[sdiy] EDP gnat keyboard

[Tim Stinchcombe]

Hi Dan,

> ALSO///when i first looked at the schematics for the GNAT i 
> was excited because it seemed they had found a simple  way to 
> convert square waves to saw...but the more i look at it, the 
> more it seems that you might need to have the cv from pin 10 
> of the 4046 feeding into the 3900, so it is not a simple matter.
> 
> what does the signal from pin 10 actually do?

IC30a is an integrator, reset by the square wave via IC26b/IC29b, so does
give the sawtooth. But, we need to integrate faster for higher frequencies,
and that is what the 'kybd voltage' (from pin 10) is doing in conjunction
with the different resistors switchable into pin 2 IC30a. It seems the
resistors deal with the 'octave' change (they are roughly 'divide by 2' at
each jump), and the keyboard voltage relates to the note within the octave
(kind of a guestimate, as without some more detailed checking I'm not 100%
sure, though it seems reasonable enough!). The resulting sawtooth wave
probably has a rather varied amplitude with each different note (but then
'accuracy' is not a trademark of EDP stuff, especially considering the
digital inverters used as 'op amps')!

Tim
__________________________________________________________
Tim Stinchcombe 

Cheltenham, Glos, UK
email: tim102 at tstinchcombe.freeserve.co.uk
www.timstinchcombe.co.uk