The invention relates to a method and an apparatus for scrambling an analogue electrical input having a certain frequency spectrum in which the signal is divided into segments and the segments re-ordered to form a re-ordered signal.
The invention will be described in the context of scrambling analogue sound signals forming part of a television signal such as in Pay Television or other so-called conditional-access television signals. The invention is particularly suitable for television signals which are recorded on a video tape recorder (or video cassette recorder--VCR) for subsequent replay. In one method of scrambling such analogue sound signals the sound signal is divided into short segments each of a fraction of a second long. The segments are grouped into blocks with a predetermined number of segments forming each block, typically four or more.
Such a signal can then be scrambled by re-ordering the segments within each block. To control the re-ordering a pseudo-random or chain code sequence can be used. This sequence has to be synchronised at the receiver with the transmitter sequence, such as by using an enciphered and dicephered code word. At the descrambler the scrambled segments are put back into the correct order to form the descrambled output sound signal.