[sdiy] Fast envelope generation

Scott Nordlund gsn10 at hotmail.com
Tue Dec 18 02:53:40 CET 2012

> Hi all,
> I had a thought about generating exponential envelopes on "small"
> microcontrollers. Actually calculating an exponential decay is
> computationally expensive, so how about using a lookup table and then
> using something akin to a phase accumulator to set the decay time?
> I'd need to work out exactly how to go from the attack phase to the
> release phase, if you released the key during attack. However for a
> simple decay/release envelope like on a TB303 it would be fine.
> I await the onslaught of "it's been done" and "it doesn't work" ;-)

It's been done, as early as 1971, in the Allen Digital Computer Organ (and derivative models like the RMI Keyboard Computer). This actually included two different tabularized variations, with the attack and decay rates controlled by different foot pedal-controlled clocks.

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