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Abstract

A key telephone system having a plurality of key telephone sets and a key service unit. Intercom lines connect the key service unit and the key telephone sets. A synchronizing system in each key telephone set is provided for synchronizing signal transmissions from the corresponding individual key telephone sets of the key service unit with the transmissions from the key service unit. The individual synchronizing systems receive a multiplex transmission from the key service unit and the transmission has a multiplexed frequency signal as a control signal and also has a voice signal. The control signal includes control transmission data to the synchronizing system and includes a start bit following a fixed number of signals having a binary state opposite to a binary state of the start bit. Circuitry is provided including a high pass filter which removes the voice signal and for converting the frequency shift signal free of the voice signal to the transmission data control signal with control transmission data. Detection circuitry detects the reception of the start bit and develops an output and other circuitry receives the output and determines it is a synchronizing signal and develops a reset pulse. A shift clock pulse generator receives the reset pulse and develops output timing shift clock pulses effecting synchronization of transmissions between the key service unit and the corresponding key telephone set.

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