| 摘要 |
PURPOSE:To reduce a probability of a faulty synchronization and to prevent malfunction caused by reading errors by switching an action attitude of a protection circuit for holding the synchronous condition at the time of disturbance of a horizontal synchronizing signal included in a video signal by means of a control signal generated at the specific position in a video frame. CONSTITUTION:When a fall edge of a composite synchronizing signal CS does not arrive in a period for generating an HG pulse, a counter 34 starts counting. If the counter 34 reaches the prescribed number of times N, this is considered to be a faulty synchronization, whereby the clear state of a D-type flip-flop 31 is released with the aid of an output Qn of the counter 34, and an arrival of the fall edge of the signal Cs is awaited. When said edge arrives during the release of the flip-flop 31, the counter 34 is cleared. Thus even if the fall edge of the CS does not exist in the period for generating the HG pulse, this is not considered to be a faulty synchronization before the counter 34 reaches the prescribed value. The synchronous condition can be held with respect to a disturbance of the signal CS caused by accidental noises and a dropout. |
