DIGITAL AFT SYSTEM ACTIVATED DURING VERTICAL RETRACE INTERVALS

摘要

A digital fine tuning system in a tv receiver includes an arrangement (30) for measuring the frequency of the intermediate frequency (IF) picture carrier by counting cycles of the IF carrier during a measurement interval and comparing the count at the end of the measurement interval to a predetermined count related to the nominal frequency of the IF carrier. To prevent counting and thereby frequency measurement errors due to the overmodulation of the picture carrier during image intervals, the frequency measurement operation is enabled during a predetermined portion of a retrace interval when the picture carrier is not overmodulated. IMAGE