@Christoph-Hart Ah ok! Thank you! So it is like setting the Algorithm then or routing, only just with 2 Ops. Lovely.

I am experimenting with making a simple FM synth.

I have a question that I do not find the answer to in the documentation.
What do the "Carrier Index" and "Modulator Index" do?
I have a DX7 and to me, it sounds like changing the values of the indexes changes the algorithm?
can anyone explain?

Thank you,

BK