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46 lines
1.8 KiB
Markdown
46 lines
1.8 KiB
Markdown
# General Instrument AY-3-8910
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this chip was used in many home computers (ZX Spectrum, MSX, Amstrad CPC, Atari ST, etc.), video game consoles (Intellivision and Vectrex), arcade boards and even slot machines!
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it is a 3-channel square/noise/envelope sound generator. the chip's powerful sound comes from the envelope...
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the AY-3-8914 variant was used in Intellivision, which is pretty much an AY with 4 level envelope volume per channel and different register format.
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# effects
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- `20xx`: **set channel mode.**
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- `0`: square
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- `1`: noise
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- `2`: square and noise
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- `3`: envelope
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- `4`: envelope and square
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- `5`: envelope and noise
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- `6`: envelope and square and noise
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- `7`: nothing
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- `21xx`: **set noise frequency.** range is `0` to `1F`.
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- `22xy`: **set envelope mode.**
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- `x` sets the envelope shape:
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- `0`: `\___` decay
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- `4`: `/___` attack once
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- `8`: `\\\\` saw
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- `9`: `\___` decay
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- `A`: `\/\/` inverse obelisco
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- `B`: `\¯¯¯` decay once
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- `C`: `////` inverse saw
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- `D`: `/¯¯¯` attack
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- `E`: `/\/\` obelisco
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- `F`: `/___` attack once
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- if `y` is 1 then the envelope will affect this channel.
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- `23xx`: **set envelope period low byte.**
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- `24xx`: **set envelope period high byte.**
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- `25xx`: **slide envelope period up.**
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- `26xx`: **slide envelope period down.**
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- `29xy`: **enable auto-envelope mode.**
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- in this mode the envelope period is set to the channel's notes, multiplied by a fraction.
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- `x` is the numerator.
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- `y` is the denominator.
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- if `x` or `y` are 0 this will disable auto-envelope mode.
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- `2Exx`: **write to I/O port A.**
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- this changes the port's mode to "write". make sure you have connected something to it.
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- `2Fxx`: **write to I/O port B.**
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- this changes the port's mode to "write". make sure you have connected something to it.
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