FREQUENCY MODULATION
VORTEX offers several ways to modulate the frequency of both VCO1 and VCO2. Not only do the V/Oct
inputs accept bipolar signals, there’s a lot of additional sonic mayhem to be explored by using through
zero linear FM and exponential FM, either with the internally normalized connections or with external
modulation sources. The flow chart for the FM bus is in the ADDENDUM section.
THROUGH ZERO FREQUENCY MODULATION / TZFM
Unlike in regular linear FM where the oscillator that’s being modulated (called the carrier) is never able
to go below 0Hz (at which point the oscillator wouldn’t oscillate at all), with TZFM it actually can go
below zero.
Instead of running forwards it just starts running backwards and this results in the waveshape being
inverted whenever the modulator pushes the absolute frequency of the carrier below zero (picture 21).
Aside from a difference in timbre from regular FM it also has the advantage of being a symmetrical
type of frequency modulation and this results in a more stable perceivable pitch as the sidebands are
centered around the base frequency of the carrier. At high amounts of TZFM (controlled by the FM
INDEX) the resulting sound will lose it’s clear pitch though.
Both VCO1 and VCO2 have a knob for setting the maximum amount of TZFM labeled THRU ZERO. The
big knob in the center labeled FM INDEX controls the global amount of frequency modulation (this also
applies to the exponential FM of both VCO 1 and VCO 2). The modulator for VCO1 is normalized to the
SINE from VCO2 and vice versa. It is however possible to use a different signal to modulate the
frequency by patching any VCO into the LINEAR input jack. These inputs accept bipolar signals.
Keep in mind that the LINEAR inputs are AC coupled and only signals with a frequency of
approximately 1.2 Hz and more result in TZFM (in other words: you can’t use static voltages or slow
LFOs for TZFM on VORTEX as these are filtered out by a hi-pass filter). The reason for this is to make
sure any DC offset in the modulator doesn’t destabilize the pitch of VORTEX.
In linear FM (through zero or regular) every volt you add to the input adds a certain amount of cycles
to the frequency of the oscillator. How many depends on the callibration and the settings. On some
systems linear FM is used for pitch tracking and this is referred to as Hz/V tracking.
VORTEX / Complex Oscillator
17
0V
0V
C
A
R
R
IE
R
M
O
D
U
L
A
T
O
R
the waveform inverts whenever the modulator goes through zero
time
picture 21: through zero frequency modulation
