The SN76489[1] Digital Complex Sound Generator (DCSG) is a TTL-compatible programmable sound generator chip from Texas Instruments.[2] Its main application was the generation of music and sound effects in game consoles, arcade video games, and home computers (TI-99/4A, BBC Micro, ColecoVision, IBM PCjr, Tomy Tutor, Master System, Game Gear, Tandy 1000), competing with the similar General Instrument AY-3-8910.
It contains:
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 clock input 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.
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). At maximum clock input rate, this gives a frequency range of 122 Hz to 125 kHz. Or typically 108 Hz to 111.6 kHz, with an NTSC colorburst (~3.58 MHz) clock input – a range from roughly A2 (two octaves below middle A) to 5–6 times the generally accepted limits of human audio perception.
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.
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.[3]
| Chip variant | Freq (max) | Audio in | |
|---|---|---|---|
| TMS9919 / SN94624 | 500 kHz | No | |
| SN76489 / SN76489A | 4 MHz | No | |
| SN76494 / SN76494A | 500 kHz | Yes | |
| SN76496 / SN76496A | 4 MHz | Yes |
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, a few small differences exist:
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.