| 摘要 |
1536851 Television RCA CORPORATION 26 Feb 1976 [10 March 1975] 9951/75 Heading H4F In a capacitance type video disc record player a two channel defect detector 70, Fig. 1, switches the player into a substitution mode upon the occurrence of instantaneous frequencies outside an expected video bandwidth. In the substitution mode a one line delayed version of the recovered video signal, which is already available for chrominance and luminance comb filtering purposes, is substituted for defective current signal by electronic switching apparatus 29. A phaselocked loop 51 to 59 compensates for disc "jitter". The defect detector 70 employs independent white and black level detection circuits 71 and 73 the outputs of which are added, 75, stretched, 77, and employed to actuate switch controlling means 63. Recovered video is converted to an FM pulse train by detector 21. These pulses trigger a monostable multivibrator 25 the output of which is delayed by an amount equal to a little more than one FM pulse width. The sum of the FM pulse width and monostable pulse with delay is chosen equal to half the period of peak white level. In this manner coincidence between a monostable pulse and the subsequent FM pulse indicates a white defect. Each monostable pulse resets a capacitive charging circuit in the black defect detector 73 which in the event of too long an interval between successive pulses exceeds a reference voltage level indicative of a black defeat. Defect indicating pulses from adder 75 are stretched at 77 so that signal substitution is maintained for a period after the actual defect has passed, in order to compensate for ringing effects following the actual defect. Details of circuitry and logic of defect detector 70 are described with reference to Figs. 2 to 4 (not shown). |
