Commodore VIC-20 User Manual page 248

234
The VIC 20 User Guide
Pressing the
F5
key POKEs 0 into the middle and high tone registers and 208
into the low tone register.
Pressing the
F7
key turns off the sound by PO KEing 0 into the volume
control register. Pressing the
F7
key again turns the sound back on.
Since the tone registers are one octave apart, pressing the different
function keys will allow you to hear 340 Hz (high register), 170 Hz (middle
register), or 85 Hz (low register). Once again, see Table 7-2 for a complete
listing of the frequencies and POKE values.
SWEEPING THE SCALES
You can change the value in a tone register while it is on. This will
switch immediately from one note to another. This is especially useful for
sliding from one note to the next. Try the following example. There will be
no pause between notes.
10 POKE 36878,15
20 OET AS: IF A ••
1111
THEN 20
30
IF A • •
CHR'(133) AND PEEK(36878)=0 THEN 10
40 IF A. • CHR'(133) AND PEEK(36878)-15 THEN POKE 36878,O: OOTO 20
50 A
a
VAL(AS'*10+163
60 POKE 36875,R: OOTO 20
This program accepts data from the keyboard in the form of single
numbers between 0 and 9. These numbers are converted into a value
between 128 and 254. The value is then PO KEd into the middle tone register
(location 36875). Pressing the
FI
key alternately turns the sound on and off.
By using a FOR-NEXT loop, you can sweep up or down the scale. The
following sequence covers the entire tone range of the middle register:
10
POKE 36878,15
20
FOR
Ta128
TO 254
30 POKE 36875,T: NEXT
40 POKE36878., 0
Try combining three loops and sweeping all nine octaves.
MULTIPLE TONES
By sounding two or more tones at once, you can add depth to many of
your sound effects. Adding this line to the sample program will play two
tones at once.
25 POKE 36876,T