[sdiy] Help with keyboard control circuit please

[Christian]

Hello Everyone

I wonder if anyone could help me out, please.

I am about to build a standard resistor chain type keyboard for
controlling the synth modules I have just built.

I believe in order to do this I will need to build two seperate
circuits - 1) To provide the correct 1V/Octave across the resistor
chain, and 2) Another circuit taking input from the keyboard bus and
outputting a buffered CV for the VCO's and a Gate signal for the
Envelope generators etc

Now, searching on the net I have found two nice circuits which are
both very similar and do just this. One is the Two-Note Moog circuit,
and the other is used in the NFS synth.

Now, my problem is lack of knowledge - I simply cannot understand the
operation of the circuits presented here, and wonder if anyone could
help me with a brief explanation of what is going on.

The circuit feeding the Resistor chain is made up of two op-amps, but
I am confused as to what exactly is going on - One Op-amp feeds into
the input of the other via a capacitor. Is it a type of oscillator and
the circuit is providing an Alternating Current across the chain?

The circuit providing the CV and Gate outputs has two diodes - Is this
rectifying the AC provided or doing something else? Also, the input to
this circuit is via a capacitor, leading me again to think that it is
an AC signal provided.

Any help anyone can provide is greatly appreciated.

Thanks
Christian