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.**
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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.** range is `00` to `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
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- 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.**