Feedback in FM synthesis

[Jeroen Proveniers]

Hello,

last night I was experimenting with FM synthesis on my PC, a little program
that calculates a sample based on four (or as many as I like ) operators,
like the DX-7,FB-01 etc. 
It's surprisingly simple to implement this type of synthesis, but when I
wanted to implement feedback, I got stuck.

Apparently on my FB-01, increasing feedback causes the operator to sound
like a sawwave, only the two highest settings seems to effect the sound. But
what is exactly fedback to what? I tried the output back to the modulation
input, but that only causes to lower the frequency and increases the duty
cyles, certainly no sawwaves! Does anyone know?

And why in a FM synth has only one operator feedback? Is it computationally
too intensive or is it just useless?

JJ
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