[sdiy] How do duophonic keyboards work?

[Pete Hartman]

Can't speak for anything but the ARP models....

The keyboard driver circuit is such that it generates low note CV + a
differential voltage between the two keys.  The differential is actually
opposite -- no second key, i.e. "0" differential causes it to output the
maximum... in the 3620 keyboard for the 2600 that's 4V.  I am *assuming*
that the Odyssey worked the same way, though it doesn't have a 4 octave
keyboard.

The service manuals for these are pretty good, and understandable.  I
haven't read the Ody one in a long while and never needed to look at the
keyboard for it.  But I have spent a lot of time with the 2600 schematics,
building various incarnations of the TTSH, and looking into ways around the
drawbacks of the 3620 circuit (there was a clone, but it was part-for-part,
and the wire contact keybeds that it was made to work with were hard to
come by.... and led to some ugly and expensive workarounds).  Section 2 o
the 3620 service manual has a pretty good description of the basic idea.

Pete
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