Yamaha YM2612

The YM2612, a.k.a. OPN2, is a sound chip developed by Yamaha. It is a member of Yamaha's OPN family of FM synthesis chips, and was developed as a stripped-down version of the YM2608.

The YM2612 is a six-channel FM synthesizer used in several game and computer systems, most notably in Sega's Mega Drive/Genesis video game console as well as Fujitsu's FM Towns computer series. It was also available in CMOS form as the YM3438, a.k.a. OPN2C.

As with the YM3438, it was used by Sega in various arcade game systems, including the Mega-Play, Sega System C and Sega System 32; the YM3438 core was also integrated into an ASIC in later revisions of the Mega Drive/Genesis.

Features

The YM2612 has the following features:
*Six concurrent FM synthesis channels (voices)
*Four operators per channel
*Two interval timers
*A sine-wave low frequency oscillator (LFO)
*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) This feature is common to the OPN family of chips.

Technical details

The YM2612 is based on the FM block featured in the YM2608 (which itself was based on the YM2203), integrating a stereo output DAC and LFO, but removes the SSG component (although the vestigial SSG envelope generator remains on the chip's core) and GPIO ports found on all other OPN chips such as the YM2203 and YM2608 (which are elements of the YM2149F that is integrated in most OPN series chips) as well as the seven ADPCM channels (one ADPCM, six Rhythm) found on its larger sibling. The YM2612 is also referred to as being based on the YM2203 according to official documentations.

While high-end chips in the OPN series have dedicated ADPCM channels for playing sampled audio (i.e. YM2608 and YM2610), 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 other OPN chips with ADPCM support, 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.

Low volume distortion of the built-in DAC

Unlike most Yamaha FM chips which require an external floating-point DAC, the YM2612 features a built-in 9-bit DAC, which uses time-division multiplexing to play one sample of each channel in sequence, similar to the YM2413.

Due to an error with the amplitude voltage in the original chip design, a peculiar form of crossover distortion known as ''low volume distortion'' (and historically given the terms "TDM distortion" and "ladder effect") is introduced in the output. In this case, any output that is sufficiently low in volume undergoes an exaggerated quantization, including any usage of fades in video game music.

Some early games in the Mega Drive/Genesis' lifespan such as After Burner II had their music written around this distortion to generate unique timbres or effects. As a consequence, games that used this musical trick would sound too quiet on later Mega Drive/Genesis consoles using the integrated YM3438 core on the ASIC. Also, because of the reduced dynamic range of the built-in DAC, additional distortion may be generated when playing sounds with a very high volume.

Variants

Yamaha YM3438

The YM3438, a.k.a. OPN2C, is a modified CMOS version of the YM2612, commonly used in Sega's arcade system boards as well as being used in Fujitsu's FM Towns Marty. This version has an improved built-in DAC that significantly reduces the presence of crossover distortion from the original chip with a higher signal-to-noise ratio than 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-powered version of the YM3438, used in later models of Fujitsu's FM Towns series (particularly the FM Towns II models). It came in a smaller 24-pin SOP package. Unlike the YM2612 and YM3438, the YMF276 requires an external DAC chip, similar to most other Yamaha FM chips. This is usually a YM3433B external DAC chip (as used in the FM Towns II models) or other equivalent DAC chips. As such, it does not produce significant low volume distortion as with the YM2612/YM3438 chips due to its usage of an external DAC chip.

Yamaha-created ASICs

The YM3438 core was also integrated in custom ASICs made by Yamaha for use with Sega's Mega Drive/Genesis, which were used in the final revision of the original model and most revisions of the revised models (FC1004, FF1004 and FJ3002) as well as the Genesis 3 released by Majesco (FJ3002 and FQ8007). The ASICs contained both the video and sound chips (and later the main CPU and sound CPU on the FJ3002 and FQ8007 ASICs) combined into one chip, which also includes the YM7101 VDP that houses the integrated Texas Instruments SN76489 PSG. The YM3438 core used in these revisions have been modified to further reduce the presence of the low volume distortion bug more so than with standalone YM3438 chips.

Emulation

With the advent of emulation, the YM2612 and its variants have been faithfully replicated in software through years of reverse engineering the chips, with various improvements being made to the emulation accuracy of the sound chips over time. Such changes included the implementation of the low volume distortion bug in the YM2612, as well as certain features such as the SSG envelope generator and undocumented registers. In recent years, cycle-accurate emulation of the YM2612 and its variants have also been done as well, using die shot reverse engineering of the chips.

Many video game console emulators (especially those for the Mega Drive/Genesis) and even arcade game emulators such as MAME implemented various emulated versions of the YM2612, with most modern Mega Drive/Genesis emulators having the option to replicate the sound outputs of the original and revised models of the Mega Drive/Genesis using the emulated YM2612 or YM3438 cores (which can also be used in conjunction with a low-pass filter to further simulate an original model Mega Drive/Genesis) or by implementing a function in the emulated chip to apply a low volume distortion effect to simulate a discrete YM2612.

Game audio

Used in conjunction with the Texas Instruments SN76489 PSG as the main sound generators of the Sega Mega Drive/Genesis console, the YM2612 was prominently utilized by numerous prolific video game music composers, most notably Yuzo Koshiro.

See also

* VGM – an audio file format for multiple video game platforms, including the Sega Mega Drive/Genesis
*Yamaha YM2203
*Yamaha YM2608
*Yamaha YM2610

References

External links

Nuked-OPN2 YM3438/YM2612 emulator

{{Yamaha soundchips

YM2612
Video game music technology