keyboard modifications

[tomg]

Hi Gene, you are absolutely right! There is no reason to rely on the old
s&h scheme to control your synth unless you just want to build a keyboard
like that for the pleasure of it. The new pic based midi to cv converters are 
very high quality, dirt cheep and a snap to use. If you have a midi capable 
kbd. If not and you just want to play with synths the kbd converter is pretty 
easy to do. 

Michael, I have used a lot of different (key-cap) keyboards mostly Casios
and they all work fine and have close to zero resistance when closed. As 
long as the resistance is a constant it won't matter if it is there. You could 
build the chain with any "realistic" resistor value as long as they were all the 
same. Think of the opamp as a positive voltage supply. The voltage is 1/2 
the supply if you use 2 100K resistors as a divider....and the voltage is still 
1/2 the supply using 1K resistors.....so 200 ohms works just as well as 100.  
Got it? Somebody (? sorry) built one using 1.3K 1%  resistors and it 
worked just fine.

It's best to cut up a dead kbd to try this with but I don't know of anyone
who has been less than successful with *any* keyboard.....Some people
have trouble with that lfo though ?? This doesn't mean you won't be the 
first "real" problem but my confidence is pretty high.

EFM Kbd solutions................

RXCV v1,  midi to CV pcb w/pic $30.00 
RXCV-Pro,  midi to CV pcb w/pic $50.00 
KBD2b ,  keyboard convert pcb $5.00

We aim to please...;-)

Tom


> 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
> >
> >
> 
>