| 摘要 |
A method and apparatus are provided for blocking the reception of a subscription television program signal while substituting for the blocked signal a replacement program signal which is received by the blocked receiver. A single radio frequency signal, serving as an interfering carrier, is provided intermediate the audio and video carriers of the subscription signal and is modulated with a substitute audio and/or video program signal. The frequency of the interfering carrier is selected so that beat frequencies (i.e. sum and difference frequencies with carrier frequencies of the subscription program), which result from the non-linear characteristics of a conventional television receiver, or harmonics of the beat frequencies which are similarly produced, will block reception of the subscription television signal. These non-linear receiver characteristics also produce replicas of the interfering carrier modulation in the bands of the normally detected audio and video intermediate frequency signals in the receiver and these replicas replace the blocked subscription television program and are reproduced by the receiver. At an authorized receiver, there is provided a band-elimination filter, or the like, which is placed ahead of the receiver and is tuned to the interference carrier. As a result, the modulated interference carrier is removed prior to being applied to the receiver, so that the subscription television program can be received in the usual manner at the authorized receiver.
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