The DX7 and similar Yamaha digisynths actually use Phase Modulation (PM) and were just marketed as FM, right? This also means that you don't really get the sidebands and some of the challenges, that (Exponential) FM comes with.
(I wrote this part, then edited it out, cause suddenly I wasn't sure sure if I am mixing it up with the Casio Synth, but they use Phase Distortion
, and now I am editing it back in, because it also helps to understand the rest of this post better.)
@standingwave: Thru-Zero means, that the oscillator is able to run "backwards", when the FM reaches a point beyond having it go to 0 hz, thus allowing for strong modulation beyond that point or also for example allowing the oscs base frequency to be 0hz,and the FM modulating it equally to both sides of that point, which deals with some of the problems (pitch shift) introduced by expo FM.
In eurorack wonderland, there have recently been some amazing inventions on this front.
Intellijels Rubicon2 made the TZFM big, SSFs Zero Point Oscillator has Lin FM, TZFM and some amazing wizardry regarding some AM coupled to the Zero Point and Joranalogues Generate3 has TZPM of plus and minus 720 degrees.
There was this very comprehensive article on all the different possible types of FM, further differentiating it into what they called it FM+ and FM-, but I haven't been able to find it in my quick search right now...