Content for YM2413

The YM2413, a.k.a. OPLL, is a cost-reduced FM synthesis sound chip manufactured by Yamaha Corporation and based on their YM3812 (OPL2). 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 other cost-cutting modifications: the number of waveforms was reduced to two, and the channels are not mixed using an adder; instead, the chip's DAC uses time-division multiplexing to play short segments of each channel in sequence, as also done in the YM2612.