From gstopp@fibermux.com Wed Sep 18 10:40:27 1996
    18 Sep 96 13:40:14 +0500
    18 Sep 96 13:27:28 +0500
From: gstopp@fibermux.com
Date: Wed, 18 Sep 96 09:52:59 PDT
Encoding: 76 Text
Subject: Re: Keyboard Control

     I've found two reliable methods of building a monophonic keyboard for 
     CV-Gate applications:
     
     1. Resistor divider plus holding cap plus buffer
     
     2. Digital scanner into latched D/A
     
     The circuit that I end up building really depends on the physical 
     keyboard mechanism that I want to build around.
     
     If the keyboard assembly is an old double-contact J-wire Pratt/Read 
     type without any diode matrixes, I'd probably build a resistor-divider 
     CV source into a cap and buffer on one set of contacts, and a gate 
     output from the other set of contacts. This is by far the easiest 
     circuit-wise, and is extremely reliable.
     
     If the keyboard assmembly has a single set of contacts without diode 
     matrixes, I'd build a digital scanner into a latched D/A. Since I'm 
     partial to low-note priority in monophonic controllers, I'd go that 
     route. The gate output can be made from a retriggerable monostable on 
     the latch strobe.
     
     If the keyboard has a single set of contacts and a diode matrix, I'd 
     use the scanner method modified slightly to read the diode matrix. 
     This is actually a little bit easier as it does away with several mux 
     chips. Gate generation works the same.
     
     The analog cap/buffer approach does suffer from charge droop, so it is 
     subject to long-term drifts on the CV output. Careful building and 
     choice of components can minimize this, but it can never be fully 
     eliminated. For melody playing, it's probably not a problem, but for 
     long sustained drones it's not a good thing. The D/A converter in the 
     digital scanner design will hold the pitch without drift indefinitely.
     
     A trigger output can be obtained by putting a sensitive differentiator 
     on the CV, tweaked so that it produces a clean trigger for a semitone 
     change of CV. In the digital design it is possible to generate a 
     trigger digitally, but it will add to the complexity of the design.
     
     One popular design that I don't use is the single-contact analog 
     sample/hold. This uses a resistor divider to derive the CV and gate 
     from the same set of contacts. This design starts to get flakey (i.e. 
     dropped notes) as the contacts get dirty over time, so I stay away 
     from it. The two-contact analog design is fairly immune to these 
     problems, and the digital design (in my experience) is 100% flawless 
     forever.
     
     - Gene
     gstopp@fibermux.com


Subject: Keyboard Control
Author:  POLARIS@vax1.mankato.msus.edu at ccrelayout
Date:    9/18/96 8:36 AM


Hello all,
     
I've been toying with the idea of putting together a modular for some 
time now.  I have acumulated a sufficient number of modules for a 
starter system and have read several books on the subject.  One thing 
has me stumped, though.  How do I control the thing?  The books I've 
read all gloss over any real keyboard circuits (just the basic voltage- 
divider idea is presented). I have a five octave keyboard with single 
contacts.  How do I use it to create control voltage, gate, and trigger 
signals?  I don't want to get into microprocessors, I just want a simple 
monophonic controller with whatever-note priority whould be appropriate. 
Could some kind soul out there point me in the right direction, or even 
possibly supply me with the schematic? 
     
Any help will be greatly appreciated.
     
Thanks,
     
David Forsyth