[sdiy] gate/trigger question

[thescum at surfree.com]

On Thu, 30 May 2002 01:34:58 -0400 Caleb Johnston <caleb_1 at msn.com> wrote:

Dear Experts,

>I am trying to build a trigger/gate signal that
>will start the ADSR.  I 
>heard that the trigger, which is a short pulse
>of high voltage, actually 
>starts the ADSR envelope.  And then the gate is
>somewhat lower in voltage 
>but controls the length of time the note is 
>held.  True?

Somewhat true, and it depends on the exact implementation of the ADSR.  The trigger is a quick pulse indicating a key has been pressed, and the exact voltage of the pulse depends on the maker.  I think 12V used to be common, but with folks driving things with microprocessors, 5V might be more common today.

The use of the gate & trigger depends on the ADSR it's driving.  The ASM-1 ADSR only needs a gate input.  I've got some ADSRs that have an AR and an ASR with summed outputs, which require triggers and gates.  The gate makes the ASR go, and the trigger makes the AR go.

>Also, any good ideas for circuits as how to get >that quick pulse for the 
>trigger before the gate?

In a simple implementation, you could run the gate into a rising-edge-detector.  Every time the gate went high, you'd get a quick pulse out.

The problem is retriggering.  If you hold one note, and then tap a second note, you'd probably want the envelope to restart.  An edge detector on the gate wouldn't catch that, because the gate would be constant.  You almost need to implement that in the controller.

Byron Jacquot