furnace/doc/7-systems/ym2610b.md

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# Taito Arcade/Yamaha YM2610B
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YM2610B is basically YM2610 with 2 extra FM channels used at some 90s Taito arcade hardware.
it is backward compatible with the original chip.
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# effects
- **`10xy`**: set LFO parameters.
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- `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.
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- `x` is the operator (1-4).
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- `y` is the multiplier.
- **`18xx`**: toggle extended channel 3 mode.
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- 0 disables it and 1 enables it.
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- 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:
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- `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.
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- `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.
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- 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.
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- 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.