> I'm on board, but...
>
> Back in the days, when I was working with an engineer on my own
> modular, I remember him saying "pitch to voltage conversion is one the
> most difficult and complicated tasks to accomplish in an analog
> voltage-controlled world. It's one of the holy grails of modular
> synthesizer design."
>
> But that was almost 30 years ago, so maybe now it's easier.

Yes and no. If you have a clean rectangle its an eays job today: nothing but
a microcontroller that measures the period (e.g. the time difference between
two rising edges of the signal), calculates the corresponding voltage and
outputs the result by means of a DAC.

But the main problem is to obtain a clean rectangle from (nearly) any
monophonic audio signal no matter how dirty (e.g. during the attack phase of
a plugged string) it is. Even if the fundamental is missing a problem
occurs. The human ear (and brain) has no problem to detect the fundamental
(or pitch) even if only higher harmonics are present. With a simple module
(i.e. not a computer with a lot of calculation power and software like
Celemony) that's nearly impossible.

Best wishes
Dieter Doepfer