furnace/doc/7-systems/ym2151.md

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# Yamaha YM2151
the sound chip powering several arcade boards, the Sharp X1/X68000 and the Commander X16. eight 4-op FM channels, with overpowered LFO and almost unused noise generator.
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it also was present on several pinball machines and synthesizers of the era, and later surpassed by the YM2414 (OPZ) present in the world-famous TX81Z.
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in most arcade boards the chip was used in combination with a PCM chip, like [SegaPCM](segapcm.md) or [OKI's line of ADPCM chips](oki.md).
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# effects
- `10xx`: **set noise frequency of channel 8 operator 4.** `00` disables noise while `01` to `20` enables it.
- `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.
- `17xx`: **set LFO speed.**
- `18xx`: **set LFO waveform.**
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- `00`: saw
- `01`: square
- `02`: triangle
- `03`: noise
- `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.**
- `1Exx`: **set LFO AM depth.**
- `1Fxx`: **set LFO PM depth.**
- `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 DT2 of operator.**
- `x` is the operator (1-4). a value of 0 means "all operators".
- `y` is the value.
- `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.**