Commodore VIC-20 User Manual page 254

240
The VIC 20 User Guide
This will allow you to hear the effects of different notes (frequencies) on each
other. Listen for the "beat" that occurs at certain frequencies. This is caused
by the interaction of the two tones.
The program plays notes that are an octave apart. This was done to
keep the program simple. Fortunately, beat frequencies work on different
octaves as well as on the same octave. Use Table 7-2 to locate the values of
notes of the same frequency in the same octave. Experiment with some of
these to see how changing octaves affects the sound produced.
USING THE NOISE REGISTER
The noise register operates exactly like the tone registers. Here's the
ping-pong program one more time, this time using the noise register.
18
POKE
36878.1~
20 FOR T-e TO
1~
30 POKE
36877.0
48 FOR F-0 TO 480:NEXT
~0
POKE
36877.240
60
POKE
35877.e:N!XT
N ow let's add some decay to the sound.
55
FOR
Y-15
TO
e
STEP-.l
57
POKE
36878.
Y:.NEXT
By also modifying the repeat rate, we can create a sound similar to a
train as follows:
40 FOR F
~
0 TO 400-26*T: NEXT
55 FOR ..,' = 15 TO 0 STEP • HIlT
By mixing sound we can also add a train whistle. To do that we'll need
to maintain the volume at a constant level. Add the following routine to
your program:
70 FOR G
=
e TO 3
80 POKE 36877, 240
90 FOR F = 0 TO 100: NEXT
100 POKE 36877, 0
110 FOR
Q
a
0 TO 10: NEXT: NEXT
120 IF X=1 THEN POKE 36875. 0: POKE 36874,0: X=0: GOTO 70
130 IF X-0 THEN POKE 36875,220: POKE 36874,240: X-l
135 FOR WW
=
0 TO 2
140 FOR G
a
0 TO 3
150 POKE 36877.. 240