furnace/doc/7-systems/sms.md

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