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.