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If you connect a long
wire to the output
terminals of your Hi-Fi
amplifier and another
long wire to the input
of another amplifier,
you can transmit music
over a short distance.
DON'T try this. You
could blow up your
amplifier.
A radio wave can be
transmitted long
distances.
To get our audio signal
to travel long distances
we piggyback it onto a
radio wave. This process
is called MODULATION.
The radio wave is called
the CARRIER.
The audio signal is
called the MODULATION.
At the receiving end the
audio is recovered by a
process called
DEMODULATION.
From the diagram below,
it can be seen that when
the carrier is
modulated, its amplitude
goes above and below its
unmodulated amplitude.
It is about 50%
modulated in the
diagram.
The maximum percentage
modulation possible is
100%.
Going above this causes
distortion.
Most broadcasters limit
modulation to 80%.
Modulating the carrier
frequency with an audio
frequency produces two
new frequencies.
At this point it would
be a good idea to read
the page on MIXERS.
These new frequencies
are called the upper and
lower SIDEBANDS.
The upper sideband is
the carrier frequency
plus the audio
frequency.
The lower side band is
the carrier frequency
minus the audio
frequency.
Since the audio signal
is not a single
frequency but a range of
signals (usually 20 Hz
to 20 KHz) the sidebands
are each 20Hz to 20 KHz
wide.
If you tune across a
station in the Medium
Wave Band you will find
that it takes up space
in the band.
This is called the
signal BANDWIDTH.
This is the space taken
by the upper and lower
sidebands.
In the the example given
above it would be 40
KHz.
Since the Medium Wave is
only 500 KHZ wide there
would only be space for
about 12 stations.
Therefore the bandwidth
of stations is limited
to 9 KHz, which limits
the audio quality.
If there are two
stations too close
together, their
sidebands mix and
produce HETERODYNE
whistles.
Since both sidebands
carry the same
information, one side
can be removed to save
bandwidth.
This is SSB, single
sideband transmission.
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