46 lines
3 KiB
Markdown
46 lines
3 KiB
Markdown
# TI SN76489 (e.g. Sega Master System)
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a relatively simple sound chip made by Texas Instruments. a derivative of it is used in Sega's Master System, the predecessor to Genesis. it has three square wave channels and one noise channel... not really.
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nominal mode of SN76489 has 3 square wave channels, with noise channel having only 3 preset frequencies to use (absurdly low, very low, low). to use more pitches, one can enable a mode which "steals" the frequency from square wave channel 3. by doing that, SN76489 becomes effectively a 3 channel sound chip. in addition, periodic noise mode can be enabled, with same caveats.
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the original iteration of the SN76489 used in the TI-99/4A computer, the SN94624, could only produce tones as low as 100Hz, and was clocked at 447 KHz. all later versions (such as the one in the Master System and Genesis) had a clock divider but ran on a faster clock, making a chip **very** high pitched... except for the SN76494, which can play notes as low as 13.670 Hz (A -1). as a result, its pitch accuracy for higher notes is compromised.
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## SN7 versions
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SN7 was extremely popular due to low cost. therefore, it was cloned and copied to no end, often with minor differences between each other. Furnace supports several of these:
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- SN94624, can only produce tones as low as 100Hz, and is clocked at 447 KHz.
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- SN76494, which can play notes as low as 13.670 Hz (A -1). it has a different noise feedback and invert masks.
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- SN76489, identical to SN94624, just without a clock divider.
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- SN76489A, identical to 76494, just with a /8 clock divider.
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- SN76496, literally identical to former. why is it even here?
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- SN76496 with a Atari-like short noise. the chip of many legend and rumours which may be a result of inaccurate emulation.
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- Sega Master System VDP version has a different, characteristic noise LFSR.
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- Game Gear SN7, identical to the above, but with stereo.
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- NCR8496, different noise invert mask.
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- PSSJ3, literally identical to the former. it just swaps "high" and "low" signals in the output, which results in no audible difference.
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## effects
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- `20xy`: **set noise mode.**
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- `x` controls whether to inherit frequency from channel 3.
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- `0`: use one of 3 preset frequencies (C: A-2; C#: A-3; D: A-4).
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- `1`: use frequency of channel 3.
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- `y` controls whether to select noise or thin pulse.
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- `0`: thin pulse.
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- `1`: noise.
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## info
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this chip uses the [SN76489/Sega PSG](../4-instrument/psg.md) instrument editor.
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## chip config
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the following options are available in the Chip Manager window:
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- **Clock rate**: sets the rate at which the chip will run.
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- **Chip type**: changes the chip type. see above for more details.
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- **Disable noise period change phase reset**: when enabled, the noise channel won't be reset every time its frequency changes. very useful.
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- **Disable easy period to note mapping on upper octaves**: Furnace maps the notes in the upper octaves to periods, for easier noise tuning. this option allows you to disable this feature.
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