AM and FM Broadcasting
Could you please explain AM and FM radio
broadcasting? Diagrams without equations will help. Thanks!
There are alot of books on this, but in simple terms:
Suppose you have one end of a rope tied to a tree, and you pull your end
of the rope back say 20 yards back. Now suppose your friend stands by
the tree and watches the rope, and you want to communicate something to
your friend. He asks you questions and you want to say yes or no. If
you move the rope up and down steadily, you will at least tell your
friend that you are there moving the rope, but not much else. But, if
you vary the amount you jiggle the rope, sometimes just a little,
sometimes alot, you can send your friend a message. Maybe a little
movement means "yes" and a big movement means "no". You have a very
simple amplidude (height of your ropes swing) modulated jump rope.
Or, maybe to say "yes" you move your arm up and down the same height,
(keep the amplitude the same), but maybe move faster than if you want to
say "no". The distance the rope moves up and down is always the same,
but the ups and downs are coming more often. This is frequency
modulatiion of the rope (FM).
When we now go back to radio, the rope just moving steadily, just to get
started, is called the carrier wave. The variation of the amplitude of
the carrier wave to send information is amplitude modulation or AM. The
variation of the frequency of the carrier wave to send information is
FM. If no change is done to the carrier wave, then very little
information is passes (it would be like the guy at the radio station
microphone just was quiet). You can still tell that the radio station
is there, but there is no other information passing.
AM means "amplitude modulation". The amplitude of the radio wave oscillates
over time. The frequency of the radio wave is constant. The peaks of the
wave get taller and shorter. The rate at which the amplitude fluctuates
corresponds to the pitch. How much it fluctuates corresponds to the volume.
FM means "frequency modulation". The amplitude of the radio wave is
constant. The frequency varies a small amount. The peaks of the wave get
closer together and further apart. The rate at which the frequency varies
corresponds to the pitch. How much it varies corresponds to the volume.
Because FM signals can maintain a large amplitude, low volume sounds are not
eliminated by static. Because frequencies can be separated, an FM signal
can be stereo: two different frequency signals added together. AM, on the
other hand, is much easier to produce. AM signals are much less complex.
When the radio is a great distance from the station, the complex FM signal
is too messed up to receive while the simple AM signal can still be
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Update: June 2012