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 4 MHz, 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 500 kHz 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:
- The speed of the external clock.
- A 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). 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 (A 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.