[sdiy] Polyphonic keyboard scanner

Jerry Gray-Eskue jerryge at cableone.net
Fri Jul 31 19:24:57 CEST 2009


Using only logic ics you are just about reduced to Boolean mathematics to
wire the thing up. It is going to have to deal with a lot of conditional

That being said the only chance you have of implementing something this
complicated in a small number of chips without a processors is a very old
school trick, an EPROM that takes all the keys as on or off (1 or 0) as the
address and at that memory location has the proper binary settings for the
rest of the system, you may also need to have the previous state included as
part of the address scheme. This is very old very low tech but extremely
complicated to get right and basically a lot art, it has to cover every
possible combination of events with a pre-calculated result.

In other words, maybe it is time to get a PICkit 2 programmer and some
Microchip micro controllers.

- Jerry

-----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: Friday, July 31, 2009 11:53 AM
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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