Automatic digital frequency control circuit

Abstract

An automatic digital frequency control circuit is used for processing a digital video signal. The control circuit includes a phase difference calculator, which samples an analog horizontal sync signal and generates a phase difference value between the corrected phase values with respect to a present scanning line and a phase value of a previous scanning line of which the error values are corrected when a digital horizontal sync signal is obtained, and a phase difference accumulator, which accumulates the phase difference value from the phase difference calculator to a phase difference accumulated value which has been already stored therein. The control circuit also includes an oscillation step corrector, which generates correction step data, a phase value generator, which increases the stored phase value by the corrected oscillation step data whenever the system clock is applied thereto and stores and outputs the increased phase value, and a carrier generator, which generates a carrier having a frequency corresponding to the phase value generated in the phase value generator. It will be noted that since the carrier used for processing the digital video signal is digitally generated, a compact system can be obtained while the difficulty in designing a voltage controlled oscillator which limits digitization of the system is resolved.