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The YM2413, also called OPLL, is a cost-reduced audio chip manufactured by Yamaha. It is based on their earlier YM3812 (OPL2) design.
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 an adder is not used to mix the channels; instead, the chip's DAC plays each channel one after the other, and the output of this is usually passed through an analog filter.
The YM2413 was used on the MSX-Music sound expansion on the MSX and on the Japanese Sega Master System (Sega Mark III). Also, the Famicom game Lagrange Point contains the Konami-designed VRC7 mapper circuit, which has an FM sound core quite similar (but not identical) to the YM2413.