Saturday, 28 September 2013

Patch Tips #26 - Let's Make an Envelope

Deconstructing the AD envelope. An extravagant but enlightening exercise in understanding how triggered slopes work.

I've been looking into the circuits used in attack/ decay envelope generators and thought it might be interesting to patch one from its basic building blocks. I took my clues from a design described by Barry Klein and René Schmitz and Ray Wilson's Skew LFO to come up with the Skew-velope. Here's how it sounds (MP3).

And here's how it works:

A trigger sets a flip-flop high. Its output is slewed. When the slew signal hits a peak threshold, a comparator turns the flip-flop off and the signal starts to fall. When that voltage hits zero volts - i.e. the envelope ends - another comparator turns the flip-flop on and the process begins anew. Feedback helps shape the envelope. Here is a PDF of the patch and modules I used. Ironically, you may find yourself turning to Maths for its secondary functions in your experiments!

So, why use 8 modules to replicate something that can be done with one? It helped me understand the characteristics of these sorts of contour generators. For example, why re-triggers are ignored during the rise phase, making use as a delay/ divider possible. It also helped demystify Maths & its Serge forbears. Analogue envelopes rely on some form of logic and switching to work. These multifunction modules make some of those processes available to the user.

For more, read Tim Stinchcombe's paper on the Serge circuit.