ARP 2600 - Page 20

14
2,1
831
Here
is a
simple example of frequency
modulation.
Suppose
we
begin with
a
simple oscillation,
say a
square wave
at a
frequency
of
100Hz:
Now,
at the
rate
of 25Hz, let's alternately
double
and
halve
again the
frequency of
this
square
wave:
If
we
draw
a
graph
in
which up means
higher
frequency and
down
means
lower frequency, and graph
the changes in
frequency of
our 1 00Hz
square
wave,
it
looks
like this:
-
5
eye!
A
look
at the first
and
last
graphs, and at
their "result" in
the
second
graph, shows what
we
mean if we
say
that
we
have FREQUENCY-
MODULATED A
100Hz SQUARE
WAVE
by
A
25 Hz SQUARE
WAVE.
If,
instead
of
shifting suddenly up
and
suddenly down
again,
we start from
100Hz
and
moved
gradually up to
say
200Hz and then
started suddenly
again
from
100Hz, a
graph
of the changes
in frequency
would look
like
this:
4
leu£ft
h
and
in
this
case
we would say
we
had
FREQUENCY^MODULATED
A
100Hz SQUARE
WAVE
WITH
A 25Hz
SAWTOOTH.
2.1832
AMPLITUDE
MODULATION,
on
the
other
hand,
of
a
100Hz
square
wave
by a
25Hz
square wave
would
produce
something
like
1
y