spectrum graphics - OpenLibra

+5V
Figure 1 Diagram of 'he 8912 PSG C. i r r u i '
RFiCtSTEK FUNCTION BIT
HO
RI
K2
R3
Channel A - Tone Period
Channel B - Tone Period
7 6 s J 2 1 0
8 Bit Fine Tone A
8 Bit Fine Tone B
K4 8 Bit Fine Tone C
4 Bit Course Tone A
4 Sit Course Tone B
R5 4 Bit Course Tone C
R6 Noise Period J Bit Control
R7 Channel Enable
I/O Noise Tone
C B A C B A
R8 Channel A - Amplitude Env 4 Bit Control
R9 Ch.-innel 8 - Ataplitudi; Env 4 Bit Control
RIO Channel C - Amplitude Env 4 Bit Control
RI t
R12
Envelope Period
8 Bit Fine Control
8 Bit Course Control
RI) Envelope Shape 4 Bit Control
R14 I/O Port 8 Bit Parallel Port
PSC REGISTERS
SOUNDS
PSG CIRCUIT
specified VOLUME. The command
is fairly complex and, to
make full use of its capabilities,
experimentation is required.
There are four variables or integers
involved in the command
with a comma separator between
each. The CHANNEL
selection is pretty straightfor
ward, selecting the channel that
is required. OCTAVE 0 starts at
two below middle C and in
creases from there. (Middle C is
taken as 246.9 Hz). NOTE
selects the required semi-tone
within the chromatic scale and is
the same as the Spectrum's
BEEP command. VOLUME
ranges from 0 to 15. the
amplitude of the output ranges
from 1 (very soft) to 1 5 {very
loud). When the volume is set to
0, control of the output is passed
to the envelope generator the
parameters of which must be
set in an accompanying " PLAY.
"MUSIC must be used with
" PLAY, which selects the channels
being used and the shape of
the sound envelope.
5 * PLAY 1.0.0,0
10 FORo = 0TO 6
20 FOR n: 1 TO 12
30 * MUSIC 1 ,o,n,5
40 PAUSE 20
50 NEXT n
60 NEXT o
70 * PLAY 0,0,0,0:REM turn
channels off
*SOUND
Format; channel, pitch,
volume.
channel 1 -6
pitch 0-65335
volume 0-15
* SOUND lacks the finesse of
* MUSIC, since its output is
not arranged in semitones and it
merely blasts out whatever
values are input. It can be used
to generate complex sounds as
well as pure tones. However, it
does include a noise generator
unlike * MUSIC which allows
the creation of sounds which are
more useful as a background to
games. CHANNEL values 1-3
select a tone channel and values
4-6 select which of the three
channels the single noise
generator will output through,
PITCH is no more than a division
68 ZX COMPUTING DECEMBER JANUARY 1985