digital keyboard controller...how to build one...?

[Doug Ferrell]

Matti,
	I've got a partially functioning prototype of such a device which I built
last year.  I used TTL because I wanted to do some more interesting things
(there are a lot more variations of the basic MSI digital building blocks in
the TTL family than in 4xxx CMOS).  For a basic scanner, CMOS would probably
be best.
	Anyway, the basic idea is to have a fairly high speed clock (I used
something about 100kHz) to drive a digital counter.  The counter generates
the addressing sequence for scanning the diode matrix.  Usually these are
laid out as an 8x8 grid for a 5 octave KB.  In this case you'll need a 6 bit
counter.  The three highest bits of the address lines drive a 3-8 decoder
which will activate one group of keys (an octave) at a time.  The lower
three address lines connect to the select lines of a 8-1 MUX.
	If a key is down in the currently selected octave, the output of the MUX
will indicate this when that key is selected.  At this point the counter's
output represents the address of the pressed key.  Latch the address and
convert this with a DAC to produce your 1V/Oct CV.  Look at the Paia web
page for an example of the DAC part.  It's in the circuit diagram for their
MIDI-CV box.
	The tricky parts include:
-Making sure you drive the diode matrix properly.  Note the orientation of
the diodes.  In mine (an SCI 4 octave KB), the matrix has to be driven low
and hence the output of the MUX is active low.  You'll have to use pullups
or pulldowns on one end of the matrix.
-Making sure you scan the matrix in a sensible order.  Trace out which pins
of the matrix connector connect to which part of the matrix.  You'll
probably want to scan from lowest to highest or vice versa.
-Being careful about designing your sequential circuit (it has memory,
remember?).  Timing glitches are a pain.  You may end up having to make a
true two phase clock.  Mind your Ps, Qs, and Ck.
-Using a good 10-bit or better DAC for best CV accuracy.  Tie the unused
lower bits low and use the upper 6 bits.

	My design will eventually be published on-line, but first I have to find
time to complete and debug it.  I got a severe case of featuritis after
playing with a friend's Prophet VS arpeggiator.  It dawned on me that with a
few extra parts I could make my digital KB scanner have an arpeggiator mode.
Think about it...it already scans the KB.  Rather than resetting the counter
after detecting a key press, it simply continues to scan all keys.  It
almost works...
	I also added the capability to scan Up, Down, or Alternate.  And the
trigger out has two modes: gate derived, or new note derived.  I think the
IC count was up to 15 or so (including the DAC and op amp for CV out).  Best
of all--NO SOFTWARE!

Hope this helps,
Doug


-----Original Message-----
From: owner-synth-diy at mailhost.bpa.nl
[mailto:owner-synth-diy at mailhost.bpa.nl]On Behalf Of Matti E. Leino
Sent: Thursday, September 02, 1999 3:52 AM
To: synth-diy at mailhost.bpa.nl
Subject: digital keyboard controller...how to build one...?


> I was surfin on hyperreal's pages, and I found a message, posted by gene
> stopp, where he tells, that that it possible to build digital cv
> controller...Ok, that's not big news, but how difficult it is to
> build...? I own this old Yamaha synthesizer ( broken one...) and there's
> quite good keyboard I wouöld like to use. It is diode matrix type, and
> has two switches per key. Now I would like to modify it like gene stopp
> sayed in that message, " digital scanner into latched D/A"

But how I can build this...? If one of you guys could help me in this
little problem, I would really appreciate it!

And schematics would be nice too....=)

Thanks in advance!

 - Matti


--
Matti E. Leino                  email:mle at nic.fi
Mäntysalontie 2                 http:www.nic.fi/~mle
28800 PORI                      gsm:+358400880666
Finland