Drum Trigger Signals

[Fraser, Colin J]

> -----Original Message-----
> From: Paul Perry [mailto:pfperry at melbpc.org.au]
> Sent: 03 September 1999 09:22
> To: thescum at surfree.com
> Cc: synth-diy at mailhost.bpa.nl
> Subject: Re: Drum Trigger Signals
> 
> 
> At 10:22 PM 2/09/99 -0400,Byron thescum at surfree wrote:
> 
> >With a Twin-T circuit like the 808, the circuit is sensitive to state
> >changes on the input.  You can drive them from a square-wave LFO, and
> >they'll trigger (really just a ringing filter) on every transition:
> >either on or off.>
> >It's more common to drive them from narrow pulses.  The circuit will
> >resonat on the rising edge, and the falling edge occurs before the
> >original drum sound is over, so it doesn't retrigger.
> >
> Is this really true?
> Surely the falling edge of the pulse must create a decaying resonant 
> wavetrain, which might interfere constructively or 
> destructively with the 
> first resonant burst.

The only difference between a rising and falling edge trigger would be the
initial phase of the resonant signal.

Even with the 1ms trigger pulse used in the 808, which always triggers the
oscillation in the same direction, if another trigger arrives while the
sound is decaying, the inital level of the new sound will vary quite
noticeably depending on where the trigger happened relative to the current
phase of the resonant signal.
This is most noticeable if you modify the 808 bass drum for extended decay
and have lots of overlapping beats.
BOOM, boom, bOOm, BOOOOM!.... pass me that compressor...

Personally, I prefer the 4069 as vc element in an integrator/trigger
oscillator used in the 909 circuits - still a low parts count, but it
behaves much more predictably.


Colin f