Duophonic Keyboard

[Mike I]

Hi Gene,
How's it going? I included a "duophonic" circuit in the keyboard 
controller PCB in my homemade synth back in 1982. Hal Chamberlin's 
book "Musical Applications of Microprocessors" details this. The 
schematics for the "Cat SRM" synthesizer (on the net) also show this. 
The concept is simple. The ordinary analog keyboard has a string of 100 
ohm or 10 ohm resistors fed from a constant current source. The low end 
of the keyboard is grounded. Each resistor has 1/12 volt across it, 
giving a one volt/octave output. To get the second control voltage, the 
voltage at the top of the resistor string is monitored. If only one key 
is down this top of string (TOS) voltage is constant. If more than 1 key 
is held down, a section of the resistor string is shorted out, and the 
TOS voltage drops. An op amp is used to subtract the TOS voltage from a 
reference voltage (equal to the TOS voltage when only one (or none) key 
is held down). This difference is added to the first control voltage to 
get the second control voltage. A sample/hold can be used to store the 
second CV. The problem is what to do when both keys are down, and then 
one is released, while the keyboard is only putting out one gate signal, 
etc. It's a neat addition, but the 2 voices are not independent.