Yamaha YM2612

Yamaha YM2612

The YM2612, aka 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, aka OPN2C. It was most notably used in the Sega Mega Drive/Sega Genesis game console and Fujitsu's FM Towns computer series. As the YM3438, it was by Sega in various arcade game systems, including the Mega-Play, System 18, and System 32.

The YM2612 has the following features:

• Six concurrent FM channels (voices)
• Four operators per channel
• Two interval timers
• A sine-wave low frequency oscillator
• Analogue stereo output (most other contemporary Yamaha FM chips require a separate external digital-to-analog converter chip)
• For channel three, operator frequencies can be set independently, making dissonant harmonics possible. (Normally, they would have a simple relation like e.g. 2x or 3x relative to a common base frequency.)

Each channel may be assigned to either left, right or both (centre) of the stereo output.

The sixth channel can act as a surrogate PCM channel by means of the 'DAC Enable' register, allowing the chip to play 8-bit PCM sound samples. Enabling the register disables FM output for that channel. PCM data is written to the channel via an 8-bit register. The YM2612 does not provide any timing or buffering of the PCM samples, so all frequency control and buffering must be done in software by the host processor.

Instead of the accumulator-equipped sound mixer used in the YM2608, it uses a simpler time-division sound multiplexer, which switches between each channel very rapidly. An external sound filter was used to reduce the effect of the simpler sound mixing, which also had a negative effect on sound quality.

Although the chip was stripped of its predecessor's SSG component, its vestigial SSG ADSR envelope generator is still functional.

Synth patches used by the YM2612 in Mega Drive/Genesis games are compatible with certain members of the Yamaha DX/TX synthesizers, such as the Yamaha DX100.

Pinout

# Pin Function 1/12 GND Ground Terminals 2-9 D0 - D7 8-bit Bidirectional data bus 10 NC Not connected 11 /IC System reset, initialize registers, active low 13 /IRQ Maskable Interrupt Request, active low 14 /CS Chip Select, active low 15 /WR Write mode on data bus, active low 16 /RD Read mode on data bus, active low 17 A0 Read/Write Enable, active high 18 A1 Part 1 / Part 2 select (low=1, high=2) 19 A.GND Analog Ground Terminal for internal D/A converter 20-21 MOL, MOR Analog sound outputs, left and right 22 A.Vcc Analog Vcc Terminal for internal D/A converter 23 Vcc +5V Power Terminal 24 øM Master Clock (7.67 MHz recommended)

/CS /RD /WR A0 A1 State of Data Bus 0 1 0 0 0 Selects Part 1 Register 0 1 0 1 0 Writes data to selected Part 1 Register. 0 0 1 1 0 Reads the status of Part 1. 0 1 0 0 1 Selects Part 2 register 0 1 0 1 1 Writes data to selected Part 2 register. 0 0 1 1 1 Reads the status of Part 2. 1 × × × × D0 through D7 become high-impedance.

See also

• Yamaha YM2608
• Yamaha YM2203
• Sound chip
• VGM
• GYM

References

• Chipdir
• The infamous SEGA2.DOC
• spritesmind.net - New Documentation: An authoritative reference on the YM2612
• YM3438 info on x68000.de
• YM3438 Application Manual (Japanese)