[sdiy] Blackmer VCA cell

[Gordonjcp]

On Sat, Oct 23, 2021 at 06:18:37PM -0400, Matthew Skala via Synth-diy wrote:
> On Sat, 23 Oct 2021, Brian Willoughby wrote:
> > The simplest envelope generator is exponential, based on pots and
> > switching. But these are more difficult to voltage-control the
> > parameters of the EG itself (at least without varistors). Great for
> > cheap systems where the pots on the front panel are the only way to
> > control the EG.
> 
> There's exponential and then there's exponential.
> 
> A capacitor charging from a voltage source through a resistor will change
> its voltage fast at first, when there's a large drop across the resistor
> and therefore a large current, and then slower as it approaches the target
> voltage.  That makes the voltage curve concave down as it's going up
> (attack), and concave up as it's going down (decay and release).  That's
> what people usually mean when they talk about an "exponential" envelope.

It's a little more complicated than that because in a "real" envelope you have the timing cap charged off the full positive rail (either 12V or 15V) but the comparator that flips to the decay phase set for 10V.

If you plot this (and it's easy to simulate, even numerically with a bit of code that implements a simple 1-pole filter) then you'll see that particularly with a 15V rail the attack phase is pretty near linear with hardly any concavity!  I've improved the overall sound of a few synth plugins in Linux I wrote ages ago by modifying the envelope code to account for this.  Instead of setting a target value of 1 for the envelope and flipping at "not quite 1", I set the target value to 1.2 or 1.5 to simulate a 12V or 15V supply rail.  This made the envelopes a lot more "natural" because they didn't seem to hang on for ages before flipping to decay.

-- 
Gordonjcp