Yamaha YM2612

The YM2612, a.k.a. OPN2, is a six-channel sound chip developed by Yamaha. It belongs to Yamaha's OPN family of FM synthesis chips used in several game and computer systems.

Developed as a stripped-down version of the YM2608, it lacks its larger sibling's ADPCM channel, Rhythm Sound System, SSG components, and GPIO ports. It also includes a simplified sound mixer with integrated DAC. It was also available in CMOS form, as the YM3438, a.k.a. OPN2C. It was most notably used in Sega's Mega Drive/Genesis video game console, as well as Fujitsu's FM Towns computer series. As with the YM3438, it was used by Sega in various arcade game systems, including the Mega-Play, System 18, and System 32.

The YM2612 has the following features:

The major difference between the YM2612 and the YM2608 is the removal of the original accumulator-equipped sound mixer, which mixed together the 14-bit floating point output of the FM channels, followed by serial output to a separate DAC chip. Instead, the YM2612 uses a simpler time-division sound multiplexer, which first truncates the 14-bit channel output to 9-bits, then loops through outputting each channel in sequence via a built-in DAC.

The DAC in the YM2612 introduces a glitch in the negative edge of waveforms, shifting that side out of place, causing a distinctive form of distortion that becomes more pronounced as tones decay. This has become known among fans as the "ladder effect". Post-YM2612 sound filtering circuitry varied in degree and quality between devices using the YM2612, affecting the sound quality even further.

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