[sdiy] what is trigger signal?

[WeAreAs1 at aol.com]

In a message dated 2/21/01 12:34:25 PM, grichter at asapnet.net writes:

<< But what if a new key is depressed before the old one is let go? There 
needs
to be a "new key" signal that will "trigger" a new attack for that note from
the envelope generator. That is the Trigger signal, for the most part it
indicates a new key has been pressed, even if the old one is not released
yet.

It is a short electrical pulse, hopefully, not shorter than 1 millisecond.
It is usually positive going. Very short trigger pulses can cause problems
when interfacing between equipment from different manufacturers.
 >>

It should also be noted that it's possible to get the above "multiple 
trigger" response from a synth that only provides a gate signal (and has no 
separate trigger signal).  It's a modification that's not too difficult to 
do.  

What is needed is a means of temporarily interrupting the gate signal 
whenever a new note (of a different pitch) is played.  This can be done by 
AC-coupling the keyboard CV into a sensitive and fast little differentiator 
circuit, the output of which will be a very short positive-going spike (about 
1ms to 5ms).  It's possible to set the differentiator to output a spike when 
the input voltage change is somewhere around .0833 volts or greater (one half 
step or greater in a 1v/octave system).

Then, put an NPN transistor across the synth's gate line (collector goes to 
the gate line, emitter goes to Ground, which will shunt the gate signal to 
Ground whenever the transistor is turned on), and turn the transistor on with 
the aforementioned spike.  The result will be a 1ms interruption of your gate 
every time you change pitch, which in turn retriggers the attack of your 
envelope generators, giving you the much-desired "multiple trigger" effect 
(and saving you years of time in pesky piano lessons and Hanon finger 
exercises, I might add).

I've installed this mod in many Minimoogs (Manymoogs?), and it works great.  
It should work just as well with any other gate-only system.  I'll send out a 
schematic to those who might be interested in trying it.

Michael Bacich