[sdiy] Starting my ASM-1

[Gene Stopp]

Electronotes issue 67 page 12 is a really good digital scanning interface
for a single bus keyboard. I've build several over the years and they are
great. If you think about it, converting a switch number to an analog
voltage and deciding what to do if more than one is pushed at a time is best
handled by a simple digital circuit. This design uses a low-note priority
latch plus a simple DAC, which allows you to keep your resistor-string-based
keyboard legato playing style.

This kind of interface is pretty much immune to dirty contacts. It has zero
drift over time after you stop playing, so you can drive the same synth with
a midi-cv converter and not have to worry about it drifting out of tune.

If you have a 2-bus keyboard and you want to get by as simply as possible,
you can use a current-source/resistor-string/holding-cap/buffer cv output on
one bus and use the other bus for the gate. This works pretty good and is
fairly immune to dirty contacts, but it does drift over time after you let
go of the last key. This would be acceptable in a synth that is never
intended for midi-cv operation while the keyboard is still hooked up. It
does require that you adjust the j-wires on all the keys so that the
contacts hit at the same time.

I would never reccommend a gated sample-and-hold design ever again. It just
takes one dropped or mis-sampled note to spoil a cool solo to make you
realize that there's a design you could've gone with that never has that
problem.

I've noticed there's a bunch of raw keyboard assemblies on ebay right now -
these would be perfect.

Best Regards,

- Gene


-----Original Message-----
From: Ian Fritz [mailto:ijfritz at earthlink.net]
Sent: Thursday, January 10, 2002 9:31 PM
To: Rob Mantel; synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Starting my ASM-1


At 08:18 PM 1/10/2002, Rob Mantel wrote:
>I built the formant keyboard years ago, and to be honest, I would never do
>it again. A lot of those keyboards, like the formant one, use double
>contacts for each key (one for the cv, one for the gate), its a pain to
keep
>those contacts work reliable and trigger both at exactly the same time over
>the years.  So if you want to build your own, try to find something that
>only uses 1 contact per key, but unless you can find the right schematics,
>it won't be easy to have a keyboard that you can play both with and without
>triggering a new gate pulse when you play the keys legato. There might be
an
>easy solution for this, but I don't know it.


See EN #67 (12)   :)