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. Later, when it was sold outside of TI, it was renamed the SN76489, and a divide-by-8 was added to its clock input, increasing the max input clock rate to, to facilitate sharing a crystal for both NTSC colorburst and clocking the sound chip. A version of the chip without the divide-by-8 input was also sold outside of TI as the SN76494, which has a max clock input rate. Tone Generators: The frequency of the square waves produced by the tone generators on each channel is derived from two factors:
Each channel's frequency is arrived at by dividing the external clock by 4, and then dividing the result by N. Thus the overall divider range is from 4 to 4096. This gives a frequency range at maximum input clock rate of 122 Hz to 125 kHz. Noise Generator: The pseudorandom noise feedback is generated from an XNOR of bits 12 and 13 for feedback, with bit 13 being the noise output. The pseudorandom generator is cleared to 0s on writes to chip register 6, the noise mode register.
Product Family
There are two versions of the SN76489: the SN76489 and the SN76489A. The former was made around 1980–1982 and the latter from 1983 onward. They differ in that the output of the SN76489 is the inverse of the expected waveform, while the SN76489A the waveform is not inverted. The SN76496 seems to be totally identical to the SN76489A in terms of the outputs produced, but features an "AUDIO IN" pin for integrated audio mixing.
Chip Variant
Freq
Audio In
TMS9919 / SN94624
500 kHz
No
SN76489 / SN76489A
4 MHz
No
SN76494 / SN76494A
500 kHz
Yes
SN76496 / SN76496A
4 MHz
Yes
Clones and successors
used real SN76489AN chips in their SG-1000game console and SC-3000 computer, but used SN76489A clones in their Master System, Game Gear, and Sega Genesis game consoles. These modified sound chips were incorporated into the system's video display processor. Although basic functionality is almost identical to that of the original SN76489A sound processor, a few small differences existed: the randomness for the noise channel is generated differently, and the Game Gear's version includes an additional flag register that designates which speaker each audio channel are output. The periodic noise is also 16 stages long on the Sega-made clones rather than 15; this makes a significant difference for music/programs which use periodic noise, as sounds will play at 6.25% lower pitch than on the TI-made chips. Another clone is the NCR 8496, used in some models of the Tandy 1000 computer. Later Tandy 1000 machines integrated the SN76496's functionality into the PSSJ ASIC.
Usage
Arcade games
These games shared a common board design by Tehkan that used three of the functionally identical SN76496.
