So one way you can do this is to use the output of an oscillator to audio-rate modulate the FM amount(s) of another oscillator. Now how close this is to what happens IRL, I don't know, but it does allow you to do more complicated FM patches. So, for instance, if you had Osc1+2 set to FM mode and Osc3 as a sine wave (doesn't have to be), by chaining Osc3 to modulate Osc2 FM Amt A and Osc2 set to modulate Osc1 Amt A, you have what would be close to a 7-operator FM chain. Of course you can modulate other FM parameters instead e.g. Amts B and/or C. This way you can go quite far from what you can do with Yamaha FM
Be sure to use your envelopes, MSEGs, LFOs to modulate the amount of oscillator modulation
The other way, which is true to Yamaha style FM is to have 1 oscillator containing my sine phase change wavetable, which I uploaded somewhere. This is the same sine wave in 64 slots of the wavetable, but with linearly increasing phase offsets. Set up oscillator modulation again, but this time modulate wavetable position. Set osc1 and 2 to the same sine phase change wavetable and osc3 to sine and you basically have the same as a single FM module in DUNE2. So what I hear you say? Well, we've moved in to the realm of wavetables. That phase changing wavetable doesn't have to be a sine wave. It can be any wave you want! For instance, I have phase changing wavetables that correspond to each of the different waveshapes of the TX81z and OPL3 FM synths. I have wavetables that apply this phase changing approach to waveforms that have probably never been FM'd ever
Finally, we can move away from Yamaha style phase modulation when we replace the sine phase change wavetable with another wavetable with completely different waves in each slot, e.g., the default wavetable: sine->saw->square. Modulating that will basically allow you to do more general waveshaping. Try to set Osc1 to default wavetable in the mid position and osc2 as a sawttooth wave set to modulate osc1 wt position with +50. Set osc reset to on for both. Now try messing with the osc2 init phase to see what different waveshapes you get. Also try changing wt position and modulation amt for osc1. Now think about how you could do a chain of osc3->osc2->osc1. Loads of possibilities
Here are some of the waveshapes possible from the default wavetable when modulated with a sawtooth and changing init phase from 0 through 16 steps of a full cycle.:
Default WT waveshaping.png
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by docam » Thu Jan 14, 2016 11:03 pm 
but it's another debate.
