furnace/doc/7-systems/ym2203.md

# Yamaha YM2203 (OPN)

a cost-reduced version of the YM2151 (OPM).
it only has 3 FM channels instead of 8 and removes stereo, the LFO and DT2 (coarse detune).

however it does contain an AY/SSG part which provides 3 channels of square wave with noise and envelope.

this chip was used in the NEC PC-88/PC-98 series of computers, the Fujitsu FM-7AV and in some arcade boards.

several variants of this chip were released as well, with more features.

# effects

- `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 from 1 to 4.
  - `y` is the multiplier.
- `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.**
- `20xx`: **set SSG channel mode.** `xx` may be one of the following:
  - `0`: square
  - `1`: noise
  - `2`: square and noise
  - `3`: nothing (apparently)
  - `4`: envelope and square
  - `5`: envelope and noise
  - `6`: envelope and square and noise
  - `7`: nothing
- `21xx`: **set noise frequency.** `xx` is a value between 00 and 1F.
- `22xy`: **set envelope mode.**
  - `x` sets the envelope shape, which may be one of the following:
    - `0`: `\___` decay
    - `4`: `/___` attack once
    - `8`: `\\\\` saw
    - `9`: `\___` decay
    - `A`: `\/\/` inverse obelisco
    - `B`: `\¯¯¯` decay once
    - `C`: `////` inverse saw
    - `D`: `/¯¯¯` attack
    - `E`: `/\/\` obelisco
    - `F`: `/___` attack once
  - if `y` is 1 then the envelope will affect this channel.
- `23xx`: **set envelope period low byte.**
- `24xx`: **set envelope period high byte.**
- `25xx`: **slide envelope period up.**
- `26xx`: **slide envelope period down.**
- `29xy`: **enable SSG auto-envelope mode.**
  - in this mode the envelope period is set to the channel's notes, multiplied by a fraction.
  - `x` is the numerator.
  - `y` is the denominator.
  - if `x` or `y` are 0 this will disable auto-envelope mode.
- `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`: +0
    - `1`: +1
    - `2`: +2
    - `3`: +3
    - `4`: -0
    - `5`: -3
    - `6`: -2
    - `7`: -1
- `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.**