[sdiy] Gate from pitch change

[Karl Ekdahl]

Hi list, i'm hacking up an old Casio and need to get a
gate signal to control an ADSR. 

The keyboard has both VCA CV output (and obviously a
VCA!) and a squarewave output, both which decays a
couple of 100 ms after key release which makes them
unusable as direct ADSR gate. The only feasible way
that i can think of to get a working gate out of this
would be to detect pitch changes but i'm not sure on
how to implement this. 

I'm thinking of a Voltage to frequency converter
(LPF?) a S&H and a comparator but that seems
unnecessarily complicated. 

Either way a pitch-change detector would only generate
a new gate signal if a new key is pressed but i guess
i'll have to live with that.

Any ideas would be appreciated!

Karl