[sdiy] Irwin dual VCA--any interest?

[synth at charlielamm.com]

OK, this makes sense a lot more sense than what was churning around in my
brain.  I feel a bit foolish for ignoring basic RC math.  Oh well, live and learn.
Thanks for the help....

On Tue, 13 Jan 2004, [ISO-8859-1] René Schmitz wrote:

> Hi Charlie and List,
>
> synth at charlielamm.com wrote:
> > What I found empirically:
> >
> > You build an ADSR using the same cap n 4066 arrangement a lot of
> > designs use (there are 2 on my site that do, for instance).  Since the
> > voltages at attack and release rise in a linear fashion over time, I
> > consider these to be "linear" devices.  Someone should correct me if I am
> > using this terminology wrong.
> >
> > You set the decay to be 5 seconds, from the end of the gate, to when you
> > want, say -100db (ie., silence).
> >
> > For argument's sake, lets say that 5V CV equals 0db (no boost or cut in
> > amplitude) when plugged into the VCA's modulation in.
> >
> > In 2.5 seconds, you have 2.5V at the ADSR output, and you want the VCA to
> > be -6db down.  In my mind, this means you have a "linear" ADSR, and so
> > you want a "linear" VCA.
>
> I disagree, with the linear decay you need (IMO) an exponential VCA.
> (I.e. one that responds in dB per Volts) And with an already exponential
> decay, you need a linear one. (Responds with a gain factor that is
> proportional to the voltage. AKA Two quadrant multiplication.)
> THe major difference in the two combinations is what happens in the
> attack phase. Exponential EGs do have a convex shape, while the
> exponentiated linear shape will result in a concave curve.
>
> > As I understand it, and I may be wrong and get cut to shreds by the group,
> > an "exponential"  VCA won't do this, since it's set to some fixed V per
> > DB, it will probably be way more than 6db down after 2.5 seconds; for
> > practical purposes, you won't hear anything after a much shorter time than
> > this.
>
> Natural instruments (i.e. plucked strings, drums) decay exponentially.
> So linear slopes with linear VCAs don't sound natural.
>
> > Hence a lot of folks say expo VCA's are "good for percussion" but I think
> > this assumes the sort of cap n' 4016 ADSR I just described.
> >
> > The Irwin VCA, by my tests, works that way.  With a REF IN of -5V, if you
> > send in 2.5V positive CV, the output is about -6db relative to where it
> > was when the CV was 5V.
> >
> > The important thing: When you hook it up to a standard, cap and 4066 type
> > ADSR, it sounds "right".  The release fades out in a satifying, slow
> > manner.
>
> This one here http://www.charlielamm.com/synth/schematic.pdf
> is haveing the exponential slopes. (When the switches are on, all that
> is in effect is an RC Stage with its usual exponential behaviour.)
> So it sure sounds "right".
>
>   Cheers,
>   René
>
>