[sdiy] Linear Analog Synth Portamento

[David G Dixon]

> Don't know why I didn't mention this before:
> 
> PAiA Fatman is a linear analog synth that has portamento 
> implemented using a capacitor and a pot as a simple RC 
> passive lowpass filter.

Yes, but a pot-and-cap does give "linear portamento".  For that, you need
something like Harry Bissell's brilliant MorphLag circuit.  In short, (at
the minimum) it requires a Miller integrator and a comparator to compare the
input voltage and the inverted integrator voltage and provide current drive
to the integrator depending on the sign of the difference.  When the input
and output voltages are equal, the comparator is forced to an output of zero
volts with a tiny bit of negative feedback.