of Microprocessors

774 MUSICAL ApPLICATIONS OF MICROPROCESSORS
FREQ I
MULTIPLYING
DAC I
:...o-j-------J SIG
STATE­
VARIABLE i=J""'-n~'"
FILTER
FREQ
Q
11 11 11 4
oOFF
SIGASIGB
VOLUME
MOAC
GAIN
AUDIO
OUTPUT
Fig. 20-8. Commodore-64 SID chip
frequency equal to the undistorted tone frequency. The shorter registers can
generate a repeating beat pattern with the tone generator and thus produce a
wide variety of droning-type sounds.
One final feature, which like the others is pure .logical bit manipulation,
is called "high-pass filtering." This amounts to a D-type flip-flop
clocked by one of the frequency dividers and an exclusive-or gate as shown. If
the flip-flop is clocked at a low frequency, the bottom input to the exclusiveor
gate is relatively stable, and thus nearly all activity at the top input will
pass to the output. As the clock frequency is increased, the two gate inputs
differ, on the average, less and less, and thus the output consists of narrow
pulses for those times when they do differ. The effect is most useful with
noise signals, although an interesting chorus-like sound can be obtained if
the flip-flop clock is very nearly equal to the tone frequency.
At the time of this writing, the Commodore-64 computer is by far the
most popular in terms of number of units sold. It also has the greatest builtin
musical capability of any microcomputer (with the possible exception of
Macintosh's unrealized potential) and sells for only $200. Like the Atari
machine, the secret lies in a custom chip, which, in this case, is dedicated to
sound generation and A-to-D conversion of the game "paddle" pots. Unlike
the Atari chip, however, the Commodore SID (sound interface device) chip is
a hybrid having both digital circuits like counters and analog circuits like a
programmable state-variable filter on the same chip.