses all life in the
> > > > sustain phase, or is cut off in an unnatural way. Layering and/or lots
> > > > of reverb may mask this to an extent but that is just stopgap. Real
> > > > synthesizers have curves!
> > > >
> > > > A linear decay combined with an exponential control/gain VCA will give
> > > > an exponential decay of amplitude, but only if the sustain level is 0.
> > > > With nonzero sustain levels the envelope will again switch from changing
> > > > to static all at once.
> > > >
> > > >
> > > > Best regards
> > > > Simon
> > > >
> > > > > Op 11 februari 2016 om 15:47 schreef Tom Wiltshire
> > > > > <tom at electricdruid.net>:
> > > > >
> > > > >
> > > > > Yeah, I've done the same thing. My PIC VCADSR has linear or
> > > > > exponential envelopes selectable. But that's not exactly what I'm
> > > > > suggesting here.
> > > > >
> > > > > Instead, I was thinking of doing a normal exponential ADSR for use
> > > > > with a linear VCA, and a "log compensated" ADSR for use with a log
> > > > > VCA. This would have a linear decay and release, but an extreme curve
> > > > > to the attack section to give the typical shape even with the log VCA.
> > > > >
> > > > > Tom
> > > > >
> > > > > On 11 Feb 2016, at 09:14, Roman Sowa <modular at go2.pl> wrote:
> > > > >
> > > > > > It was too easy not to implement it, so I did that on my EG too. One
> > > > > > switch determines exponential or almost-linear curve for attack and
> > > > > > the other one for 1st decay. It simply changes the voltage charging
> > > > > > the timing capacitor.
> > > > > > Here's example scopeshot of both settings:
> > > > > > http://www.sowa.synth.net/modular/addsr_punch.jpg
> > > > > >
> > > > > > Roman
> > > > > >
> > > > > > W dniu 2016-02-10 o 23:27, P Maddox pisze:
> > > > > >> Tom,
> > > > > >>
> > > > > >> On 06/02/2016 19:08, Tom Wiltshire wrote:
> > > > > >>> 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?
> > > > > >> That's what we do with 001/002/002R, you have the option for
> > > > > >> "linear" or
> > > > > >> "exponential" Envelopes.
> > > > > >> Exponential "sound" quicker than linear but give the typical
> > > > > >> "synth" sound.
> > > > > >>
> > > > > >> P
> > > > > >>
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