Importance

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Abstract

A method and apparatus for blocking the reception of a subscription television program while substituting a replacement program at a blocked receiver, provide security against detuning an unauthorized receiver to receive the subscription program. A security carrier signal is provided in a subscription television signal at the frequency of the audio carrier of the lower adjacent channel and is alternately amplitude modulated with bursts from first and second interference frequencies at a third or keying frequency. The television signal also includes an interfering carrier signal intermediate the audio and video carriers in the channel and the interfering carrier is modulated with a substitute television program. When a receiver is properly tuned to the channel, the security carrier has no effect on reception and either the subscription or substitute program is received, depending on whether the receiver is authorized or not. Should an attempt be made to receive the subscription program in an unauthorized receiver by detuning the receiver, the security carrier interferes with or blocks video reception in the detuned receiver and the modulation of that carrier produces a moving visual pattern which is extremely annoying to a viewer.

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First Claim