Through-Zero FM

[Martin Czech]

I've never heard of that BEFORE entering this list, you are not allone.

It's simple:

1. Phase modulation and Frequency modulation
   are closely connected: d/dt (phase(t)) = frequency
2. The waveform shaping depends primarily on PM.
   It also depends on FM (1), but as you can see
   some integral operation is implied, that means
   less PM for higher frequency FM.

Now, you want as much PM as you can get for bright sounds.
Using the normal expo input or (if you are so lucky to have)
the linear modulation input allows only for positive frequencys,
the current sources in the vco are unipolar.

Not so, if you set up FM or PM eg. with gnuplot etc.
Strong modulation can reverse the phase pointer, ie.
negative frequencys. This means "through zero".

A usuall vco can't do this. 
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.
As a result you get almost the same waveform as your mathematical
model of PM or FM for deep modulation.


That's it.


m.c.