I'm away from home for a couple of weeks still so i cannot try anything.
The suggestion from SirPrimalform in the first message of this thread sounds good to me.
Here are some clues.
I don't think you need new variables.
Add a new Constant here :
https://github.com/Ixox/preenfm2/blob/master/src/synth/SynthState.h#L250Modify SynthState.cpp
https://github.com/Ixox/preenfm2/blob/master/src/synth/SynthState.cpp#L198To add your UI string for this new "Frequency type" value.
Line 204 replace OSC_FT_FIXE by the new constat you added in SynthState.h.
At this point the value should show up in the UI.
Modify Osc.cpp and add a "case" in the switch to calculate your frequecy when the new Ftype is selected.
Use the variable detune for your Hertz detune calculation.
I don't think you need to modify FMDIsplay.cpp but you can search for "OSC_FT_" and see there are some hack there to have the display show/hide ftune depending on FType.
Make sense ?