I checked out the patch. The easiest way to explain it is that regular oscillators don’t go to 0 Hz unless you power them off. In Audulus though the starting frequency is 0 Hz and you add to it to modulate the frequency.
You might be thinking that you are multiplying that signal by some value, what’s that about? That would be the index of modulation.
So to review, modulation is added to the frequency but modulatibe by ±1 Hz isn’t that amazing most of the time so we multiply the signal by an index amount.
Fm demo patch.audulus (124.0 KB)