# 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.

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 13.670 Hz (A -1). consequently, its pitch accuracy for higher notes is compromised.

# 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.

# chip flags
## 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, might 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 masks
- PSSJ3, literally identical to the former, it justs swaps "high" and "low" signals in the output, which results in no audible difference

TODO: all these checkboxes