DETECTING DEVICE FOR TIME BASE FLUCTUATION

PURPOSE:To start quickly the detection of the time base fluctuation after this fluctuation is over by adding the reference voltage to a PLL loop to a reset it in a synchronism detecting circuit if the disturbance of synchronism is detected for the PLL loop which performs the phase matching between a reference horizontal synchronizing signal and the output signal of a voltage control oscillator. CONSTITUTION:The 1st PLL loop comprising a divider 18, a phase comparator 13, an adder 7 a VCO 4 and a divider 19 performs the control to secure the phase matching between a reference horizontal synchronizing signal and the output signal of the VCO 4. While the 2nd PLL loop consisting of a phase comparator 3, a changeover switch 21, an LPF 5, an adder 7 and the VCO 4 carries out the control to secure the phase matching between the output signal of the VCO 4 and a reproduced horizontal synchronizing signal within a low frequency range where the signal passes through the LPF 5. When a synchronism disturbance detecting circuit 20 detects the disturbance of both PLL loops, the LPF 5 is connected to a reference voltage generating circuit 22 via the switch 21. Thus the state of the 1st PLL loop is quickly reset. Thus it is possible to ensure the stable correction of a time base even in case a video signal having the inferior S/N is reproduced.