mirror of
https://github.com/tildearrow/furnace.git
synced 2024-11-30 08:23:01 +00:00
36 lines
2.4 KiB
Markdown
36 lines
2.4 KiB
Markdown
# TI SN76489 (e.g. Sega Master System)
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
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... except for the SN76494, which can play notes as low as 13670 Hz (A -1). as a result, its pitch accuracy for higher notes is compromised.
|
|
|
|
## SN7 versions
|
|
|
|
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:
|
|
- SN94624, can only produce tones as low as 100Hz, and is clocked at 447 KHz.
|
|
- SN76494, which can play notes as low as 13.670 Hz (A -1). it has a different noise feedback and invert masks.
|
|
- SN76489, identical to SN94624, just without a clock divider.
|
|
- SN76489A, identical to 76494, just with a /8 clock divider.
|
|
- SN76496, literally identical to former. why is it even here?
|
|
- SN76496 with a Atari-like short noise. the chip of many legend and rumours which may be a result of inaccurate emulation.
|
|
- Sega Master System VDP version has a different, characteristic noise LFSR.
|
|
- Game Gear SN7, identical to the above, but with stereo.
|
|
- NCR8496, different noise invert mask.
|
|
- PSSJ3, literally identical to the former. it just swaps "high" and "low" signals in the output, which results in no audible difference.
|
|
|
|
## effects
|
|
|
|
- `20xy`: **set noise mode.**
|
|
- `x` controls whether to inherit frequency from channel 3.
|
|
- `0`: use one of 3 preset frequencies (C: A-2; C#: A-3; D: A-4).
|
|
- `1`: use frequency of channel 3.
|
|
- `y` controls whether to select noise or thin pulse.
|
|
- `0`: thin pulse.
|
|
- `1`: noise.
|
|
|
|
|
|
## info
|
|
|
|
this chip uses the [SN76489/Sega PSG](../4-instrument/psg.md) instrument editor.
|