keyboard modifications

[Debby and Gene Stopp]

I've used rubber-button keyboards for monophonic CV-output keyboards, with
excellent results. I use scanning interfaces myself - ever since I built the
one out of Electronotes back in the 70's, I've never gone back. I've even
added one to my Minimoog so it always plays correctly.

I don't have experience with the priority encoder approach, but sometimes I
think about trying it. I did make a programmer for another Minimoog with a
priority encoder, and I had some problems latching the word output reliably.
Fortunately there's a technique that I can use to get a good latch - push
the button firmly and hold it for a second. On a keyboard it might be an
issue however.

One of my scanning interfaces uses a single chip! I programmed the circuit
into an Altera part once.

The advantages of digitizing a keyboard versus using a resistor string are
numerous. The DAC never drifts over time, unlike an analog S/H. This means
you can leave the keyboard connected while you control the synth externally,
like in MIDI-CV setups. The scanning circuitry never makes a mistake - with
an analog S/H you will probably get the occasional dropped note. Also with a
scanning circuit you can do neat tricks like switchable hi-note or low-note
priority, by making the scan counter up count or down count. One thing I've
never tried is triggering the DAC latch with an LFO, which would
theoretically provide a random voltage output that is already quantized.

Is there a cascadeable priority encoder with a latched output? That would
make for a real simple keyboard circuit. I've only used the TTL version,
which needs an external latch.

- Gene

----- Original Message -----
From: "harry" <harrybissell at prodigy.net>
To: <WeAreAs1 at aol.com>
Cc: <mysynth at hotmail.com>; <synth-diy at node12b53.a2000.nl>
Sent: Monday, December 11, 2000 5:35 PM
Subject: Re: keyboard modifications


> Hello Michael (et al)
>
> Correct... the EFM style needs a low contact resistance. You could
possibly
> trigger
> a good analog switch... but the DAC is the best way to go.  If you don't
want to
> have the keyboard scan, try the 4532 8 input priority encoder. Very easy
to
> generate
> a word with... uhh... priority ???
>
> H^) harry
>
> WeAreAs1 at aol.com wrote:
>
> > In a message dated 12/11/00 4:29:03 PM, mysynth at hotmail.com writes:
> >
> > << What kind of keyboards are people modifying for DIY use?
> >
> > I have three different old keyboards here... one Sequential Circuits
> > Six-Track and two Casios.  I disassembled them with an eye towards
> > cannibalization and found identical rubber-bubble type contacts.
> >
> > On exploring further, I tried to ohm-out the switches on the keys and
found
> > a fairly consistant 150 to 200 ohms is the best I could do.  Is this
typical
> > for this style switch?  My guess is this resistance would reign havoc
with
> > the EFM keyboard controllers (asking as they do for 100 ohm @ 1%
tolorance).
> > >>
> >
> > If you wish to use this type of rubber-contact keyboard for generating
> > control voltage, the best way would probably be to connect the keyboard
to a
> > DAC, and let the DAC create the accurate CV that you need.  Another way
would
> > be to remove the rubber contact assemblies and build your own
wire-contact
> > switches to mount underneath the keys.  Neither of these solutions are
neces
> > sarily very elegant, but are workable nonetheless.
> >
> > Michael Bacich
>
>