[sdiy] Favorite VCO options

[David G. Dixon]

> > In any case, any module which can be
> > synced will have an input capacitor to differentiate the incoming master
> > signal anyway,
> 
> Says who? :-)

Me.

> > and other circuitry to make it compatible for that specific
> > device, so there's really no need to bring out the comparator pulse.
> 
> There is. For one thing, the standard for triggers is generally a
> short +5V pulse.
> Compare: http://www.synthesizers.com/gates.html
> 
> This means that a +15V rising saw wave would trigger at the wrong point.

Huh?!?  I don't get it.

> For another thing, what if the gear only works on rising-edge
> detection? In that case, there's no rising edge in the positive-going
> ramp (or falling edge, in case you're talking about a negative-going
> saw). Nothing gets triggered. Sure, hardware can have a rectifier to
> convert broken trigger signals, but a diode delays the trigger a bit
> in that case, and the circuit becomes more complicated, so I shun away
> from doing something like that.

...?  Diodes delay triggers?  That's news to me!  What's a "broken trigger
signal"?

> More importantly: if you have two oscillators that are in sync, you
> want them to start resetting at the same time. In your example, the
> master oscillator would reset, and once it's halfway done we get a
> (falling) edge that can trigger the slave oscillator to reset. This
> means the two oscillators will not actually be in phase. This matters
> especially at high frequencies - the higher the frequency, the bigger
> part of its cycle is taken up for core resetting.

The master resets the slave when the master has a rising edge.  Period.
Otherwise, there's no pulse to trigger the sync.

> But most importantly: if the oscillator is FM'd, the saw output could
> have steep rising edges without the integrator resetting. So, the saw
> output is definitely not suitable.

The saw only has an edge when the integrator resets.  If the integrator
doesn't reset, the ramp rises to the positive rail and rides there
indefinitely.  All FM does is modulate the slope of the ramp so that it
reaches to the comparator threshold voltage sooner or later.

I don't know where you're going with all of this...