[sdiy] Polyphonic keyboard scanner

[thx1138]

On 7/31/09 10:23 AM, "Eric Brombaugh" <ebrombaugh1 at cox.net> wrote:

> David G. Dixon wrote:
>> 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.
> 
> No scorn or derision here - sounds like an interesting exercise in
> creative anachronism. I imagine that a basic 64-key scanner would need a
> 6-bit counter with the top 3 bits coupled to a 3->8 decoder for a column
> driver. A switch matrix with diodes at each crossing so that multiple
> keys could be down without creating phantom closures. That feeds into an
> 8->1 mux driven from the low 3 bits of the counter to detect closures in
> each row.
> 
> At that point you've detected all the key closures but you still have to
> prioritize them and convert them to CVs. That requires some sort of
> memory stack for the priorities as well as an accurate DAC and
> potentially some sort of lookup-table ROM for the CV conversion.
> 
> When all is said and done you've probably got at least 15 - 20 sq in of
> PCB space, along with $5 - $10 worth of logic ICs. Contrast that with
> the $2 you'd pay for a low-end MCU that could do the job and you've got
> to ask yourself if you're doing it to scan a keyboard, or for the 'stone
> knives & bearskins' experience. Both are perfectly valid in a hobby
> context (witness the all-transistor clock projects that are floating
> around).
> 
> Eric
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Hi Eric and SDIY members,

The old Oberheim TVS-1a (2 Voice) and the Oberheim 4voice / 8voice based
upon SEM-1A and Keyboard electronics with all logic no CPU.

There may be others, but the Oberheim electronics were based upon an early
E-Mu design between Dave Rossum and Tom Oberheim.

Regards,

Terry