Texas Instruments SN76489
The SN76489 Digital Complex Sound Generator (DCSG) is a TTL-compatible programmable sound generator chip from Texas Instruments. It contains: Its main application was the generation of music and sound effects in game consoles, arcade games and home computers (such as the Texas Instruments TI-99/4A, BBC Micro, ColecoVision, IBM PCjr, and Tandy 1000), competing with the similar General Instrument AY-3-8910. 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. 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 (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). This gives a frequency range at maximum input clock rate of 122 Hz to 125 kHz (or typically 108 Hz to 111.6 kHz, with an NTSC Colorburst clock input – a range from roughly A2 (two octaves below middle A) to 5–6 times the generally accepted limits of human audio perception). 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 (with the feedback bit set to 1) on writes to chip register 6, the noise mode register. Product Family. 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 1980–1982 and the latter from 1983 onward. They differ in that the output of the SN76489 is the inverse of the expected waveform (the waveform 'grows' towards 0 V from 2.5 V), 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 (on pin 9) for integrated audio mixing. Clones and successors. Sega used real SN76489AN chips in their SG-1000 game 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 systems' video display processor (VDP). 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(s) each audio channel are output (left, right, or both). 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 (notably the SL, TL and RL series) integrated the SN76496's functionality into the PSSJ ASIC. Texas