Patent GB-1533291-A (GB)
Title: Blanking circuit for a video signal
IPC: H04N5/228
Filed: 1976-03-31
Abstract: 1533291 Video blanking circuit NORTH AMERICAN PHILIPS CORP 31 March 1976 [3 April 1975 24 June 1975] 12951/76 Heading H4F Errors present in a video signal, due to straylight in a television camera, are corrected by a blanking circuit. Video input 12 receives the video signal which is input to amplifier 10 the output of which is applied to input 18 of amplifier 20 via FET 14 during blanking intervals. Amplifier 20 also receives the integrated 28, 30 output of amplifier 10. Amplifier 20 applies a D.C. signal to an input of amplifier 10 that is a function of the difference voltage between inputs 18 and 22. At input 18 it receives the original blanking level, and at input 22 the adjustable bias voltage from the potentiometer 24 and the output voltage of amplifier 28. Thus potentiometer 24 is adjusted until the nominal black level at the output of amplifier 10 is at zero volts with the lens capped so that there is no straylight and therefore no straylight correction signal from amplifier 28. However, because of the straylight, when the cap is removed, the real black level in the video signal is not at zero volts, but at some other value. Since amplifier 28 also applies to input 22 a D.C. value in accordance with the straylight, the real black level will be set to zero volts, thus compensating for the straylight. The amount of the compensation is determined by potentiometer 26 in accordance with the amount of straylight present. In order to maintain the change in the black level in the subsequent video processing amplifiers, which may be A.C. coupled, it is essential to blank the video after the D.C. has been added. For this purpose a video output amplifier is provided, which substitutes during blanking, a blanking level, which is at ground potential. During the blanking time, a negative going blanking pulse is applied to terminal 68 which cuts off transistor 64. Therefore there is no current through resistors 62, 42 and 44, and thus since resistor 44 is connected, to ground, resistor 42 is at ground potential, as is the cathode of diode 41. Diode 41 will then block from the output terminal 40 any signal at the emitter of transistor 38 that is lower than ground potential. An extra current to the base of transistor 34 is provided during blanking by resistor 46 and diode 54. This causes the voltage at the collector of transistor 34, the base and emitter of transistor 38, and the anode of diode 41 to go lower than ground. Hence diode 41 clips the blanking portion below the ground potential, and therefore, eliminates the original signal during the blanking interval including any noise present during the blanking interval. Therefore the clipping action is exactly at the black level, during the blanking time. During the video signal portion transistor 64 conducts and draws current through resistor 62. In addition, the bias current through resistor 46 is diverted from the diode 54, and flows through resistor 76 and transistor 64 to the minus 10 volts source. This forward biases diodes 60 and 41, lowering the clipping level to slightly below black level. Therefore, if the actual signal is at black level, no clipping action will take place. The video signal is then amplified by a factor of two to compensate for losses in resistors 42 and 44. The amplification is determined by the ratio of resistors 58 and 31. The amplification is linearized for the non-linear characteristic of diode 41 by the feedback resistor 58. Diode 56 closes (i.e. establishes) the feedback loop during clipping, since otherwise transistor 34 would saturate and there would be a slow recovery. In addition, the base of transistor 34 would not remain at virtual ground, which would cause a feed-forward signal to flow through resistor 58 during clipping.