Importance

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Abstract

A transmission system for a key telephone system, in which speech signals and control signals are transmitted through two pairs of transmission conductors between a key service unit and each of a plurality of key telephone sets, and in which DC electric power is supplied from the key service unit to each of a plurality of telephone sets. In accordance with the present invention, the speech signal and the control signal are transmitted through a first transmission line of high frequency band provided by one pair of the transmission conductors from the key telephone set to the key service unit, and the speech signal and the control signal are transmitted through a second transmission line of high frequency band provided by the other pair of the transmission conductors from the key service unit to the key telephone set. A third transmission channel is provided by the use of respective neutral points of the two pairs of transmission conductors established at the key service unit and by the use of respective neutral points of the two pairs of transmission conductors established at the key service unit to form a voice-frequency two-way transmission line between the key service unit and the key telephone set. DC electric power is supplied from the key service unit to the voice-frequency two-way transmission line from a DC power source, so that the DC electric power supplied is separated from the voice-frequency two-way transmission line by a power separating circuit, such as a smoothing circuit.

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First Claim