The guidelines I keep in mind when using frequency and phase modulation are:
•
With frequency modulation: The modulating frequency must be below the carrier
frequency (or it will be attenuated or disappear completely).
•
With frequency modulation: Low frequency (below audio rate, including DC
offsets) modulators will result in a change in pitch of the oscillator.
•
With phase modulation the modulating frequency either needs to be in audio
range or you need to mix multiple outputs with different phases to hear the
modulation. Slow differences in phase are not audible without a reference.
•
With audio rate modulation (either frequency or phase) simple ratios are required
to create harmonic sounds. This may or may not correspond to musical ratios (or
scales).
Through-Zero Effects
The phase and frequency modulation in the Three Body are both implemented with
signed addition (or subtraction). This means the waveform can reverse direction if it
changes signs (or goes through zero). For phase modulation this is part of the
wavefolding action mentioned above. For frequency modulation this occurs when the
modulation index is large relative to the carrier frequency and can be seen on the
negative side of the modulating cycle.
FM EQUATION (FROM WIKIPEDIA) SHOWING MODULATION
INDEX DIVIDED BY MODULATION FREQUENCY
Consider that phase modulation is just adding two waves
before a waveshaper, if they are not audible beforehand they
will not be audible afterwards (unless they are compared
against a reference, like in a phaser implementation).
SELECTED FREQUENCY RATIOS WITHIN A SCALE
1:1 UNISON
2:1 OCTAVE
3:2 PERFECT FIFTH
4:3 PERFECT FOURTH
5:3 MAJOR SIXTH
5:4 MAJOR THIRD
8:5 MINOR SIXTH
6:5 MINOR THIRD
