| 摘要 |
A copy guard system is provided for protecting software in a video tape recording (VTR) system. The copy guard system includes a positive edge detection circuit for detecting a positive edge of an input VTR head switch pulse signal, a negative edge detection circuit for detecting a negative edge of the input VTR head switch pulse signal, a counter for generating a fixed duration pulse signal in response to the positive edge, a clamp circuit for clamping an input image signal, a gate pulse generating circuit for generating a pulse signal before the end of the fixed duration pulse signal, and a level detector for determining whether a copy guard signal is set in response to an output from the clamp circuit and the pulse signal. A pulse detection generating circuit is coupled to the level detector for transmitting a rectangular waveform to a signal killer pulse generating circuit. The signal killer pulse generating circuit receives the rectangular waveform and the signal from the positive edge detection circuit and generates a killer pulse in order to control a signal only during the period of having an image signal in one field. A signal killer circuit receives the clamped output and the killer pulse to control the output of the image signal.
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