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Definition: FDM


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

(2) (Frequency Division Multiplexing) Transmitting multiple data signals simultaneously over a single channel. Each data stream (text, voice, video, etc.) is modulated onto a carrier with a unique center frequency by the multiplexor and demodulated at the receving end (see modulation and carrier). FDM works in both wired and wireless applications (see FDMA).

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.

Analog FDM Vs. Digital TDM
FDM separates signals by using different carrier frequencies, whereas TDM (time division multiplexing) interleaves signals as in the following illustrations.