Digital arrangement for detecting a correct television synchronization signal output useful in a signal seeking tuning system

摘要

A detector for determining the validity of a composite synchronization signal to indicate the presence of a picture carrier includes two threshold comparators set just below and just above the maximum and minimum amplitude excursions, respectively, of the synchronization pulses. The output pulses of the threshold comparators are counted during a reference interval to determine if the frequency of the synchronization pulses is correct. Due to the threshold comparators, noise transients are detected and counted so as to cause an incorrect frequency indication. In addition, reference pulses are counted during the duration of the output pulses generated by one of the threshold comparators to determine if the width of the synchronization pulses is correct. The number of incorrect counts of the frequency or width is counted during a measurement interval. If the number of incorrect counts exceeds a predetermined limit, the composite synchronization signal is not a valid one. Hysteresis is also provided to prevent sporadic transitions of sync presence indication.