[sdiy] Polyphonic keyboard scanner

[Jerry Gray-Eskue]

You do not have to put all the logic in a single EPROM, like most systems it
is simpler to break the logic out into stages so that the decision matrix is
smaller for each stage.

if you allow for 61 keys 6 bits gives you 64 combinations or 6 bits to
identify each key if you use a logic filter to "see" only one key at an
evaluation pass. Just because you do not have a processor does not mean you
cannot use clocked logic.

You may have up to 8 keys active this count is another 3 or 4 bits depending
on what you do with count 0. The total address count is now around 10 bits
to identify notes with priority.

Like I said this is almost a lost art, with the little bit of understanding
of the tricks that were used I would not attempt to create this logic in
Eproms. There was a lot of high level mathematical reductions of the logic
sets to make it feasible for complicated systems. Using the same tricks I
have seen the entire game of Tic Tac Toe encoded into a few rotary switches.

- Jerry

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of Ingo Debus
Sent: Saturday, August 01, 2009 8:09 AM
To: synth diy
Subject: Re: [sdiy] Polyphonic keyboard scanner



Am 31.07.2009 um 19:24 schrieb Jerry Gray-Eskue:

> 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,

Ugh.
How big an EPROM (or how small a keyboard) would you want to use? A
27C160 has 21 address lines (in 8 bit mode), that's less than two
octaves. You couls cascade EPROMS for more keys, but every additional
key doubles the number of EPROMS.
Also, you need pitch and gate information for each synth voice, thus
(at least) one eight bit EPROM per synth voice: 7 bit for pitch and
one for gate. Ok, for a small keyboard you'd only need 5 bits for
pitch, thus you could stuff five voices into four 8-bit-EPROMs or two
16-bit-EPROMs.

Or did I get your idea completely wrong?

Another point, David asked for a solution without programmable logic
chips (what does that P in EPROM again stand for?). While the EPROMs
don't contain a computer program in the strict sense, you'd surely
need to write a program of some sorts to generate that huge amount of
data.

Ingo
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