| 摘要 |
PURPOSE:To obtain an extremely precise gate pulse by providing a resetting circuit resetting a vertical synchronizing signal detection circuit and a gate pusle output circuit and using the output of the gate pulse output circuit as a gate signal. CONSTITUTION:Due to the rise signal of an AND gate 12 an output shown by figure (e) shows up from a flip flop circuit 13. Said output is transmitted as a gate signal to an output terminal 15. Simultaneously the 2nd counter 4 is cleared, and counting a horizontal synchronizing signal from a NOT circuit 3 starts. When the output of the 2nd counter 4 comes to (0,0,1,1), that is, 12 horizontal synchronizing signals are counted, a reset signal is outputted from the resetting circuit 16 to reset flip flop circuits 8 and 13. Accordingly the output of a NAND circuit 9 rises as figure (c) shows, and the output of the flip flop circuit 13 falls as figure (e) shows. Therefore the gate signal that appears only when teletext data exists is formed as fiture (e) shows.
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