| 摘要 |
PURPOSE:To extract a vertical synchronizing signal stably with a high precision by setting a time window about one field after the trailing edge of the just preceding vertical synchronizing signal and defining a point of time, when the level exceeds a threshold in this time window, as the start point of a vertical synchronizing signal. CONSTITUTION:A demodulated television signal 12 is divided into two; and when the trailing edge of a vertical synchronizing signal is detected by an extracting circuit 13 of the trailing edge of the vertical synchronizing signal, a one-field delay circuit 15 counts clocks inputted from a terminal 16 and outputs leading edge information 18 to a DC level clamping circuit 19 and a vertical synchronizing signal start point extracting circuit 21 after the elapse of a one-field time. When leading edge information 18 is inputted, the DC level clamping circuit 19 clamps it to 0V and outputs a clamp output 20. The vertical synchronizing signal start point extracting circuit 21 is set to the holding state when receiving leading edge information 18; and since the time window is set, the circuit 21 outputs a vertical synchronizing signal start point pulse 22 corresponding to a certain number of clocks when the clamp output 20 exceeds the threshold. |
