| 摘要 |
A system for securing a pay-TV channel by coherently moving the RF components of the channel (with or without an encoding signal) beyond the AFC holding range of commercially available tuners. The system is comprised of means for mixing the channel carrier with a signal that is varied beyond that AFC holding range at a low rate between approximately 0.75 Hz and 1000 Hz. For greater security, an amplitude-modulated encoding signal is injected at an IF frequency that is between the audio subcarrier and the video subcarrier. The encoding signal is preferably a multiple of the line scanning frequency in order to interfere with both video and audio components. For decoding, a converter with adequate AFC holding range at the receiver of a subscriber is coupled to the receiver by a sharply tuned filter which traps the injected encoding signal.
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