Digital method and apparatus for evaluating a frequency parameter of an IF signal

Abstract

A tuning system of a television receiver includes a digital automatic fine tuning (AFT) unit for generating a digital AFT signal representing the deviation of the frequency of the picture carrier of the IF signal from a nominal frequency. The digital AFT unit includes a counter for counting cycles of picture carrier during repetitive counting intervals which occur during a measurement period having a duration at least as long as the duration of one field of a television signal. The duration and spacing of the counting intervals are selected so that at least one counting interval occurs during the vertical blanking interval in which the picture carrier is not likely to be over-modulated and therefore not likely to result in erroneous low counts. The contents of the counter are evaluated at the end of each counting cycle to determine if a first count corresponding to a nominal frequency or a second count corresponding to the frequency having a positive frequency deviation have been produced. At the end of the measurement interval, the frequency of the picture carrier relative to the nominal frequency is evaluated by determining which one, if any of the first and second counts have been produced. A low frequency is indicated by the absence of both. A frequency within specified limits is indicated by the presence of the first but not the second counts A high frequency is indicated by the presence of both counts. The technique employed does not require that the timing signal be synchronized with the vertical blanking interval for reliable counting.

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