From Wikipedia, the Free Encyclopedia:

Frequency modulation synthesis (or FM synthesis) is a form of sound synthesis whereby the frequency of a waveform is changed by modulating its frequency with a modulator. The (instantaneous) frequency of an oscillator is altered in accordance with the amplitude of a modulating signal.

FM synthesis can create both harmonic and inharmonic sounds. To synthesize harmonic sounds, the modulating signal must have a harmonic relationship to the original carrier signal. As the amount of frequency modulation increases, the sound grows progressively complex. Through the use of modulators with frequencies that are non-integer multiples of the carrier signal (i.e. inharmonic), inharmonic bell-like and percussive spectra can be created.

The YM2608, a.k.a. OPNA, is a sound chip developed by Yamaha. It is a member of Yamaha's OPN family of FM synthesis chips, and is the successor to the YM2203. It was notably used in NEC's PC-8801/PC-9801 series computers.

The YM2612, a.k.a. OPN2, is a sound chip developed by Yamaha. It is a member of Yamaha's OPN family of FM synthesis chips, and was developed as a stripped-down version of the YM2608. The YM2612 is a six-channel FM synthesizer used in several game and computer systems, most notably in Sega's Mega Drive/Genesis video game console as well as Fujitsu's FM Towns computer series.