The SN76489 Digital Complex Sound Generator (DCSG) is a TTL-compatible Programmable Sound Generator chip from Texas Instruments. It contains three square wave tone generators and one white noise generator, each of which can produce sounds at various frequencies and sixteen different volume levels.[1] 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), existing alongside the competing and similar General Instrument AY-3-8910.
The SN76489 was originally designed to be used in the TI-99/4 computer, where it was first called the TMS9919 and later SN94624. Later, when it was sold outside of TI, it was renamed the SN76489.
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.[1]
There are two versions of the SN76489: the SN76489 (Narrow DIP version labeled SN76489N) and the SN76489A (Narrow DIP version labeled SN76489AN). The former was made around 1980-1983 and the latter from 1984 onward. They differ in that the linear feedback shift register used to generate periodic and pseudorandom noise in the non-A version is 15-stage, whereas in the A version it is 16-stage. This makes a significant difference for music/programs which use periodic noise, as sounds will play at 6.25% lower pitch on the SN76489A. The pseudorandom noise feedback in the A version is generated from an XNOR of bits 12 and 13, with bit 14 being the noise output, while the non-A version uses an XOR of bits 12 and 13 for feedback, and bit 13 being the noise output. This affects the first 16 or so samples only after the LFSR is reset, which clears all the bits and sets the carry in bit on all chip versions.
Sega used real SN76489 chips in their SG-series computers, but used SN76489A clones in their Master System, Game Gear, and Mega Drive/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 extension for stereo audio output. The periodic noise is also 16 stages long, as usual for an SN76489A. [2]
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.[citation needed]
It is worth noting that the SN76489A seems to be totally identical to the SN76494 and SN76496 in terms of the outputs produced, but they additionally feature an "AUDIO IN" pin (on pin 9) for integrated audio mixing.[3]
References
- ^ a b Engineering staff of Texas Instruments Semiconductor Group. "SN 76489 AN" (PDF). ftp://ftp.whtech.com/datasheets%20&%20manuals/Datasheets%20-%20TI/SN76489.pdf.
- ^ Maxim (April 27, 2005). "SN76489 notes". SMS Power!. http://www.smspower.org/maxim/docs/SN76489.txt.
- ^ "SN76494, SN76494A, SN76496, SN76496A programmable tone/noise generator" (PDF). Texas Instruments. 1984, 1989. http://www.datasheetarchive.com/preview/3178523.html.
External links
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