Yamaha YM2151

The Yamaha YM2151, also known as OPM (FM Operator Type-M) is an eight-channel, four-operator sound chip. It was Yamaha's first single-chip FM synthesis implementation, being created originally for some of the Yamaha DX series of keyboards (DX21, DX27, and DX100). Yamaha also used it in some of their budget-priced electric pianos, such as the YPR-7, -8, and -9.