PHASE MODULATION
The phase modulation in VORTEX is actually a part of the circuit that also takes care of the wavefolding
and waveshaping. It’s capable of phase modulation by taking the SAW wave and adding a DC offset.
It’s only true phase modulation when using the SAW from VCO1 shaped into a SINE wave*. For the
other waves (SAW, TRAPEZOID and SINE) it is psuedo phase modulation. Interesting timbres can be
achieved by both types of phase modulation with the added benefit of providing a more stable
perceivable pitch than for example exponential FM.
Phase modulation is a kind of modulation that doesn’t affect the core pitch of the oscillator. It merely
shifts the waves on the horizontal axis creating a kind of Doppler-effect. At slower MOD rates this
sounds like a vibrato but the perceived pitch change is a result of the compression and stretching of
the waves, just like when a firetruck passes by. The phase modulation doesn’t affect the TRIANGLE
core of VCO1 and therefore has also no influence on the SINE and SQUARE outputs that are derived
from the core and are used for syncing or modulating VCO2.
The offset used in the circuit is provided by the SINE of VCO2 (when nothing is patched into the MOD
input) and attenuated by the PHASE MOD slider which further controls the DEPTH of the phase
modulation. It’s possible to break the normalization by patching a signal into the MOD input. This input
accepts bipolar signals.
It’s also possible to modulate the DEPTH with the SINE from VCO2
(normalized). The knob is an attenuator for this modulation. You can override this normalization by
patching a signal into the DEPTH input. This input accepts bipolar signals and the knob is an attenuator
for this external modulation.
The range of pseudo phase modulation is +720° for MOD signals around 2.5V and -540° for MOD
signals around -2.5V (with the signal completely unattenuated i.e. with the PHASE MOD slider in the
top position). Anything beyond -2.5V to 2.5 V results in silence. The internal normalized modulator
(SINE of VCO2) is therefore capable of surpassing the audible phase modulation range but by using
the PHASE MOD slider it’s possible to attenuate the signal so it stays within the audible range.
Maximum DEPTH modulation is achieved (with the PHASE MOD slider set-up in such a way that the
phase modulation stays in the audible range i.e. somewhere in the middle or a little bit higher) from
-5V to 5V but this is greatly dependent on the MOD source and PHASE MOD slider settings. It’s
possible to send higher voltages into the DEPTH input to exceed the range capable with the internal
DEPTH modulation but this might result in silence (as explained in the previous paragraph).
* It’s possible to do true phase modulation by folding the SAW into a SINE and filtering
out the glitch in the wave. The max. range of this phase modulation is ± 360°
VORTEX / Complex Oscillator
10
