| 摘要 |
In a VIF circuit for synchronously detecting a picture signal from an AM-modulated PIF signal, the accuracy of a picture signal to be demodulated is maintained even in a state of overmodulation. A comparator (68) determines whether the picture signal exceeds VTH, which is a state of overmodulation. In a period in which the signal is less than or equal to VTH, a switch circuit (56) inputs an output voltage of an APC filter (54) to a VCO (48), and a PLL is formed. The PLL operates so as to lock onto the PIF signal, and reproduces a carrier wave. On the other hand, in a period in which the signal exceeds VTH, the switch circuit (56) inputs an output voltage VSH of an SH circuit (58) to the VCO (48). The SH circuit (58) holds the voltage of the APC filter (54) that is in a locked state in a horizontal synchronizing period and uses the voltage to control the VCO (48), whereby free run and frequency drifting are minimized. |
