Other common FM structures include “multiple modulator FM” and “feedback FM”.
Several STK instruments (Rhodey, Wurley, and TubeBell) use two summed FM pairs, though in all cases one of the modulators feeds back on itself (and is not a sinusoid).
The BeeThree STK instrument sums 4 oscillators which are modulated by an LFO to simulate vibrato. Again, one of the oscillators feeds back on itself (and is not a sinusoid).
The following link
provides many more details and algorithms, including some theory on feedback FM.
