| 摘要 |
Video information is transmitted encrypted as part of a video stream, so as to provide conditional access to the video information. The video stream comprising mutually interspersed first and second subsets of the image frames. The information for the image frames of the first subset is encrypted. A stream is broadcast ( 10 ) that contains, interspersed with one another, the information for the first subset in encrypted form and the information for the image frames of the second subset at least partly in plain form. This permits receivers ( 12 ) to detect the unencrypted parts without decryption and to generate and store pointer information ( 124 ) to the unencrypted parts. The pointer information can be used later for the purpose of trick play ( 126 ) at the receiver.
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