[sdiy] Good Expo curve for VCA use..

[Scott Nordlund]

> Precisely, it's just for fading things in... And also, it needs to
> start at zero - which an exponential growth never passes - so the
> first part of the attack curve is chosen for instance as a linear
> increase, followed by the exponential increase from some well-chosen
> level.

A decent way to do this is to make a line segment between 1 and 0, and take it to the 4th power.  It works well as an exponential-like curve that reaches zero in a controllable amount of time.

Of course this is a little easier to implement in software...