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 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 using a built-in DAC. The truncation of the output samples is done in such a way that it introduces a glitch in the negative edge of waveforms, shifting that side out of place, causing a distinctive form of distortion that has become known among fans as the "ladder effect". Post-YM2612 sound filtering circuitry varied in quality between devices using the YM2612, affecting the sound quality even further. The "ladder effect" was not always an undesirable flaw, however, as some games and musicians (notably Yuzo Koshiro with the intro theme for Streets of Rage 2) took advantage of this distortion by playing decaying sounds at low volumes to simulate a chorus effect.

Along with the mixer changes, the chip was stripped of its predecessor's SSG component, although its vestigial SSG envelope generator is still functional.

Whereas the high-end OPN chips have dedicated ADPCM channels for playing sampled audio, the YM2612 does not. However, its 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. This replaces (disables) FM output for that channel. PCM data is written to the channel via an 8-bit register. 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.

The YM3438, aka OPN2C, the CMOS form of the YM2612, changed the Channel 6 DAC output to the same 9-bit output in FM mode. The chip also had higher output impedance, requiring heavier external noise filtering circuitry but outputting louder sound. Sega used an improved version of the OPN2C for the Model 2 version of the Mega Drive/Genesis, which does not perform the bit-depth truncating seen in the original OPN2 and OPN2C as described above, and as such does not suffer from the "ladder effect".

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.

Example circuit

An example schematic for the YM2612 can be found here.

See also

References