thru zero VCO questions

[Grant Richter]

Hi Guys,
> 
> Hm, why switching at the zero crossing ?! To me it makes only sense when
I
> rectify the FM input to put it into the expos ref. input. But one can
also
> directly integrate the FM input. Much like in two opamp LFOs. As I see it
I
> have to toggle the direction when either the upper or the lower threshold
> is exceeded. When the FM input changes sign, so does the integration
> direction, I don't need to switch anything for that. I only need the FF
to
> remember the last direction. When either of the thresholds is met, then
the
> polarity of the FM input signal is inverted. 
> 

I wonder if the flip flop and comparators are required at all.
Since the effect of them is to reverse the phase of the oscillator
output, why not just switch the phase with the classic "invert-non-invert"
stage as an independent output?

I can see the reason for the full wave rectifier, you can't actually
drive the frequency below 0 Hz. The zero crossing detector
preserves the polarity of the modulation signal. The flip-flop
and dual comparators are used to work around the threshold
problem with the hysteric comparator.

I think you could bypass all that if you just switch the polarity
of the triangle output independent of the hysteric comparator.
Now you wouldn't get the through zero FM effect on the square
wave output, but it would be simple.