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

[Simon Brouwer]

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
> > > >>
> > > >> _______________________________________________
> > > >> Synth-diy mailing list
> > > >> Synth-diy at dropmix.xs4all.nl
> > > >> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
> > > >>
> > > > _______________________________________________
> > > > Synth-diy mailing list
> > > > Synth-diy at dropmix.xs4all.nl
> > > > http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
> > >
> > > _______________________________________________
> > > Synth-diy mailing list
> > > Synth-diy at dropmix.xs4all.nl
> > > http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
> > _______________________________________________
> > Synth-diy mailing list
> > Synth-diy at dropmix.xs4all.nl
> > http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20160211/9f3a2c50/attachment.htm>