CROSS MOD: This fader controls the amount of cross modulation applied between the first and second oscillators. This parameter is useful for creating deep FM (frequency modulation) sounds. When applying cross modulation, DDS 2 is modulating DDS 1 with exponential FM.
When you enable the oscillator sync option for DDS 2, the default cross modulation behaviour will be reversed, i.e. DDS 2 will be frequency modulated by DDS 1.
Note: Since DDS 2 is forced to restart its waveform cycle with DDS 1 and DDS 1 is responsible for the base frequency when oscillator sync is enabled, any changes applied to DDS 2’s settings will change the harmonic content of the sound the depth of which can be controlled by the CROSS MOD fader. De facto, this allows for a well-behaved type of frequency modulation whose sonic outcome might resemble the results of wave folding or phase modulation.