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Carrier Telephony

Early in the history of telephony efforts were made to develop methods for transmitting more than one telephone conversation over a pair of wires. This led to the use of carrier transmission,27 defined1 as "that form of electric transmission in which the transmitted electric wave is a wave resulting from the modulation of a single-frequency wave by a modulating wave." Carrier transmission is employed in systems of carrier telephony, defined1 as "that form of telephony in which carrier transmission is used, the modulating wave being a voice-frequency wave," Ordinarily, the term is applied only to wire telephony. Carrier telephone systems were used first in about 1918.

The principle of operation of the usual multichannel carrier telephone system is as follows: For each channel the incoming voice-frequency signals are used to amplitude-modulate a carrier wave of higher frequency. As will be explained (page 413), the modulation results in the creation of two sidebands, each of which is at a higher frequency than the voice-frequency signals, and each of which contains all the signal variations necessary for the transmission of speech.

In most carrier telephone systems one sideband only for each channel is transmitted simultaneously with sidebands from other channels over the same pair of wires or cable circuit. Carrier repeaters are used to offset circuit attenuation.

At the receiving end the signals are separated by filters, and each signal is demodulated by a process fundamentally the same as that of modulation. As a result, the desired voice-frequency component is created from the otherwise unintelligible carrier signal. The voice-frequency component is transmitted over the subscriber line to the listener.

Last Update: 2011-05-30