FDM

(′frē·kwən·sē di′vizh·ən ′məl·tə′plek·siŋ)

(communications) A multiplex system for transmitting two or more signals over a common path by using a different frequency band for each signal. Abbreviated fdm; FDM. Also known as frequency multiplexing.

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(1) (Fused Deposition Modeling) See 3D printing.

(2) (Frequency Division Multiplexing) Transmitting multiple data signals simultaneously over a single wire by using multiple carriers, each having a unique center frequency. Each data stream (text, voice, video, etc.) is placed into a separate carrier that is modulated by various methods (see modulation).

It Started in the 1930s

In the 1930s, the telephone companies began to use FDM to combine multiple analog voice signals over one line to maximize efficiency of their long distance trunks. During the 1960s, FDM was replaced with TDM, its digital counterpart (see TDM and channel bank).

Cable TV companies use FDM to transmit hundreds of channels of analog and digital TV as well as Internet over a single coaxial cable. The set-top box or TV tuner locks onto a particular frequency (channel) and demodulates the carrier to derive the data for the TV screen. The cable modem both modulates and demodulates the appropriate carriers to provide bi-directional operation for Internet access. See FDMA, OFDM, circuit switching and WDM.

FDM Is Where It Started

Starting in the 1930s, the analog telephone system employed FDM to combine multiple voice conversations onto one line. The analog telephone backbones gave way to digital, thus FDM has been replaced with TDM for backhaul trunks.

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