| 摘要 |
A video distribution system (e.g., CATV or MATV) employs at least one channel having a reception-defeating jamming signal which changes frequency periodically or, preferably, randomly. The signal originating head end distributes information characterizing the operative frequency of the jamming wave. At controller apparatus for authorized viewers, the jamming frequency specifying data is employed on a frequency agile basis to heterodyne the protected channel into a proper (fixed) frequency relationship with a notch band suppression filter to remove the jamming signal. The secure channel with the jamming wave removed is then again shifted to its original or other intended frequency to permit normal reception either directly by a standared television receiver, or via a frequency converter.
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