| 摘要 |
Scrambling of a video signal in a pay TV system is effected in dependence upon a first variable which is changed recurrently, e.g. monthly, and a second variable which changes from field to field. For each field the second variable and other data are transmitted simultaneously with the video signal together with check data produced from the transmitted data and a third variable which is changed recurrently. The first and third variables are recurrently transmitted to each authorized subscriber station, encoded in dependence upon a subscriber number stored in the subscriber station. In the subscriber station, unscrambling is effected in dependence upon the stored first variable and the transmitted second variable after the transmitted data is checked using the check data and the stored third variable. The first variable is conveniently different for different types of television programs, the type of television program being identified by the other data transmitted simultaneously with the video signal.
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