View source for Yamaha YM2612

[[File:Yamaha YM2612 chip.jpg|thumb|right|Yamaha YM2612]]
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The '''YM2612''', aka '''OPN2''', is a six-channel [[sound chip]] developed by [[Yamaha Corporation|Yamaha]]. It belongs to Yamaha's OPN family of [[frequency modulation synthesis|FM synthesis]] chips used in several game and computer systems.

Developed as a stripped-down version of the [[Yamaha YM2608|YM2608]], it lacks its larger sibling's ADPCM channel, Rhythm Sound System, [[General Instrument AY-3-8910|SSG]] components, and [[General Purpose Input/Output|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 Mega Drive/Sega Genesis]] game console and [[Fujitsu|Fujitsu's]] [[FM Towns]] computer series.  As the YM3438, it was used by Sega in various [[arcade game]] systems, including the [[List of Sega arcade system boards#Sega Mega-Play|Mega-Play]], [[List of Sega arcade system boards#Sega System 18|System 18]], and [[List of Sega arcade system boards#Sega System 32|System 32]].

The YM2612 has the following features:
*Six [[monophonic]] FM channels (voices)
*Four operators per channel
*Two [[Programmable Interval Timer|interval timer]]s
*A sine-wave [[Low frequency oscillation|low frequency oscillator]]
*Analogue stereo output (most other contemporary Yamaha FM chips require a separate external [[digital-to-analog converter]] chip)
*For channels 3 and 6, 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.)
*Per-channel programmable stereo sound (left, right, or both left and right resulting in centre)
*Patch compatibility with Yamaha DX/TX synthesizers, for example [[Yamaha TX81Z|TX81Z]], and others, once the user accounts for the differing parameter ranges displayed by the synths as a convenience to the user. This of course does not account for differences such as these synths not offering a sine wave for the LFO, differing clock rates and hence envelope speeds, and some particular sound signatures of the YM2612 as described next.

The major difference between the YM2612 and the YM2608 is the removal of the original [[Accumulator (computing)|accumulator-equipped]] sound mixer, which mixed together the 14-bit floating point output of the FM channels, followed by serial output to a separate [[Digital-to-audio converter|DAC]] chip. Instead, the YM2612 uses a simpler [[Time-division multiplexing|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.{{citation needed|date=October 2014}}

Along with the mixer changes, the chip was stripped of its predecessor's SSG component, although its vestigial SSG [[Synthesizer#ADSR envelope|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 [[Pulse-code modulation|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{{Clarify|date=October 2014}}. 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 [[Sega Mega Drive|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".{{citation needed|date=October 2014}}

==Pinout==
{{col-begin|width=auto}}
{{col-break}}
{| class="wikitable"
|-
! #
! 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&nbsp;MHz recommended)
|}
{{col-break|gap=1em}}
[[File:Yamaha YM2612 pinout.svg|300px|center]]
{| class="wikitable"
|-
!/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
|&times;
|&times;
|&times;
|&times;
|D0 through D7 become high-impedance.
|}
{{col-end}}

== Example circuit ==

An example schematic for the YM2612 can be found [http://sue.niko.to/ps98/ym2612_sch.png here].

== See also ==
* [[Yamaha YM2608]]
* [[Yamaha YM2203]]
* [[Sound chip]]
* [[VGM (file format)|VGM]]
* [[GYM]]

== References ==
* [http://www.chipdir.nl/pinusr/ym2612.txt Chipdir]
* [http://www.smspower.org/maxim/Documents/YM2612 The infamous SEGA2.DOC]
* [http://gendev.spritesmind.net/forum/viewtopic.php?t=386 spritesmind.net – New Documentation: An authoritative reference on the YM2612]
* [http://www7.plala.or.jp/kikekike/fm/YM3438_APL.pdf YM3438 Application Manual (Japanese)]

{{Yamaha soundchips}}

[[Category:Yamaha sound chips|YM2612]]
[[Category:Video game music]]