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. A version was made for sales outside TI, the SN76489, which added a divide-by-8 to the clock input, allowing a clock input rate up to which allowed it to use the crystal for the
NTSC color burst which many machines of the era already included. A version of the chip without the divide-by-8 input was also sold outside of TI as the SN76494, which had the original max clock input rate. It contains: • 3
square wave tone generators • A wide range of frequencies • 16 different volume levels • 1 noise generator • 2 types (
white noise and periodic) • 3 different frequencies • 16 different volume levels
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 10-bit 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). 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 color burst (~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.
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==