Chapter 7 Sound 239
produce a wavering effect. Enter the following line to produce vibrato:
57
FOR
V • 0
TO
S
.58
FOR
W.
15
TO
8
STEP
-1:
POKE
36878,
W:
NEXT
59
FOR
W • 8
TO
15:
POKE
36878,
W:
NEXT:
NEXT
Tremolo
is
a fast vibrato.
If
you change lines
58
and
59
to
57
FOR
V-0
TO
S
58
FOR
W
II
15
TO
8
STEP
-J:
POKE
36878
..
W:
NEXT
59
FOR
W - 8
TO
15
STEP
J:
POKE
36878,W:
NEXT:
NEXT
and add an input line to enter the vibrato rate
1
INPUT
"VIB.
RATE";
J
you will be able to vary these parameters
at
the start
of
each tone. Entering
larger numbers will increase the speed
of
the vibrato,
and
smaller numbers
will slow it down.
Do
not enter numbers larger than
15;
they will have no
effect. Entering a 0 will result in
an
endless loop.
Mixing
Tones
Earlier in this chapter
we
put
three harmonizing tones together to
create a chord.
It
is
also possible to combine tones to create entirely new
sounds. Let's take a look at some
of
these.
BEAT
FREQUENCY
A beat frequency
is
a sound
that
is
produced when two tones
that
are
very close together are played at the same time. Try the following simple
experiment:
10
PRINT
":1"
20
INPUT
"STARTING
VALUE"jS
30
IF
S(138
OR
S)243
THEN
20
40
FOR
D = S -
10
TO
S +
10
50
POKE
36876,8:
POKE
3687S,D
60
POKE
36878,15
70
FOR
T = 0
TO
500:
NEXT
80
POKE
36878,0:
NEXT
This program starts with a note value between
138
and
243
for the
middle tone register. The program will then play all
of
the notes within a
range
often
above and ten below the starting value in the high tone register.
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