| 摘要 |
PURPOSE:To simplify the constitution of a discriminating circuit by discriminating whether or not a prescribed number of synchronizing signals included in a composite video signal are incoming within a prescribed time so as to discriminate the presence or absence of a TV signal without using a pseudo flyback pulse. CONSTITUTION:The horizontal synchronizing signal is separated and extracted from the composite video signal from an input terminal 1 by a synchronous separation circuit 2. The high frequency noise component of the synchronizing signal is eliminated by an LPF 3, waveform-shaped and applied to the inverting input of a comparator 5 of a comparator circuit 4. The comparator 5 compares the voltage with a reference voltage divided by resistors R6, R7 so as to output the horizontal synchronizing signal having a proper waveform without noise to the output side. This horizontal signal is applied to a counter 6 to which a clock is inputted from a clock terminal 7. Then, the count value of the counter 6 is applied to a digital comparator 9 via a latch 8 and the presence or absence of the TV signal is discriminated by the horizontal synchronizing signals incoming for a prescribed number so as to simplify the constitution of the discriminating circuit. |
