| 摘要 |
1325866 Colour television RCA CORPORATION 6 Nov 1970 [28 Nov 1969] 58395/69 Heading H4F A PAL video signal having colour reference bursts whose phases are at + 45 and - 45 degrees relative to the -(B-Y) axis on alternate lines, and where some of the colour bursts may be defective has the colour bursts removed at 3 and these bursts are used to synchronize the generation of more reliable bursts which are added at 7 to the rest of the video signal which has passed through a processor 6, the added bursts being altered in phase if necessary to compensate for delay in processor 6. The separated colour bursts from separator 3 are passed to a phase detector 4 which compares their phase with the phase of a crystal oscillator 9 operating at subcarrier frequency, the phase detector 4 controlling a variable reactance network 10 which controls the oscillator 9. The oscillator 9 is connected to the phase detector 4 via a 90 degree phase alternator which alternates the oscillator output through 90 degrees and is operated by a squarewave pulse source 12 operating at half line frequency and timed by the horizontal sync pulses. A PAL identification circuit 14 ensures that the pulse source 12 operates so that the output bursts are in phase and not antiphase with the input bursts. The output from the loop 5 which includes the oscillator 9 is passed through a control network 16 which alters the phase to allow for any delay in processor 6 and then to adder 7 via a gate 17 which gates in the bursts at the correct times. The 90 degree phase alternator 11 may be formed by a resistor and capacitor connected in series as shown in Fig. 2, the resistance (R), capacitance (C) and subcarrier frequency (W/2#) being related by WCR = 1. Instead of the switch 23 of Fig. 2, an input may be provided to each end of the R-C series, a switch being provided in each input to earth each input on alternate lines, the switches being operated by the pulse source 12, Fig. 1. Either the resistor or capacitor or both may be made variable and the attenuation of the output relative to the input may be corrected for and this attenuation may be checked to ensure correct phases of the inserted bursts. In this case the positive and negative values of any fluctuation about the theoretical value for correct phase (attenuation 1/V 2 if WCR= 1) are detected and fed to a differential amplifier which controls the variable resistor or capacitor, which latter may be formed by two opposed varactor diodes. The pulse source 12 may drive inhibitors to earth each output of the differential amplifier on alternate lines. |
