Receiving scrambled signals.

摘要

A receiver receives scrambled signals which are subjected to a descrambling process under the control of control words periodically conveyed to the receiver from a sub-system, a fresh control word being required evey 256 television frame periods. In order to reduce the time taken to initiate the descrambling process at the selection of a particular service or an interruption therein the first control word conveyed to the receiver is that required for the following 256 frame interval followed by the control word for the current 256 frame interval. Further control words conveyed to the receiver are those required for the following 256 frame interval to which they are conveyed.