[sdiy] Roland CR-8000 triggering

[Adam Inglis]

I'm looking at installing a midi to trigger interface in my CR-8000,  
but I  want to retain the internal trigger function. I need a way of  
isolating the two trigger sources from each other.

The internal trigger system is from the CPU, via some pull-up  
resistors and a buffer, giving negative-going triggers from 5 - 0  
volts, seen in this pic here
http://www.adambaby.com/images/tech%20pics/CR8000/CR8k_CPUtrig.png
These triggers go to the various analogue voice circuits, here's a  
couple of examples, bass drum and hand clap
http://www.adambaby.com/images/tech%20pics/CR8000/CR8k_BDtrig.png
http://www.adambaby.com/images/tech%20pics/CR8000/CR8k_claptrig.png

I tried duplicating the input capacitor seen at the trigger entry in  
these voice circuits, for each trigger source, however this attenuated  
the  pulse too much resulting in a different voicing (proving that  
these circuits can be "velocity" or voltage sensitive).
Elby Designs, who make the MIDI2SDS16 interface that I'm testing, say  
that in general you can't have both connected at once.
Any thoughts?

cheers
Adam

(BTW, Roman asked re a previous post whether there was any change of  
voicing with pulse width variation. In the first pic you can see they  
quote a huge range of variation as the width varies with the tempo,  
from 4 - 70 milliseconds. So I guess the pulse width can't be  
critical. I haven't tested below 4 ms though.)