| 摘要 |
A monitoring apparatus connected to a TV set selects, from a plurality of internal sources, such as VHF/UHF and BS tuners and built in said TV, and a plurality of external sources connected to the TV set from the outside thereof, such as a CATV tuner and a VCR, the source of video currently displayed on a cathode-ray tube of the TV set by comparing a sync signal of the video signal applied to the cathode-ray tuber and a sync signal of the video signal output from each source. Then the monitoring apparatus determines the currently selected channel by comparing a sync signal of the video signal output from the currently selected source and a sync signal of a video signal of each of broadcast channels generated by reference receivers in the monitoring apparatus independently of the TV set. When the channels cannot be narrowed down to one currently selected channel by the comparison of sync signals, the video signals output from the currently selected source are compared with the video signals of the broadcast channels generated by the receivers to determine one of the channels as being currently selected.
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