Equalizer!
Weell, if you don't know what FM synthesis is - I'll try to explain.
FM is a very mathematical form of synthesis. The simpler form of FM synthesis uses sine waves as a starting point. Those sine waves modulate one another to produce more complex waveforms. The sines are divided into groups of carriers and modulators. By changing frequency and amplitude of the modulators you re-shape the carrier waveform from a sine to something totally new. Comparing to analog, FM synthesis is a little unpredictable and non-linear, really. FM is for people, who like to use their head, or simply don't expect instant results.
If you really want to know more about FM - search the web for a DX7. Almost all the DX sites have an explanation on how FM works.
ED