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frequency division multiplexing
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communications (FDM) The simultaneous transmission of
multiple separate signals through a shared medium (such as a
wire, optical fibre, or light beam) by modulating, at the
transmitter, the separate signals into separable frequency
bands, and adding those results linearly either before
transmission or within the medium. While thus combined, all
the signals may be amplified, conducted, translated in
frequency and routed toward a destination as a single signal,
resulting in economies which are the motivation for
multiplexing. Apparatus at the receiver separates the
multiplexed signals by means of frequency passing or rejecting
filters, and demodulates the results individually, each in the
manner appropriate for the modulation scheme used for that
band or group.
Bands are joined to form groups, and groups may then be joined
into larger groups; this process may be considered
recursively, but such technique is common only in large and
sophisticated systems and is not a necessary part of FDM.
Neither the transmitters nor the receivers need be close to
each other; ordinary radio, television, and cable service are
examples of FDM. It was once the mainstay of the long
distance telephone system. The more recently developed timedivision multiplexing in its several forms lends itself to
the handling of digital data, but the low cost and high
quality of available FDM equipment, especially that intended
for television signals, make it a reasonable choice for many