| 摘要 |
PURPOSE:To prevent the blanking interruption of signals such as a VIT signal and a character one and the omission of some part of then by detecting a change in the frequency of a horizontal synchronizing signal and changing the blanking pulse width in accordance with the detection output. CONSTITUTION:The DC output E1 of the horizontal synchronizing signal 1 detected from a composite synchronizing signal is outputted through a frequency- voltage converter 2 and an LPF 3. Level detectors (I) and (II) detect the change in the DC output E1 as changes in frequencies f1 and f2, and the detection output is applied to a decoder (binary - ternary) 9. In sorts out the detection output of the level detector into three frequency bands to obtain switching outputs 10, 11 and 12. A switching signal output detection circuit 13 detects the switching outputs 10-12, which are inputted to a blanking width control circuit 14. Thus a vertical blanking pulse is controlled and eventually the vertical blanking pulse corresponding to the pulse width of the vertical blanking pulse is generated.
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