Aha! Thanks for that, Brandon. That's much clearer now.

--- In Doepfer_a100@yahoogroups.com , "Brandon Daniel" <bdu@...> wrote:
>
> 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
>