what is a thru zero VCO??

[Haible Juergen]

Paul,

there is a *lot* about that in the archives.
In short, it's about modulating a VCO "below" 0Hz, to
"negative" frequencies. 

Negative frequencies ??

Think of a spinning wheel that slows down and then starts
to spin in the opposite direction.
Back to music: You cannot discern any difference between 
+1kHz and -1kHz in the static case, of course. Even for
slow modulations, you won't notice more than the freuency
going down to 0Hz, and starting to rise again.
But for *audio rate* modulation, there's a whole new world
of sounds to explore.

Just inverting the VCO output signal will not do the trick.
Oscillating means steady increase of phase with time.
As frequency slows down to zero, the oscillator will stop
at any random phase, say 128 degree. Starting again
in the "negative" direction means to go on with 127, 126, 125 degrees.
Just inverting the output would mean a jump to (128+180) = 308 degrees.

JH.

	>   am I missing something here? what is meant by thru zero?
	>is it that the input voltage can go from say -10v to +10v
	>and the vco will produce an output, or does it invert the output
	>when fed with a -ve CV?