[sdiy] Polyphonic keyboard scanner

[George Hearn]

Polyphonic voice assignment is non-trivial.  Especially if you would like to
choose between different note priority schemes.  Also the best schemes have
memory to hold as many notes as are pressed down even if they are not
playing, thus when a note that is playing is released, the next highest
priority note takes over the voice etc.
  I think if you try to do this with logic alone it might end up a nightmare
birds nest situation!  There are some places where microprocessors really
are the only way.  For those that do not like programming there is software
out there that generates code for things like PICs from flow-chart diagrams.

  That said, some early synths had basic duophonic keyboard scanners that
used only logic like the Octave CAT.  I'm not sure how it worked in the Korg
MonoPoly, but this maybe somewhere to look as well.  George 

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of David G. Dixon
Sent: 31 July 2009 17:53
To: 'synth diy'
Subject: [sdiy] Polyphonic keyboard scanner

I know this will earn the scorn and derision of the programmers on the list,
but I'm wondering:  Does anyone here know of a polyphonic keyboard scanner
circuit based on logic IC's (non-programmable)?  Is this even feasible
without a massive pile of chips?  If so, I'd be very interested to learn
what the algorithm is.  The approaches I've been thinking about are all
fairly complicated (one scanner (counter + 2 multiplexers), but 8 separate
latches and ladders with data comparators to compare the count to the
latched data for preventing double latching and controlling note-off -- I
haven't worked out all the gory details yet).  I'd like to design something
that will send out 8 CV's, 8 triggers, and one gate.


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