of Microprocessors

776 MUSICAL ApPLICATIONS OF MICROPROCESSORS
VOICE
I
ADDRESS
~
I
2
3
4
S
6
F7 F6 FS I F4 F3 F2 FI F0
FIS FI4 FI3 FI2 FII FlO F9 F8
1'17 1'16 WS 1'14 Vl3 1'12 I'll 1'10
>< >< >< >< 1'111 WID 1'19 WB
NOISE n..n /Vi AA TEST R MOO SYNC GATE
A3 A2 AI Ail' 03 02 01 00
S3 S2 SI S0 R3 R2 RI Rlil
FREQUENCY LOW
FREQUENCY HIGH
PULSE WIDTH LOW
PULSE WIDTH HIGH
WAVE CONTROL 11 ENV GATE
ATTACK, DECAY TIME
SUSTAIN LEVEL, RELEASE TIME
VOICE
2 7-13
VO~CE 14-20
FILTER {~~24
READABLE {~;
2B
'>< >< >< X F2 FI F0
FlO F9 F8 F7 F6 FS F4 F3
03 02 01 OiJ FILTE)( FILT 3 FILT 2 FILT I
3 OFF HP BP LP VOL 3 VOL 2 VOL I VOL 0
X7 X6 XS X4 X3 X2 XI X0
Y7 Y6 Y5 Y4 Y3 Y2 YI _~
T7 T6 T5 14 13 T2 TI TO
E7 E6 E5 E4 E3 E2 EI Elil
FILTER CUTOFF LOW
FILTER CUTOFF HIGH
FILTER 0, SOURCE SELECT
FILTER RESPONSE, VOLUME
JOYSTICK X POT
JOYSTICK Y POT
TONE 3 WAVEFORM VALUE
TONE 3 ENVELOPE VALUE
ATTACK-DECAY-RELEASE TIME COOES
CODE ATTACK
DECAY/
CODE ATTACK
DECAy/
RELEASE
RELEASE
0000 0.002 0.006 I 000 0.100 0.300
000 I 0.008 0.024 1001 0.2S0 0.750
00 I ° 0.016 0.048 1010 0.500 I.S
001 I 0.024 0.072 I °I I 0.800 2.4
0100 0.038 0.114
1.0 3.0
I I °°
°I 0 I 0.056 0.16B I 10 I 3.0 9.0
°I I ° 0.068 0.204 I I I ° 50 15.0
°I I
I 0.080 0240 I I I I 8.0 24.0
Fig. 20-9. SID register layout
routines that display an analog synthesizer "console" complete with patch
cords and control knobs operated by the game controls. The sound quality
from the SID chip is generally quite good, although there are a few problems.
One of these is a noticeable leakage from silent voices into the output, which
creates a continuous background drone unless a zero frequency is selected as
welL
Although the chip's exact internal structure is secret" it is probably
based on the liberal use of multiplying D-to-A converters like the 7530
described in Chapter 7. The basic waveforms are undoubtedly produced by
simple processing of the upper 8 bits of a 24-bit accumulator-divider dtiven
by the computer's I-MHz clock. The selected waveform is then converted into
its analog equivalent by an 8-bit DAC The digital envelope generator
controls the amplitude of this analog signal with a multiplying DAC;
probably 8 bits as welL The analog signals from each of the three voices are