Re: [AH] Envelope generators, triggers and gates

From Florian Anwander
Sent Sun, Apr 22nd 2018, 19:32

Hello



short version:

A trigger describes a time stamp, a gate describes a duration between 
two timestamps.


long version:

A trigger is NOT(!!!) a short pulse. A trigger is the transition between 
to levels of voltage. And in detail: it is the moment when the changing 
voltage passes a dedicated voltage level (the so called threshold voltage).

There are two kinds of triggers: positive and negative. We speak about a 
  positive trigger, when a the transition is from a low level to a high 
level. And we speak about a negative trigger, when the transition is 
from a high level to a low level.

So in fact a gate defines the duration between a positive trigger 
(usually=note on) and a negative trigger (note off).

see: http://fa.utfs.org/diy/Trigger_vs_Gate.jpg

For a trigger it does not matter whether the transition in the relevant 
direction is immediate or not, and it does not matter, how the change 
back looks like

see: http://fa.utfs.org/diy/Trigger_vs_Gate_2.jpg left graph.

It is nive to know that a saw-like signal my trigger more safely than a 
pulse, because a pulse thends to overshoot and to swing back. This can 
cause doubletriggering (in the graphics the position [b] is the correct 
trigger timestamp, and the position [c] is the unwanted  double-trigger 
caused by the overshoot of the negative slope of the "trigger"-signal.






Of oourse when there is transition from low to high (= a positive 
trigger) there must come the day, when we go back from high to low. So 
every trigger signal is in fact a kind of gate signal. But the trick is: 
whether the signal is recognized as gate (event with duration) or as 
trigger, decides the receiver(!).

There are receiver-functions in a synth, which recognize only the 
positive transition (=the positive trigger). For example an AD-envelope, 
which starts an the positive ramp and does its increase/decreas run - no 
matter what happens else at the trigger-input.

And there are receiver-functions, which use the positive transition to 
start an event and the negative tranistion to recognize the duration of 
the event. For example an ADSR, which starts the attack phase with the 
positive transition, then does the decay and holds at the sustain level 
and would stay there until the end of times if here wouldn't be a 
negative trigger, which says: do "release" now.


I hope this clears the confusion about the two terms "Trigger" and "Gate".

Florian