[sdiy] Blackmer VCA cell

[mskala at northcoastsynthesis.com]

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.

But a linear envelope controlling an exponential VCA will change fast when
the voltage is high, slowly when the voltage is low.  The output amplitude
in concave up in all three of attack, decay, and release.

> A linear EG into a exponential VCA should sound very natural. Varying
> between linear and exponential would be difficult in the analog domain,
> though.

Not terribly difficult if you are already doing the work to make the rate
voltage-controlled.  You can feed back either the output voltage at the
present moment, or the difference between that and the target voltage
(depending on which kind of "exponential" you want), to whatever is
controlling the rate - and do this to a varying degree, in relation to a
constant voltage, depending on how much like an exponential or like a
linear ramp you want it to be.

-- 
Matthew Skala
mskala at ansuz.sooke.bc.ca                 People before tribes.
https://ansuz.sooke.bc.ca/