of Microprocessors

LOW-COST SYNTHESIS TECHNIQUES
773
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AUDIO
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Fig. 20-7. Atari-800 sound-generator block diagram
ultimate that has been achieved with I-bit D-to-A conversion. Since the
Atari computers were primarily game machines, the ability to produce sound
effects figured heavily in the sound-generator's design.
Basic pitched sounds are generated by four independent sound channels,
each with an 8-bit divide-by-N counter, a noise modulator, and a 16­
level volume control. To increase the range of accurate frequency generation
from 8-bit dividers, a basis frequency of 16 or 64 kHz can be selected.
Additionally, the channels can be combined pairwise to produce one 16-bit
divider and two 8-bit ones or two 16-bit dividers. A useful feature is that a de
level can be selected rather than the frequency generator output, which then
makes the volume control act like a 4-bit D-to-A converter.
For sound-effects use, the noise modulator serves to distort the pure
square-wave tone in a number of different ways. Pulses from a channel's
frequency divider are routed through an AND gate prior to toggling the
output flip-flop. The other input to the gate can be a logic ONE (for pure
tones), or various combinations of 4-bit, 5-bit, and 17-bit shift-register
random-bit generators that are constantly clocked at 1. 79 MHz. The effect is
to delete some of the pulses that would otherwise reach the flip-flop and thus
distort the sound. With the 17-bit generator, which can also be truncated to
9 bits, the resultant spectrum resembles bandpass-filtered noise with a center