The amplitude of the modulator wave is used to vary the phase of the carrier wave. This carrier wave is generated by calculating a phase value from the operator clock (the frequency of which is controlled by the fnumber / block values) and passing that phase value to the wave lookup table. Modulation occurs simply by adding a modulation component derived from the amplitude of the modulating wave, to that phase value which is fed into the carrier's wave lookup.
Last night I did a series of tests with the aim of determining exactly how the amplitude of the modulator is related to the depth of modulation. In other words how do we get from the modulator amplitude to that additional phase value.
I selected 4operator algorithm 4 which looks something like this:
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v  
+> OP1 
 

  v
> OP2 > OP3 +>
  ^

 
> OP4 

I set operator 1 and operator 4 to a square wave with a high attenuation.
These will serve as a reference signal. The square waves can be easily recognised and removed from the output signal, but the rising and falling edges of the waves provides represent the start and midpoint of the modulator wave as well. Operator 2 is also a square wave, and the attenuation for this wave is varied in the experiment. It is this variation which will be compared to the output result. Finaly operator 3 is a sine wave with a low attenuation. This is the carrier wave and the distinct phase shift which occurs twice in each cycle is caused by the square wave modulator.
I haven't had a chance to study the results yet, but a cursory examination shows that the data appears to be good. I Have a distinctive sine wave shape, with a discontinuity in two places as the phase shifts abruptly.
When I get a chance to go through the figures, I should get a clear answer as to how the wave amplitude is converted to a phase shift value.