| 摘要 |
PURPOSE:To improve stability for the switching between an automatic synchronization and a direct synchronizaton with a vertical synchronizing circuit of a TV picture receiver, by deciding whether the received vertical synchronizing signal is in accordance with the broadcast standard by setting the detecting margine of a counter within a certain range. CONSTITUTION:In case the vertical synchronizing pulse Vsync (Hz) is identical with the signal Vsync =2fH/525 as defined by the broadcast standard, a counter shown in the diagram counts 2625 or 2626 according to the difference of phase. On the other hand, it is very hard to decide whether the vertical synchronizing signal is in accordance with the broadcast standard if the said signal receives the interference of a video wave detection signal. In this connection, a detection margin is secured for a vertical synchronizing pulse deciding circuit as shown in the diagram. In other words, the counter is connected to an NAND circuit N3N4 with each relation as shown in the diagram. As a result, the broadcast standard signal can be decided up to 2625+ or -3 for the detection of the counter. Thus the frequent switching is avoided between the automatic and direct synchronizations to secure the extremely high stability of switching. |
