[PD] triggered oscilloscope/bang on zero crossing?

[Derek Holzer]

I'm trying to put together a simple oscilloscope abstraction that 
imitates the "trigger" function of a real one, so that the waveform can 
be seen from its starting point.  In other words, a [tabwrite~] would 
graph on the upward swing of a zero crossing. To do this, I'd like to 
know if there is an audio object which gives a bang at a zero crossing, 
or perhaps even only at a crossing from the negative to the positive 
domain of the signal. I've thought of something with [expr~ $v1 < 0], 
but what comes out is still in the audio domain and isn't the bang which 
[tabwrite~] needs.

Other suggestions welcome.

best!
Derek

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