Scrambling and unscrambling video signals in a pay TV system

A method of scrambling a video signal, comprising video signal lines and vertical intervals, to produce a scrambled video signal which comprises said video signal lines and dummy video signal lines in place of said vertical intervals, said method comprising the steps of:

• storing the video signal lines of the video signal to be scrambled sequentially in a memory and reading them sequentially from the memory to constitute the video signal lines of the scrambled video signal;
• re-reading video signal lines from the memory to produce said dummy video signal lines of the scrambled video signal, different numbers of video signal lines being re-read from the memory to produce dummy video signal lines to replace different vertical intervals, whereby the field length of the scrambled video signal is varied;
• and
• separately providing information relating to the timing of the replaced vertical intervals.

Apparatus for scrambling a video signal, comprising video signal lines and vertical intervals, to produce a scrambled video signal, said apparatus comprising:

• a memory unit;
• means for storing the video signal lines sequentially in the memory unit;
• means for providing an indication of the timing of each vertical interval relative to the video signal lines stored in the memory unit;
• means for reading the video signal lines sequentially from the memory unit, the reading means being responsive to said indication to re-read video signal lines from the memory unit to produce dummy video signal lines in place of each vertical interval, the reading means re-reading different numbers of video signal lines to replace different vertical intervals, said video signal lines sequentially read from the memory unit and said dummy video signal lines constituting the scrambled video signal;
• and
• means for providing information relating to the timing of each replaced vertical interval of the scrambled video signal.

A subscription television system comprising:

• a scrambling apparatus comprising means for replacing each vertical interval of a video signal, comprising video signal lines and vertical intervals, by a plurality of dummy video signal lines to produce a scrambled video signal, different vertical intervals being replaced by different numbers of dummy video signal lines whereby the scrambled video signal has a variable field length, and means for providing information relating to the timing of each replaced vertical interval of the scrambled video signal;
• means for distributing said scrambled video signal and said information to at least one unscrambling apparatus;
• and
• at least one said unscrambling apparatus comprising means responsive to said information for generating vertical intervals each having a timing coincident with the timing of said dummy video signal lines of the scrambled video signal, and means for replacing said dummy video signal lines of the scrambled video signal by said vertical intervals to produce an unscrambled video signal.