FM synthesis 
frequency modulation synthesis 
There are two main ways to synthesize music: frequency modulation and sampling from a wavetable. FM synthesis uses a signal to modulate the frequency of another signal, and the results sound like arcade games from the 1970s or one of the first wave of poker-faced European technobands. Each FM voice needs at least two signal generators (also called oscillators or operators), and more-sophisticated FM systems may use four or six signal generators per voice. In general, however, FM synthesis is not as desirable an asset in your sound circuitry as wavetable synthesis.