| 摘要 |
PURPOSE:To detect video signal through a digital circuit by counting up the number of horizontal synchronous pulse for a fixed period longer than the sum of a vertical synchronous period and an equivalent period. CONSTITUTION:A horizontal synchronous pulse is impressed to an input 21 of an AND circuit 23 and a control pulse longer than the sum of the vertical synchronous period and the equivalent period is impressed to an input 22. During the period of T, a counter 24 counts up the number of horizontal synchronous pulse. The counted value is latched by a latch circuit 26. The contents of the latch circuit 26 are compared with prescribed values N1 and N2 by the 1st and 2nd comparators 28, 29 respectively. The comparator 28 outputs the ''H'' level when the contents of the latch circuit 26 are >N1 and the comparator 29 outputs the ''H'' level when the contents are <N2. Consequently, a video signal detecting output can be digitally obtained from an AND circuit 31. |
