| 摘要 |
Copy protection normally included in a video signal to prevent unauthorized copying is dynamically modified, by changing an attribute of the copy protection to be added to or present in the video signal. The copy protection can include any signal that affects playability of an attenuated video signal, for example, by triggering an early or late horizontal or vertical retrace (such as caused by sync narrowing or the checker signals described in U.S. Patent 5,583,936). The attribute change (53) is related to scene content carried by the video signal, so that bright and dark scenes carry different copy protection. The attribute being changed can be, for example a checker's low level that is reduced to below the blanking level when the scene content is darker than normal, and increased to above the blanking level when the scene content is brighter than normal. Alternatively, the attribute being changed can be width of a synchronization pulse normally present in the video signal. The brightness of a scene carried by the video signal can be measured directly (52), or alternatively as the change in attenuation in the synchronization pulse received at the input of a sync separator of a typical VCR or TV set.
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