Video switching detecting circuit

Abstract

The object of the invention is to provide a video switchover detection circuit that reduces the circuit scale and allows high-accuracy detection with a smaller-scale configuration. According to the invention, the video switchover detection circuit comprises a PLL circuit composed of a phase comparator, an LPF, a VCO, and a frequency divider. In the phase comparator, the phase of an external HD as an input signal obtained by shaping the pulse of a horizontal synchronization signal is compared with the phase of an internal HD as a reference signal obtained by dividing the frequency of the output pulse of the VCO in the PLL circuit. In case the phases of the signals differ from each other, an error signal is output from the phase comparator. A first counter counts the period in which the error signal is active, or the time when the phase difference persists. A horizontal synchronization decision section determines the lock state of the horizontal synchronization signal depending on the length of the phase difference duration and in case out of synchronization is detected, a video switchover detection signal is output from an output circuit.