Texas Instruments SN76489
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The SN76489 Programmable Sound Generator (PSG) is a TTL compatible four-channel sound chip from Texas Instruments. Its main application has been the generation of music and sound effects in game consoles, arcade games and home computers (such as the BBC Micro and the IBM PCjr).
Features of the SN76489 are
- Three square wave, programmable tone generators;
- One programmable white noise generator;
- Programmable attenuation;
- Mono audio output.
The frequency of the square waves produced at each channel is derived by two factors: the speed of the external clock and a value provided in a control register for that channel (called N). Each channel's frequency is arrived at by dividing the clock by 32 and then dividing the result by N.
An SN76489 clone is included 'on-chip' in SEGA's Video Display Processors (VDP) for their Master System, Game Gear, and Mega Drive (Sega Genesis in North America), game consoles. The basic functions are the same, but the randomness for the noise channel is generated differently, the "periodic noise" output is a 1/16 duty cycle (as opposed to 1/15 on the original) and the implementation in the Game Gear allows for stereo audio output.
Another clone is the NCR 7496, used in the Tandy 1000 computer. It again has a different white noise pattern but is otherwise functionally identical to the SN76489.
It is worth noting that the SN76489 seems to be totally identical to the SN76494 and SN76496 in terms of the outputs produced, but they additionally feature an "AUDIO IN" pin for integrated audio mixing.
