So I tried to use the frequency modulation in the FDS and I was thinking if one could tune the modulation rate for every individual note to make real FM sounding instruments. (I gave up in the attached ftm but you can see what I'm going for)
First, use the beta, if you aren't. The beta has a different (more correct) modulation algorithm.
Second, there are number of things that make this different (i.e. unsuitable) for FM synthesis.
The primary reason is that it actually modulates frequency. In the YM series of chips, what is referred to as FM is actually phase modulation. These two synthesis methods can produce similar sounds, but only phase modulation is practical for stable, repeatable sounds (and is easier to implement).
In addition to this, the modulator phase does not synch with the note, and even if it did, it's not possible to synch the modulator frequency to an integer of the (the FDS doesn't have the accuracy for it, and the frequency modulation is not properly logarithmic). It will always have a sliding phase effect. (Actually, YM chips tend to have a sliding phase sometimes too for the same reason, but they have a lot more accuracy at their disposal so it is more subtle.)
I had a test of this somewhere, but I can't seem to find it when searching the forum (also might not work the same way anymore with the beta).
For example, if you have a table of all -2s, then a mod depth of $18 (or, you could jut leave the intrument's mod depth blank and use the H18 effect), you'll get stable FM notes
http://mmlshare.com/tracks/view/303
Xyz: those tones aren't stable, they're phasing like crazy.
Also, putting an all - or all + mod table just causes the mod value to wrap periodically. If you turn the mod speed and mod depth up high enough you get a saw wave modulator, essentially.
So with that out of the way...I don't really get what their intention was with the modulator unit anymore. If it can't be used in music (unless you count vibrato), why would they bother to include features like that? Sound effects maybe?
Vibrato and sound effects aren't a good enough reason?
The other thing is, especially back then, all of these chips were experiments. You'd have an idea of what you might want to use it for, but a lot of times it will get used differently in practise. The 2A03 has several rarely used features in it, for example, and then on the other side you have things like sunsoft bass, which wasn't intended at all by the hardware design.
