Poly Keyboard Interfaces

[gstopp at fibermux.com]

     JDM, you speak much wisdom....
     
     I think that a basic 8-voice keyscan/sequential assign discrete logic 
     interface would be relatively easy. In fact a working, tested design 
     using the word comparator method in each voice is published in some 
     distant issue of Electronotes - I haven't looked at it in a while 
     however.
     
     This type of interface would have been cool back in '79, but what good 
     would it be without velocity? Great, add a second set of contacts per 
     key and some kind of 7-bit counter for each voice to time the distance 
     between contact closures, plus add the DAC support. And aftertouch - 
     some CMOS foam under the whole keyboard assembly would work for this. 
     Then there's release velocity... and what about poly aftertouch?
     
     Time to get real. In the light of all this, I would say that this 
     would be a good project for the synth-DIYer who has everything and 
     needs something to keep his hands busy. I myself however fall into 
     some middle ground, where I like to design things but have limits. I 
     call it a fascinating idea yet I don't think I'll ever go through with 
     it. Not with all these MIDI keyboards lying around - the keyscan front 
     end is already take care of and then some, broken down into a 
     well-defined serial stream. Off the top of my head I can count four 
     5-octave velocity/release velocity/aftertouch/bend/mod/volume MIDI 
     controllers in my posession, plus another that has everything except 
     aftertouch.
     
     The thing that really needs developing is a good multi-output 
     MIDI-to-CV converter. When I say "good" I mean different assignment 
     options, programmable split points, multiple MIDI channel support, 
     things like that. When I say "multi-output" I mean 8 CV's, 8 
     velocities, 8 release velocities (maybe), bend, mod, aftertouch, 
     volume, and some continuous controllers. Heck maybe 16 CV's and 
     velocities. If you want to get really fancy you can put an arpeggiator 
     in there somewhere (although I'd prefer to leave that in the 
     controller I think).
     
     Sounds like a pair of cascaded MPU-101's. Too bad they're so expensive 
     now.
     
     So now we're back into that need for a CPU and a editor/compiler/EPROM 
     burner setup. But the hardware is back to a slim CPU/RAM/EPROM/DAC 
     plus a big honkin' analog mux. If 4067's are used then we could easily 
     go 64 or 128 CV's. The MIDI parsing code becomes the fun part, as you 
     say.
     
     - Gene
     gstopp at fibermux.com


______________________________ Reply Separator _________________________________
Subject: Re: Poly Keyboard Interfaces
Author:  "J.D. McEachin" <jdm at synthcom.com> at ccrelayout
Date:    5/2/96 12:55 PM


On 30 Apr 96 at 11:00, Gene wrote:
     
>      I think that the idea of a CPU-less polyphonic CV interface is 
>      fascinating, especially now that I will have many voice cards to play 
>      with. I have been thinking about it....
     
I think it's lunacy, but since you're so dead set on it, I'll try to lend my 
experience w/ the software-based polyphonic interface to your cause.  
     .
     .
     .
     .
>      So - will this work? Any thoughts? Maybe someday I'll draw this up on our
>      Altera station. I could do all of this on one chip. After hours, of
>      course.
     
I could do it all on one chip too - an 8751!
     
Good luck Gene, if you decide to accept the challenge.  You'll earn my respect 
for being even crazier than me.  Now, tell us again how you're going to add 
MIDI and an Arpeggiator? ;-}