| 摘要 |
A video signal is modified so that a television/receiver still produces a normal picture from the modified signal, whereas videotape recording of this signal produces generally unacceptable pictures. Videotape recorders have an automatic gain control circuit (Fig. 2) which measures the sync pulse level in a video signal and develops a gain control correction signal for keeping the video applied to an FM modulator in the videotape recording system at a fixed predetermined level. A portion of the back porch intervals following the trailing edges of a substantial number of sync pulses are significantly raised and lowered to improve the playability of the original signal without reducing the effectiveness of the copy protection of the copied signal. This is achieved most simply by adding pulses to selected lines of the video signal during the back porch interval, which pulses significantly raise or lower the voltage amplitude during the blanking interval so to provide a signal where the average value is approximately equal to the blanking interval. The sync level and the sync duration on the selected lines are also reduced to enhance the effectiveness of the process.
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