Ive seen VST plugins or soft synths that have a FM option on inputs that are not just simple oscillators. The inputs i talk about dont have any frequency parameter you can access. Think about for example pre-recorded audio. Heres an example. https://www.youtube.com/watch?v=9pn_b7OUO6I&t=177s
I wondered how the FM in these cases is implemented. Later i thought about it and i guess its done something like this.
I thought it was pretty interesting, but i couldnt find any reading material on this. Idk if theres more to say about this other than what is already known about “regular” FM. But if there is, i’d like to hear it.
For sine waves, the modulations of frequency, phase and delay are basically equivalent, only the input has to be given in a different form, see the conversion formulas above and the limited usability of the index. With recorded output, you can work with rate modulation (as in your example) or phase modulation (BufRd), with an arbitrary incoming signal you can use delay modulation. In all of the latter cases you get the typical sideband effects.