Patent GB-1477686-A (GB)
Title: Reproducing recorded vidio signals
IPC: G11B20/06
Filed: 1974-08-02
Abstract: 1477686 Colour television NORTH AMERICAN PHILIPS CORP 2 Aug 1974 [7 Aug 1973] 34178/74 Heading H4F Dropouts in a video signal, including a carrier frequency-modulated by a luminance signal, reproduced from a record medium are detected utilizing first and second frequency dividers 8, 9 receiving the modulated carrier Ey and providing respective signals Ey( +), Ey(-), Fig. 2, of half the carrier frequency whose consecutive zero crossings correspond to the zero crossings of the rising and trailing edges respectively of the modulated carrier, these signals being fed via respective frequency demodulators 12, 13 to a comparator 14; a difference between the signals fed to comparator 14 indicates a dropout. Indication of a dropout may cause operation of a conventional compensation circuit or comparator 14 may be connected to a logic circuit 18 controlling a switch 19. The switch 19 receives the output signals of frequency demodulators 12, 13 and, in the absence of large dropouts, the output of a frequency demodulator 3 receiving the modulated carrier. Logic circuit 18 also receives the output of further comparators 15, 16 each receiving the demodulated carrier from 3 and an output from a respective one of the demodulators 12, 13. If comparator 14 detects equality between its inputs switch 19 connects the demodulated carrier from 3 to output 23. If comparator 14 detects inequality between its inputs, indicating a drop-out, comparator 15 detects equality and comparator 16 detects inequality, the output of demodulator 13 is connected to the output 23, as comparator 15 indicates that the outputs of demodulators 3 and 12 experience the same disturbances. If comparators 14 and 15 detect inequality and comparator 16 detects equality, the output of demodulator 12 is connected to the output 23. If the comparators 15 and 16 both detect inequality the output of demodulator 3 remains connected to the output 23. To compensate for large dropouts a one-line delay 5, frequency demodulator 6 and switch 4 controlled by comparator 14 or a separate dropout detector 7 may be included, detection of a large dropout causing operation of switch 4 to supply the delayed signal from demodulator 6 to the switch 19 which is controlled to pass this signal to the output 23. To compensate for dropout in the chrominance component of a signal in which a low frequency chrominance component has been added to the carrier frequency-modulated by the luminance signal and the resulting signal limited the apparatus may include, in addition, a circuit which includes two frequency dividers (34, 35, Fig. 5, not shown) operating as do frequency dividers 8, 9 each connected to a respective frequency demodulator (34, 35) which supply signals Y( + )-Ec<SP>1</SP>(+) and Y(-) + Ec<SP>1</SP>(-) respectively, Y(+) and Y(-) being the same as the signals supplied by frequency demodulators 12, 13, Fig. 1, Ec<SP>1</SP>(+) and Ec<SP>1</SP>(-) being the derivatives of the chrominance information Ec(+) and Ec(-) contained in the zero crossings of the rising and falling edges respectively of the reproduced composite signal. The luminance components are removed in respective subtracters (36, 37) receiving signals from the demodulators 12, 13, respectively, the resulting signals being integrated (38, 39) to provide the chrominance information Ec( +) and Ec(-). The chrominance component Ec derived from the reproduced signal by a filter (31) is fed, together with the chrominance information Ec( +) and Ec(-) to a switch (45) similar to switch 19, Fig. 1, which is likewise controlled by a logic circuit (44) receiving the outputs from three comparators (40, 41, 42) receiving respectively the signals Ec<SP>1</SP>(+) and Ec<SP>1</SP>(-) or Ec(+) and Ec(-); Ec and Ec(-); Ec and Ec(+).