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

Fri Feb 12 09:36:41 CET 2016

Hi Tom, 

I was under the impression that your curves were from a proper exponential EG.
On second thought I am convinced they aren't. 

In the classic EG, the envelope corresponds to the voltage on a capacitor which,
in the decay phase, is discharged via a decay pot towards the sustain voltage. 

Suppose the peak voltage of the EG is 1V (0dB) and the sustain voltage is 0.5V
(-6dB). It then takes a certain time to discharge the capacitor from 1V to e.g.
0.707V (-3dB)

If the sustain voltage is set to 1/16 V (-24dB), it should take exactly the same
time to discharge the capacitor from 1/8V (-18dB) to 0.707*1/8V (-21dB). 

It's a bit difficult to determine exactly from your diagrams, but there I see a
time of ca. 600 in the first case, and ca. 300 in the second case. So these
diagrams certainly don't correspond with an EG that has exponential decay
towards a sustain level. What kind of circuit are they from?

Best regards
Simon

> On 12 February 2016 at 01:30 Tom Wiltshire <tom at electricdruid.net> wrote:
> 
> 
> Hi Simon,
> 
> > 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
> 
> No, you won't. That's because this is a picture of a exponential EG into a
> linear VCA seen on a log scale.
> 
> My point was that as the sustain level drops, the "exponential approach to
> sustain" becomes less and less true. By a sustain level of 1/8th, there is a
> clear angle between the decay and the sustain section. It only looks smooth
> viewed on a linear graph, and that viewpoint doesn't hold much validity when
> we are discussing volume.
> 
> > 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.
> 
> This is more-or-less inevitable with the digital envelopes we've been
> discussing. Most digital envelopes trim the end of the decay curve at some
> point simply to save memory and reduce the number of samples that all say "1"
> at the end of the curve. My example was fairly harsh, but not unrealistic. In
> the video posted earlier in this thread,  Nigel Redmon proposes a -80dB level
> for the "end" of the curve, a 0.0001 level. Beyond that, the digital
> resolution provides a limit. 
> 
> Regards,
> Tom
> 
> >  
> > 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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