On Sun, Jan 11, 2009 at 2:39 PM, madrayken < dene.carter@... > wrote:

> That sounds perfect. Out of interest, I presume I'd still need to do
> this even with a vactrol Low Pass Gate (i.e. vactrols would still
> need a 'trigger' to achieve the same effect)

I think you may be confusing two different ways to achieve a similar
(yet different in critical ways) percussion sound. I'll describe both,
and that should make clear the "trigger" usage for both.

1. spiked filter percussion: This is similar to what's used in many
old drum machines. You set a filter (usually bandpass) on the edge of
self-oscillation, then feed the audio input a trigger, this impulse
pushes the filter into oscillation temporarily until it decays
naturally back to silence. The end result is a transient click (from
the trigger) combined with a sinusoidal signal that decays out to
silence. If you tried to use a square gate in this case instead of the
trigger, you'd get new "spikes" into oscillation at each transition of
the waveform, with positive DC offset on every other percussive hit.
Triggers are better ;0)

2. Lowpass gate percussion: This is usually done with a pair of
oscillators in an FM configuration, the output of the carrier
oscillator going into either a single lowpass gate in "both" mode or
serial lowpass gates, first the lpf and then the vca. The trigger
signal is used here as the cv into the lpgs. This quick transient
allows the naturally-slow and interestingly-shaped characteristics of
the vactrol response to shine, and results in that classic lpg bonking
sound.

-Brandon