The concepts of ratios and tuning are of fundamental importance in FM synthesis.
Ratios determine the relationship between the incoming note(s) frequency and the frequency of a signal, be it carrier or modulator. Using this technique we can change the pitch of carrier signals on either a constrained (octave) or unconstrained (free select) basis.
The ratio of a modulating signal to the incoming note frequency determines the effect created by the frequency modulating – the best way to understand this is to download a copy of RedLion and try for yourself!
The fine tuning knob in RedLion can be used in conjunction with the ratio knob to tune to particular intervals or can be used to move two (or more) signals slightly out of tune and produce a range of beating and phasing effects.
This video demonstrates all of these concepts using the RedLion FM synth VST/AU plug-in. Enjoy!