of Microprocessors

766 MUSICAL ApPLICATIONS OF MICROPROCESSORS
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12 ,------T-ON-E--, 1A SELECT
FREQ ===> A
A
GEN.
AOUTPUT
FREQ
B
B OUTPUT
FREQ
C
COUTPUT
NOISE
FREQ 4
ENVE LOPE
GENERATOR
Fig. 20-4. AY-3-8910 Programmable sound generator
also trigger the envelope generator. By suitable selection ofcomponent values
and setting of the configuration bits, quite a number of sound effects such as
a chugging train, breathy whistle, "saucer lands," ray gun, gunshot, bird,
clock, etc., are possible. In fact a popular toy consisting of the chip, several
rorary switches, some pots, a few slide switches, a battery, and speaker was
designed and sold as "The Sound Studio."
Another common sound chip is the General Instruments AY-3-891O.
This by contrast is intended to be driven exclusively by a microprocessor and
has been used in countless add-on music synthesizer boards for personal
computers. Besides the sound generation circuit, its 40-pin package includes
two 8-bit general-application I/O ports, although a 28-pin version
(AY-3-8912) with one port and a 24-pin version (AY-3-8913) with none are
also available. Figure 20-4 shows the internals of this chip, which are
essentially three square-wave tone generators, a pseudorandom noise generator,
three mixers, and a single envelope generator driving three multiplying
DAC gain controls. Twelve-bit values control rhe frequency of the tone
generators, while a 5-bit value controls the noise sample rate. A full 16 bits
control the envelope generator rate, but the multiplying DACs themselves are
very coarse with only 16 logarithmically distributed amplitude levels. The
envelope generator can operate either in the normal single-shot mode or it
can constantly recycle. Aside from rhe fixed square-wave rimbre, it is possible
to get some decent sounding music from one of these chips, considering the