| 摘要 |
A scrambled TV signal is provided by adding thereto a signal having an amplitude which changes pseudorandomly in successive increments of time, the duration of each of which is much less than the horizontal line duration of the TV signal. This signal encompasses substantially the entire duration of the TV signal and has an amplitude during horizontal and substantially the entire vertical sync interval to suppress the horizontal and vertical sync pulses to picture level. The pseudorandom signal is generated synchronously with the TV signal on a frame basis and synchronizing pulses are transmitted during the vertical interval together with a pulse for amplitude calibration. A matching pseudorandom signal is generated in synchronism with the pseudorandom component of the scrambled TV signal. The synchronizing pulses and the calibration pulse operate circuits which effect synchronization and amplitude control in the generation of the matching signal. The matching signal is subtracted from the scrambled TV signal to provide a descrambled signal for reception. The incremental change in level in a random direction of the scrambled signal, either increasing or decreasing in amplitude, constitutes a random walk which is not readily duplicable by unauthorized parties in that the pattern may be different for different programs and may be changed by coding in the video portion of the TV signal or by code signals which are separately transmitted over spare channels or telephone lines or by the use of replaceable integrated circuit chips containing different codes.
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