Duophonic Keyboard

[WeAreAs1 at aol.com]

Hello guys,

About the ARP/Moog/Octave CAT duophonic keyboard method, Mike Irwin wrote:

> 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.

It should be mentioned that the old Oberheim Two-voice synth (TVS-1) had a 
duophonic keyboard with completely independent duophonic gate articulation.  
That is to say, if you played just one key, you'd get only one voice.  If you 
played another key, you'd get the second voice.  If you then released both of 
the keys (and had your envelopes set for a long release time), the two notes 
would continue to sound at their current note as their sound decayed, and 
would not have both of the notes jump to the last or lowest note that your 
finger happened to touch, as it would on the ARP system.  (I'll bet ARP sales 
people described that note-jumping thing as a "feature" back then.  "Yeah, 
it's our duophonic keyboard with Monophonic Auto-Fat!", or something like 
that.)

I know the TVS-1 thing doesn't sound like a really big deal these days, since 
this is exactly how all polyphonic synths have behaved since the introduction 
of microprocessor-scanned polyphonic keyboards (around 1977 or 1978), but 
they did it with simple analog circuits (a kind of variation of the ARP 
method, with a little more sophisticated logic controlling the S&H's).  
TVS-1's were a lot more expensive than Odysseys, back then.  I guess this 
keyboard was one of the reasons.

When Oberheim introduced the Four-voice, they implemented a discrete-logic 
version of the Dave Rossum/Scott Wedge-invented polyphonic scanning keyboard 
(still no microprocessor).  Those who are interested in seeing how it worked 
can view the Oberheim FVS-1 schematics at Kevin Lightner's Synthfool.com 
site, or look at Juergen Haible's customized OB four-voice clone keyboard 
scanner at his site (linked at the Synthfool site).

It would be interesting to modify an Odyssey keyboard to function like the 
TVS-1 keyboard.  It wouldn't be too difficult, I think.  (of course, it would 
still be only two oscillators, unlike the TVS-1's four, so you might miss the 
"Auto-Fat"...)

Michael Bacich