SCRAMBLED TELEVISION SIGNALS

摘要

A television receiver for receiving scrambled television signals has an arrangement (3) which provides scrambled vision components (V) and packet arranged sound/data components (D) some of which are also scrambled. The components (D) are applied to a packet demuitiplexer (4) where the packets are assembled from the data in several lines in the signal to provide a first output (P) the respective packets carrying sound/data information and at a second output (E) encrypted control data which is applied to a sub-system (6) to produce a vision scrambling control word (VS) for each line of the vision components. The sub-system (6) during each line produces further bits which are applied to a descrambling control word generator (7) which assembles these further bits from three lines to form a packet scrambling control word. This control word and the packets associated with it are added in an adder (5) and applied to a main packet buffer store (8) from which each packet in read out by a packet selector (9) and the packet is applied to a packet descrambler (12) whilst the packet scrambling control word is selected by a register (11) and used to control the generation of a binary sequence by a pseudo random binary sequence generator (13) which sequence is added to the packet in the descrambler (12), modulo-two, to achieve descrambling. A packet identifier (10) examines the packet address to ensure the packet requires descrambling and inhibits the generator (13) should descrambling not be required. With such a receiver the data used for controlling descrambled of the sound/data components is obtained indirectly from the control data which improves security.