Duophonic Keyboard

[Gene Zumchak]

Mike,

    This is a super consise description.  Maybe I could have done it if I
had tried.  (This is it, Harry).  As you point out, the CV's are not
independent and there is no second trigger.  This could be added by
differentiating the change in the second control voltage.  In other words,
this has its limitations, but it's practically free.  My name is on the
patent.  I think it was about '71.

    As I mentioned in another email today, I still hope to write this up for
the non-techs among us.

    Thanks for this very nice description.

Gene



Mike I wrote:

> 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.