Yamaha YM2413

The YM2413, a.k.a. OPLL, is an FM synthesis sound chip manufactured by Yamaha Corporation. It is related to Yamaha's OPL family of FM synthesis chips, and is a cost-reduced version of the YM3812 (OPL2). It has nine concurrent FM channels with two operators per channel, and features a percussion mode (or rhythm mode) that turns the last three channels into percussion/rhythm channels to produce 5 percussion/rhythm sounds.

To make the chip cheaper to manufacture, many of the internal registers were removed. The result of this is that the YM2413 can only play one user-defined instrument at a time; the other 15 instrument settings are hard-coded and cannot be altered by the user. There were also some other cost-cutting modifications: the number of waveforms was reduced to two, the additive mode was removed along with the 6-bit carrier volume control (channels instead have 15 levels of volume), and the channels are not mixed using an adder; instead, the chip has a built-in DAC that uses time-division multiplexing to play short segments of each channel in sequence, a practice that would be done with the YM2612 later on.