Cross modulation is basically another term for FM. Except where Frequency Mod synthesizers are primarily digital, Cross modulation is implemented in analog circuits. Its when you take the output of an oscillator module and patch it into the pitch of another oscillator. You can detune said oscillators and change their volume or mod depth to get some really cool, really grimey results.
I was messing around with the Cross modulation built into the oscillators in Retrologue and was surprised to find that the Cross option doesn’t work between the oscillators but instead uses a static sine wave locked to the pitch of the modulated oscillator. This means you can’t change the pitch of the modulator or it’s waveform, the only parameter you have access to is the modulation depth via the ‘shape’ knob. This is more like Korg’s VPM algorithm if anyone is familiar.
Thought I’d clear that up since the manual says: “This algorithm provides a combination of two oscillators where a *master oscillator is modulating the pitch of a slave oscillator (Sine, Triangle, Saw, or Square) at audio rate. The Shape parameter adjusts the **pitch ratio between slave and master oscillator, resulting in a sound close to frequency modulation.”
**not the pitch ratio, otherwise the modulator would jump between harmonic multiples of the carrier, what you are adjusting here is the modulation depth applied to the “slave oscillator”.