AW: Through-Zero FM

[Haible Juergen]

	>A triangle vco can be easily modified to do this:
	>Modulate with full wave rectified modulation signal (this 
	>way negative values are avoided), detect the sign of modulation
signal,
	>if it is changing toggle the integrator charging direction.

Almost. Actually, you have to add both, the initial CV (keyboard etc.)
and the modulation signal, and do the FWR and zero cross detection on
the sum of both. Now that would be the case in a linear (V/Hz) VCO.

On an expo VCO, the "normal" reference current of the expo converter
takes over the role of the keyboard voltage. So you must add some
positive (or negative) reference voltage and your modulation voltage,
and apply the FWR and zero cross detection to that sum.

JH.