| 摘要 |
PROBLEM TO BE SOLVED: To solve a problem that where frequency is changed for every kind of a modulation signal transmitted from terminal equipment every frequency signal has to be received and demodulated by a center side units which are installed for each frequency. SOLUTION: In polling system communication between the center side unit and more than one kind of terminal equipment in a CATV system, two kinds exist in the modulation signal, i.e., the one obtained by performing data- modulation in a carrier after a prescribed time elapses after a carrier transmission starts and the one obtained by performing sub-modulation in the carrier before data modulation. In this case, the two kinds of modulation signals are received by one center side unit. Existence of sub-modulation is detected in the received modulation signal or a time from carrier detection in the signal to the detection of the first modulation signal is detected, so that the kind of the modulation signal is identified by the existence of sub-modulation or the length of the detection time. More than or equal to two kinds of modulation signals are of NRZ format and NRZI format. The NRZI format signal identified by the center side unit is converted into the NRZ format signal at the side of a center.
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