| 摘要 |
<p>A line selector circuit for a television receiver identifies a predetermined horizontal line. The line to be identified desirably occurs during the vertical blanking interval subsequent to he vertical sync interval contained therein. A line of particular current interest is line 19 which contains reference information which can be used for automatic control of the receiver. The predetermined line is identified through the generation of a timing period which is initiated by a sync pulse synthesized by the summation of horizontal and vertical rate information. The timing period is terminated by horizontal sync information at the beginning of the predetermined line. The timing period is generate by a one-shot multivibrator having a first threshold circuit responsive to the synthesized sync pulse designed to initiate the timing period the same line in the vertical sync interval of either field. The timing period is terminated prior to the natural delay of the one-shot when the amplitude of the timing waveform of the one-shot which determines the natural delay of the one-shot is modified by added horizontal sync pulses to trip a second threshold circuit in the one-shot. The natural constant and the amplitude of the horizontal sync pulses are set so that the horizontal sync pulse at the beginning of the predetermined line will terminate the timing period.</p> |
