Importance

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Abstract

A conditional access system for over-air transmission and reception of scrambled television signals improves the reliability of the reception by transmitting a key signal for use in descrambling the television signal in a block of information which is itself encyphered by the key signal. On reception, the receiver after decyphering of the block of information compares the key signal recovered from the block with the key signal provided at the receiver for decyphering the block. Descrambling will only be allowed if comparison shows the two key signals to be the same. The system also provides for information relating to the credit status of each user to be transmitted over-air. In order to ensure rapid operation, the credit status signal is sent repeatedly and a further signal is appended which is used at the receiver to prevent repeated accumulation of credit. An alternative arrangement is for the transmitter to transmit a signal indicative of the total sum of credit ever purchased by a user and for the user's receiver to include a counter for accumulating all charges for programs viewed. A simple comparison between the two signals is sufficient to establish whether or not the viewer may view a program.

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First Claim