| 摘要 |
PURPOSE:To attain the phase comarison sensitivity and lock in range on synchronization similar to those of the NTSC system by allowing an APC synchronism circuit to change alternately the 1st and 2nd phase comparator circuit inputs at each 0 deg., 90 deg., 180 deg. and one horizontal scanning period so as to prevent a phase error of 45 deg. with different polarity at each horizontal scanning to an output of the phase comparator circuit from being generated. CONSTITUTION:A color burst signal and an outut signal of a control oscillator 12 are compared at the 1st phase comparator circuit 13 with nearly 90 deg. of phase difference at the synchronism state the same as the NTSC system by using the 1st phase shift circuit 19 and they are compared at almost in-phase by the 2nd phase comparator circuit 21 provided separately from the 1st phase comparator circuit by using the 2nd phase shift circuit 20. Moreover, a polarity discriminating circuit 23 and a polarity switching circuit 24 switch the polarity of an output of a frequency divider circuit 22 so that the phase shift amount of the 1st and 2nd phase shift circuits is 0 deg. and 90 deg. when the phase of a color burst signal is +135 deg., and 90 deg. and 180 deg. when the phase of the color burst signal is -135 deg., and the phase of the PAL color burst signal is discriminated using the output of the said 1st and 2nd phase discriminating circuits 13, 21. |
