| 摘要 |
PURPOSE:To prevent a video signal from being intercepted by correctly reproducing a picture only when the video signal having the same frame synchronizing signal pattern as a preset synchronizing signal pattern. CONSTITUTION:A binary data string D1 obtained in a shift register 81 and a binary data string D2 set by the dip switch of a frame synchronizing signal pattern setting circuit 82 are inputted to a coincidence decision circuit 83 and only when both the data strings coincide, the pulse P of a positive polarity is obtained from the coincidence decision circuit 83. A synchronizing signal forming circuit 84 forms a horizontal synchronizing pulse HP and a vertical synchronizing pulse VP by defining the pulse P to be a reference and a clamping pulse forming circuit 9 forms a clamping CP from the horizontal and vertical synchronizing pulse HP, VP and is outputted to a clamping circuit 2. Accordingly, when the binary data strings D1 and D2 are different, since the clamping pulse CP of a correct timing is not obtained, the picture cannot be reproduced. Thereby, a danger that the interception is carried out by a third party is eliminated. |
