| 摘要 |
PURPOSE:To delete an unnecessary video appearing on the top part of a screen by using an oscillation circuit to which the differential signal of a first vertical blanking signal which has the time width of a conventional vertical fly-back time is set to a trigger input, and generating a second vertical blanking signal which has the desired time width. CONSTITUTION:A first vertical blanking signal generation circuit 3 generates the first vertical blanking signal (d) from a vertical output (b) from a vertical deflection output circuit 2 and an output (a) from a vertical oscillation circuit 1. The signal (d) is inputted to a differential circuit 5 in a second vertical blanking generation circuit 4 and the differential signal (e) is obtained. In the differential signal (e), only a part in which the vertical fly-back time starts is sampled and is inputted to the oscillation circuit 6, whereby the differential signal (e) is oscillated as the trigger. Since the time constant of the duty in the oscillation circuit 6 is variably constituted, the second blanking signal (f) with the optional time width can be generated with changing the time constant. Thus, the video is not projected on the top part of the peripheral frame of a projection screen.
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