[sdiy] Linear Analog Synth Portamento

[David G Dixon]

> The Buchla MARF does that. The length of the stage is voltage 
> controlled. The clock is a ramp. There's a sample and hold 
> that grabs the starting voltage and then it crossfades from 
> the S&H output to the destination voltage. It's an 
> extraordinarily complicated circuit compared to a pot and cap.

Egads!  I've got a simpler idea, but still reasonably complicated.  Start
with a Bissell MorphLag in linear mode, replace the pot with an OTA or a
linearized 2164 VCA, generate a start-of-attack pulse from the comparator,
use that to trigger a sample-and-hold sensing the output voltage, then take
this stored output voltage and the input voltage to a full-wave rectified
differential amp, and use the output of that to control the CV of the
OTA/VCA.  For added complexity and better performance, replace the single
S&H on the output with a two-step analog shift register on the input.

Thoughts?