Yamaha YM2612
The YM2612, a.k.a. OPN2, is a sound chip developed by Yamaha. It belongs to Yamaha's OPN family of FM synthesis chips used in several game and computer systems.
Overview
The Yamaha YM2612 is a six-channel FM synthesizer, derived from the Yamaha YM2203c.[1] It was most notably used in Sega's Mega Drive/Genesis video game console,[2] as well as Fujitsu's FM Towns computer series.[3] As with the YM3438, it was used by Sega in various arcade game systems, including the Mega-Play, Sega System 32, and Sega System 32.
Features
The YM2612 has the following features:[1]
- Six FM channels
- Four operators per channel[4]
- 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)
Technical details
The YM2612's FM synthesis block is an extended version of the FM block featured in the YM2203C, adding two channels and integrating a stereo output DAC.[1] The YM2612 removes the SSG component and I/O port of the YM2203C[1]. It was also available in CMOS form, as the YM3438, a.k.a. OPN2C.[1]
Whereas the high-end OPN chips such as the YM2608B 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.[1] 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.[2]
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.
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.[1]
Variants
Yamaha YM3438
The YM3438,[1] a.k.a. OPN2C, is a modified CMOS version of the YM2612. It is not a direct, drop-in replacement for the YM2612 however, as the sound outputs have higher impedance.
Yamaha YMF276
The YMF276, a.k.a. OPN2L, is a low-power version of the YM3438. Unlike YM2612 and YM3438, the YMF276 requires external DAC.
Game audio
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.[6][7]
See also
- VGM – an audio file format for multiple video game platforms, including the Sega Mega Drive/Genesis
References
- YM3438 Application Manual (Japanese)
- 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.
- https://www.theguardian.com/games/2020/feb/13/super-sonic-creating-the-new-sound-of-sega-hedgehog-hit-sonic-the-hedgehog-masato-nakamura-tom-holkenborg
- https://console5.com/techwiki/images/5/57/Sega_Service_Manual_-_Genesis_II_-_Mega_Drive_II_%28PAL-B-I-G%2C_RGB%29%2C_No_001%2C_June%2C_1993.pdf
- https://www.theguardian.com/games/2020/feb/13/super-sonic-creating-the-new-sound-of-sega-hedgehog-hit-sonic-the-hedgehog-masato-nakamura-tom-holkenborg
- https://www.theguardian.com/games/2019/sep/19/sega-mega-drive-mini-review-a-legacy-truly-honoured