Texas Instruments SN76489
programmable sound generator chip
The Texas Instruments SN76489 is a programmable sound generator (PSG) chip released in 1979, used to create music and sound effects on computers and video game systems. Initially developed by Texas Instruments for its TI-99/4A home computer, it was later updated and widely adopted in systems like the BBC Micro, ColecoVision, IBM PCjr, Sega's Master System and Game Gear, and the Tandy 1000.
The SN76489 offered three tone generators for musical notes and a noise generator for sound effects like static and explosions, all with adjustable frequencies and volume levels. It competed with the General Instrument AY-3-8910; it lacked some of the 8910's more advanced features, like an envelope generator, but was inexpensive and easy to implement. TI had released the more powerful Texas Instruments SN76477 in 1978, but this was difficult to implement and did not see widespread use.
Overview
The SN76489 was originally designed to be used in the TI-99/4 computer, where it was first called the TMS9919 and later SN94624, and had a 500 kHz max clock input rate. A version was made for sales outside TI, the SN76489, which added a divide-by-8 to the clock input, allowing a clock input rate up to 4 MHz which allowed it to use the crystal for the NTSC color burst which many machines of the era already included. A version of the chip without the divide-by-8 input was also sold outside of TI as the SN76494, which had the original 500 kHz max clock input rate.
It contains:
3 square wave tone generators
A wide range of frequencies
16 different volume levels
1 noise generator
2 types (white noise and periodic)
3 different frequencies
16 different volume levels
Tone Generators
The frequency of the square waves produced by the tone generators on each channel is derived from two factors:
The speed of the external clock
A 10-bit value provided in a control register for that channel (called N)
Each channel's frequency is arrived at by dividing the external clock by 4 (or 32 depending on the chip variant), and then dividing the result by N. Thus the overall divider range is from 4 to 4096 (or 32 to 32768). At maximum clock input rate, this gives a frequency range of 122 Hz to 125 kHz.
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