| 摘要 |
In illegal cable decoders, adjustable thresholds are used to provide an illegal stable picture. By adding and/or inserting jamming, i.e., unreliable vertical signals in the vertical blanking interval and its vicinity, the illegal cable decoder can not lock onto a reliable vertical signal. As a result of the jamming signals, the illegal cable decoder outputs a vertically unstable picture or signal and concealment is maintained. The jamming signals are compatible with an authorized cable decoder and can be in the form of a track and hold signal throughout the vertical interval and its vicinity. In one embodiment of an unreliable vertical signal, a modulated signal with a range from about blanking level to about peak white level is inserted and/or added in the vertical blanking interval and/or its vicinity. To cause horizontal instabilities in the illegal cable decoder, the horizontal blanking interval is modified such that regardless of threshold level (i.e. either below blanking level for sync detection or above blanking level for a generally stable horizontal signal), a position and/or modulated signal is sliced or sensed.
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