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Frequency-division multiplexing (FDM) is a form of signal multiplexing where multiple baseband signals are modulated on different frequency carrier waves and added together to create a composite signal.
Non telephone FDM can also be used to combine multiple signals before final modulation onto a carrier wave. In this case the carrier signals are referred to as subcarriers: an example is stereo FM transmission, where a 38 kHz subcarrier is used to separate the left-right difference signal from the central left-right sum channel, prior to the frequency modulation of the composite signal. A Television channel is divided into subcarrier frequencies for video, color, and audio. DSL also uses different frequencies for voice and for upstream and downstream data transmission on the same conductors. Where frequency division multiplexing is used as to allow multiple users to share a physical communications channel, it is called frequency-division multiple access (FDMA). FDMA is the traditional way of separating radio signals from different transmitters. Telephone In the middle 20th Century, telephone networks used FDM to carry several voice channels on a single physical circuit. In Single-sideband modulation schemes, 12 voice channels would be modulated onto carriers spaced 4 kHz apart. The composite signal, occupying the frequency range 60 – 108 kHz, was known as a group. In turn, five groups could themselves be multiplexed by a similar method into a supergroup, containing 60 voice channels. In long distance systems, supergroups were multiplexed into mastergroups of 300 voice channels (Europe) or 600 (AT&T Long Lines L-Carrier) for transmission by coaxial cable or microwave. There were even higher levels of multiplexing, and it became possible to send thousands of voice channels down a single circuit. Level 1 Diagram Level 2 Diagram Level 3 Diagram For shorter distances, cheaper balanced pair cables were used for various systems including Bell System K- and N-Carrier. They didn't allow such large bandwidths, so only 12 voice channels (Double Sideband) and later 24 (Single Sideband)were multiplexed into four wires, two for each direction. By the end of the 20th Century, FDM voice circuits had become rare. Modern telephone systems employ digital transmission, in which time-division multiplexing (TDM) is used instead of FDM. The concept corresponding to frequency division multiplexing in the optical domain is known as wavelength division multiplexing. See also | ||||||||
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