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The Yamaha YM2612 is a six-channel FM synthesizer, based on the Yamaha YM2203C. 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:
- Six FM voices
- Four operators per channel
- Two interval timers
- A sine-wave low frequency oscillator
- Integrated stereo output digital-to-analog converter (most other contemporary Yamaha FM chips require a separate external D/A converter chip)
- Per-channel programmable stereo sound (left, right, or both left and right resulting in centre)
- For channel three, operator frequencies can be set independently, making dissonant harmonics possible. (Normally, they would have a simple relation like e.g. 2× or 3× relative to a common base frequency)
The YM2612 is an extended and integrated derivative of the FM synthesis block from the YM2203C, removing the SSG component (though retaining its envelope generator) and I/O port, and integrating a stereo output DAC. It was also available in CMOS form, as the YM3438, a.k.a. OPN2C.
Whereas the high-end OPN chips such as the YM2608 have dedicated ADPCM channels for playing sampled audio, the YM2612 does not. However, its sixth channel can act as a basic PCM channel by means of the 'DAC Enable' register, disabling FM output for that channel but allowing it to play 8-bit pulse-code modulation sound samples. Unlike the other OPNs with ADPCM, the YM2612 does not provide any timing or buffering of samples, so all frequency control and buffering must be done in software by the host processor.
Output DAC peculiarities
Unlike most Yamaha FM chips which require an external floating-point DAC, the YM2612 features a built-in 9-bit DAC, which utilizes time-division multiplexing to play one sample of each channel in sequence, similar to the YM2413. This method introduces a peculiar form of crossover distortion in the output, which has become known as the "ladder effect" among fans.
Additionally, because of the reduced dynamic range of the built-in DAC, additional distortion may be generated when playing sounds with a very high volume.
The YMF276, a.k.a. OPN2L, is a low-power version of the YM3438. Unlike YM2612 and YM3438, the YMF276 requires external DAC.
Yamaha Fx1004 and FJ3002
As the main sound generator of the Sega Mega Drive/Genesis console, the YM2612 was prominently utilized by numerous prolific video game music composers, such as Yuzo Koshiro.
- Sega Genesis Technical Manual - YM2612 section (updated with errors corrected)
- Games vs. Hardware. The History of PC video games: The 80's. p. 527. Retrieved 14 December 2017.
- YM3438 Application Manual (Japanese)