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 also some other cost-cutting modifications: the number of waveforms was reduced to two, 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's built-in DAC uses time-division multiplexing to play short segments of each channel in sequence, which was also done in the YM2612 much later. Applications The YM2413 was used in: the FM Sound Unit add-on for the Sega Mark III sold exclusively in Japan, that improved the sound quality of all compatible games. The Japanese model of the Sega Master System came with this add-on built-in; an arcade board design produced by SNK and Alpha Denshi in the late 1980s for a number of their games, including Time Soldiers, Sky Soldiers, and Gang Wars; the Atari Games Rampart arcade game; the Yamaha PSS-170 and PSS-270 keyboards in 1986; the Yamaha SHS-10 shoulder keyboard in 1987, and the Yamaha PSS-140 and Yamaha SHS-200 in 1988; the Yamaha PSR-6 keyboard in 1988; several sound enhancement cartridges for MSX computers. It is also built into select MSX2 and MSX2+ systems, and all MSX Turbo R machines, as part of the MSX-Music standard; and JTES Japanese teletext receivers. Variants and clones Yamaha YM2420 (OPLL2) is a variant with slightly changed registers (intentionally undocumented to avoid hardware piracy), used in Yamaha's own home keyboards. It has the same pinout and built-in FM patches as the YM2413, but several registers have parts of the bit order reversed. Yamaha DS1001 (Konami VRC7 MMC) contains a YM2413 derivative, used on the Family Computer game Lagrange Point, as well as on a few redemption arcade machines. It has 6 FM channels instead of 9, lacks the rhythm channels, and has a different set of built in FM patches from the YM2413. The VRC7 also has an undocumented debug mode that exposes YM2413-compatible control signals while also re-adding 3 extra FM channels (which were also used for the rhythm channels).