AW: Through-Zero FM

[Haible Juergen]

	>I have read the term "through-zero FM" several times, in previous
posts found
	>here.  Usually with the writer referring to it in glowing terms
lavish with
	>praise.  I, too, now wish to wallow 'n' revel in the wholesome
goodness of
	>through-zero FM.   So, could someone please be so kind to explain
the concept
	>to me?  (and to the rest of the list, of course!)  Also, what are
some musical
	>applications and sonic characteristics of through-zero FM?

Think of a spinning wheel that rotates clockwise with 100 cycles per second.
(or a VCO that runs at 100Hz)
Now you want to modulate this spinning rate symmetrically around these 100
Hz.
Modulating from 80 Hz zo 120 Hz for example works fine. Even from 0 Hz to
200 Hz
is still possible on a "normal" VCO. If you want to further increase the
modulation
depht you can go higher than 200 Hz, but not lower than 0 Hz, i.e. the
modulation
is not symmetrical anymore. A modulating sine wave would be clipped, with a
very strong effect on the spectrum of the modulated signal.
The idea of Thru Zero Modulation is that your wheel can also rotate
"backwards",
i.e. in the opposite direction. So again starting from 100 Hz (clockwise)
you can
for example go up symmetrically to 1100 Hz (clockwise) and down to 900 Hz
(backwards 
or counterclockwise). This is obviously possible with a spinning wheel, and
it is
also possible to build such VCOs. Consider that "frequency" means change of
phase angle
in time, so you can decrease phase as well as increase it, and thus you can
speak of positive and negative frequency. (You will only notice the
difference if
you see a VCO *change* from positive to negative, not as long as it stays in
one
direction.)

Sonically it means that you just (finally) get all the effects of ordinary
linear FM. That is,
you get side bands that are defined by modulation strength and modulation
frequency.
The new thing is that you can actually make *use* of this concept for the
first time.
Without going "thru zero" you're limited to a rather small modulation depth,
and so
you don't leave the "rather boring" aera of FM with low modualtion amount.
So you don't gain something completely new - it's just that you get rid of a
stupid
limitation.

JH.