[sdiy] Keyboard Circuit - Seperate Keyboard and Gate buses?

[Gene Stopp]

You're right - you should only need one bus. The resistor string circuit in
Electronotes (#45 or thereabouts) is a single-bus design that holds the
busbar slightly negative with a very high value resistor to V-, and when a
key is pressed the bus jumps to whatever that resistor tap in the string
happens to be. A comparator watches the bus and when it goes above ground, a
gate is generated for as long as the key is pressed. A FET-switch
sample-and-hold captures the string tap voltage so it persists after the key
is released. A differentiator watches the CV for any jump in voltage and
generates a trigger as well.

Minimoogs and ARP Odysseys use a FET-switch S/H as well, with a second bus
for the gate. The Odyssey also has a third bus for a trigger.

The Moog 951 has two busses, one for the resistor string and one for the
gate, with a S/H switch made out of a DIP relay and controlled by the gate.

The Electonotes digital scanning interface (#68 or thereabouts) uses a
single bus. The gate is derived with a one-shot monitoring the scan pulses
from the keyboard mux. An optional trigger circuit is provided that uses a
bunch of logic to figure out when the key has moved, but a CV differentiator
would be just as effective with much fewer parts.

If you have two busses and you want to keep it really simple, use the
resistor string bus as the S/H switch. Connect the bus straight to the
holding cap, and buffer it. The wires need to be short and everything needs
to be clean to avoid the droop. Use the other bus for the gate.

The FET-based designs have a flaw - sooner or later you will get a bum note.
What I mean is a note that is not the one you played, and most likely one
that is not even on a semitone boundary. The dirtier the contacts the more
likely to happen. The digital design will never do this, no matter how dirty
the contacts get. You might not get a note at all, but if you do get one, it
will be the right one. The simple bus as the S/H switch design is almost as
resilient. For some reason, in my experience, the Moog 951 is also extremely
resistant to bum notes.

If you only have one bus, my advice would be to go digital.


- Gene
 


-----Original Message-----
From: owner-synth-diy at dropmix.xs4all.nl
[mailto:owner-synth-diy at dropmix.xs4all.nl] On Behalf Of Chris Manders
Sent: Friday, February 24, 2006 1:49 PM
To: synth-diy at dropmix.xs4all.nl
Subject: [sdiy] Keyboard Circuit - Seperate Keyboard and Gate buses?

Hi folks

I am planning on building a simple keyboard circuit
for my Synth.

Looking at some synth schematics, I notice that a
large amount of them make use of two seperate bus bars
- One for the CV (using a resistor ladder) and one for
the gate. Indeed the (later?) ARP Odyssey used three -
One further to generate an additional trigger pulse.

For the ones that use two, why should they choose
this? Surely (and I what I have in mind) is that if
any voltage is present on the CV input line as a key
touches the resistor ladder, then that would be
sufficient to generate the 'gate'? Surely just one bus
would suffice?

I feel I am missing something obvious (story of my
life, really).   :-)

Chris


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