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

[Richard Wentk]

At 21:48 24/02/2006, Chris Manders wrote:
>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?

Potentially you could have the lowest note = 0V, which would cause a 
problem for any amp+clamp gate circuit.

Also there's often quite a bit of bounce on keyswitches, and it's easier to 
deal with this with two circuits.

But that aside there's no reason a single line design wouldn't work. It's 
just not traditional. If you wanted to get tricky about this you could 
something like checking for an open circuit, maybe by having a nominal 
pull-down to - say - -12V with a big resistor+diode for a tiny current. 
When a key was pressed a much larger current from the resistor ladder would 
swamp this, giving you enough information to split the data into gate+CV 
from that one input.

This would be more complicated to design, and you'd have to put more 
current through the ladder than you'd normally use. But it might possibly 
easier to put together mechanically.

Richard