[sdiy] How do duophonic keyboards work?

[Michael E Caloroso]

US patent 3,733,955 covers the duophonic keyboard for the MuSonic (later
Moog Music) Sonic 5/6.  The patent has a good description on how the
circuit works.  It is similar to the ARP circuit but it predated the ARP
patent on their duophonic keyboard.  I'm surprised the patent reviewers
missed the prior art.  The MuSonic system originated the single keyboard
buss concept of carrying DC current modulated by a high frequency triangle
generator for detecting both CV level and trigger from a single key contact.

MC

On Sat, Aug 2, 2025 at 3:54 PM Pete Hartman via Synth-diy <
synth-diy at synth-diy.org> wrote:

> I re-read the Ody manual a bit and it has to do with the current
> generating circuit being used to also generate that differential voltage.
> It's driving a constant current through the resistor chain of the keyboard
> (such that each resistor drops 1/12 V for each key).  When you close two
> keys, you're effectively short circuiting the resistors between the two
> keys. Since the circuit is generating a constant current, this changes the
> voltage output proportionally to the part of the circuit being cut out, so
> that current is maintained with the smaller number of resistors.
>
> Pete
>
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