furnace/doc/7-systems/ym2610b.md

# Taito Arcade/Yamaha YM2610B

YM2610B is basically YM2610 with 2 extra FM channels used at some 90s Taito arcade hardware.
it is backward compatible with the original chip.

# 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 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:
  - `00`: square
  - `01`: noise
  - `02`: square and noise
  - `03`: nothing (apparently)
  - `04`: envelope and square
  - `05`: envelope and noise
  - `06`: envelope and square and noise
  - `07`: 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.