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

[Donald Tillman]

> On Feb 6, 2016, at 11:08 AM, Tom Wiltshire <tom at electricdruid.net> wrote:
> 
> I've been thinking about linear and logarithmic VCAs and ADSR envelopes. We're usually told that the standard exponential cap-charging/discharging ADSR curve into a linear VCA produces a "natural" sound, because our ears understand volume logarithmically - e.g. we hear in dB. This is why we use log pots for volume controls, after all.

There's always been confusion in this area because the exponential characteristic applies in different ways.

For an RC charge or discharge operation, the voltage goes from the start voltage to the goal voltage, at a rate that slows down as you go, Zeno's Paradox style.  So the *difference* between the start voltage and the end voltage is proportional to exp(-t).  

For a decay portion from, say, +10V to 0V with a linear VCA, that operates like a linear curve and a log VCA.

But for the attack portion, the effect of the curve is upside down, and it's very, very different.

  -- Don

--
Don Tillman
Palo Alto, California
don at till.com
http://www.till.com