# Yamaha YM2612 (OPN2)

one of two chips that powered the Sega Genesis. it is a six-channel, four-operator FM synthesizer. channel 6 can be turned into 8-bit PCM player, that via software mixing, thanks to Z80 sound CPU, can play more than one channel of straight-shot samples at once. 
Furnace also offers DualPCM, a Z80 driver that splits channel 6 into two individual PCM channels with variable pitch. using the console's Z80 processor, these are mixed together in software and streamed to channel 6 in PCM mode with a mix rate of 13750 Hz. VGM export requires the "direct stream mode" option to be enabled, and resulting files will be very large.

## effects

- `10xy`: **set LFO parameters.**
  - `x` toggles the LFO.
  - `y` sets its speed.
- `11xx`: **set feedback of channel.**
- `12xx`: **set operator 1 level.**
- `13xx`: **set operator 2 level.**
- `14xx`: **set operator 3 level.**
- `15xx`: **set operator 4 level.**
- `16xy`: **set multiplier of operator.**
  - `x` is the operator (1-4).
  - `y` is the new MULT value.
- `17xx`: **toggle LEGACY sample mode.**
  - this only works on channel 6.
  - **this effect exists only for compatibility reasons! its use is NOT recommended. use Sample type instruments instead.**
- `18xx`: **toggle extended channel 3 mode.**
  - 0 disables it and 1 enables it.
  - only in extended channel 3 chip.
- `19xx`: **set attack of all operators.**
- `1Axx`: **set attack of operator 1.**
- `1Bxx`: **set attack of operator 2.**
- `1Cxx`: **set attack of operator 3.**
- `1Dxx`: **set attack of operator 4.**
- `30xx`: **enable envelope hard reset.**
  - this works by inserting a quick release and tiny delay before a new note.
- `50xy`: **set AM of operator.**
  - `x` is the operator (1-4). a value of 0 means "all operators".
  - `y` determines whether AM is on.
- `51xy`: **set SL of operator.**
  - `x` is the operator (1-4). a value of 0 means "all operators".
  - `y` is the value.
- `52xy`: **set RR of operator.**
  - `x` is the operator (1-4). a value of 0 means "all operators".
  - `y` is the value.
- `53xy`: **set DT of operator.**
  - `x` is the operator (1-4). a value of 0 means "all operators".
  - `y` is the value:
    - `0`: -3
    - `1`: -2
    - `2`: -1
    - `3`: 0
    - `4`: 1
    - `5`: 2
    - `6`: 3
    - `7`: -0
- `54xy`: **set RS of operator.**
  - `x` is the operator (1-4). a value of 0 means "all operators".
  - `y` is the value.
- `55xy`: **set SSG-EG of operator.**
  - `x` is the operator (1-4). a value of 0 means "all operators".
  - `y` is the value (0-8).
    - values between 0 and 7 set SSG-EG.
    - value 8 disables it.
- `56xx`: **set DR of all operators.**
- `57xx`: **set DR of operator 1.**
- `58xx`: **set DR of operator 2.**
- `59xx`: **set DR of operator 3.**
- `5Axx`: **set DR of operator 4.**
- `5Bxx`: **set D2R/SR of all operators.**
- `5Cxx`: **set D2R/SR of operator 1.**
- `5Dxx`: **set D2R/SR of operator 2.**
- `5Exx`: **set D2R/SR of operator 3.**
- `5Fxx`: **set D2R/SR of operator 4.**
- `60xy`: **set operator mask.**
  - enables or disables operators.
  - if `x` is `0`, `y` ranges from `0` to `F`. it is a bitfield, so `y` is the sum of the active operators' bits:
    - OP1 is +1, OP2 is +2, OP3 is +4, and OP4 is +8.
    - for example, having only OP2 and OP4 on would be 2 + 8 = 10, resulting in an `xy` value of `0A`.
  - if `x` is `1` to `4`, the effect targets that operator; `y` turns it off with a value of `0` and on with a value of `1`.
    - for example, the effect `6031` enables OP3.
- `61xx`: **set algorithm** (0 to 7).
- `62xx`: **set LFO FM depth** (0 to 7).
- `63xx`: **set LFO AM depth** (0 to 3).

## info

this chip uses the [FM (OPN)](../4-instrument/fm-opn.md) and [Generic Sample](../4-instrument/sample.md) instrument editors.

the maximum PCM playback rate is 26633 Hz.

## extended channel 3

in ExtCh mode, channel 3 is split into one column for each of its four operators. feedback and LFO levels are shared. the frequency of each operator may be controlled independently with notes and effects. this can be used for more polyphony or more complex sounds.

all four operators are still combined according to the algorithm in use. for example, algorithm 7 acts as four independent sine waves. algorithm 4 acts as two independent 2op sounds. even with algorithm 0, placing a note in any operator triggers that operator alone.

## CSM

CSM, or "Composite Sine Mode", involves a timer matching the frequency of the note in the "CSM Timer" channel. each time it triggers, it generates key-on and key-off commands to reset the phase of all operators on channel 3 and force their envelopes to restart at the release point. this can be used to create vocal formants (speech synthesis!) or other complex effects. outside this chip's specific implementation, the technique is known as "oscillator sync".

working with CSM is beyond the scope of this documentation. for more information, see this [brief SSG-EG and CSM video tutorial](https://www.youtube.com/watch?v=IKOR0TUlnWU).

## DualPCM

thanks to the Z80 sound CPU, DualPCM can play two samples at once! this mode splits channel 6 into two individual PCM channels with variable pitch. these are mixed together in software and streamed to channel 6 with a mix rate of 13750 Hz. VGM export requires the "direct stream mode" option to be enabled, and resulting files will be very large.

## SSG-EG

SSG-EG is short for "Software-controlled Sound Generator – Envelope Generator". it is the AY-3-8910/YM2149 envelope generator applied to each individual operator. it makes the operator's envelope play through attack, decay, sustain, and decay 2 until it reaches zero amplitude, at which time SSG triggers. according to the shape of the SSG envelope, the operator's envelope may then either loop or hold, and either of these can be set to invert the envelope (attack decreases and decay increases) when triggered.

a full guide to SSG-EG is beyond the scope of this documentation. for more information, see this [brief SSG-EG and CSM video tutorial](https://www.youtube.com/watch?v=IKOR0TUlnWU), this [detailed technical explanation](https://gendev.spritesmind.net/forum/viewtopic.php?t=386&start=106), and this [chart of tunings](https://docs.google.com/spreadsheets/d/1HGKQ08CnLGAjA1U0StJFldod3FkQ3uq86rYy1VBIuZc/).

## chip config

the following options are available in the Chip Manager window:

- **Clock rate**: sets the rate at which the chip will run.
- **Chip type**: sets the chip type.
  - YM3438: found in Model 2 and 3 Genesis. integrated version which fixes DAC distortion present in YM2612.
  - YM2612: found in Model 1 Genesis. has DAC distortion.
  - YMF276: found in FM Towns. uses external DAC and has higher output resolution.
- **DAC interrupt simulation**: simulates the short interrupts that occur on each frame with quirky drivers (e.g. as found in Sonic 2 and 3).
  - not supported in VGM export!
