> many thanks, dieter. so this is how a pitch to cv converter works

Yes, in principle. But this procedure is not very fast because of the
required low pass filter. Our planned pitch-to-CV converter will work a bit
different. The period of the incoming (rectangle) signal is measured by a
microcontroller. It calculates the voltage that corresponds to the measured
period very fast and outputs the voltage by means of a DAC (probably with 12
or 14 bit resolution). The main problem is not this part of the circuit but
the "anything"-to-rectangle converter. "Anything" means any kind of
monophonic sound source, like human voice, guitar, bass, flute, violin ...

Best wishes
Dieter Doepfer