| 摘要 |
918,424. Gain control circuits. PHILIPS ELECTRICAL INDUSTRIES Ltd. (Philips' Gloeilampenfabrieken N.V.). April 24, 1959, No. 14069/59. Class 40 (5). [Also in Group XL (b)] The invention relates to a television receiver for negatively-modulated signals, in which the video signal including the D.C. component is applied from detector 4 to a video amplifier 5 which feeds the picture-tube 9 and also an automatic gain control valve 13, to which it is direct-coupled over potentiometer 11, 12. The valve 13 is biased back by cathode bias potentiometer 14, 15 and its anode supply comprises positive-going sync. pulses applied over condenser 18; it is driven conductive by the corresponding positive-going pulses applied to its grid from the anode of the valve 5 during sync. pulse periods, to a degree dependent upon the signal amplitude. The negative potential appearing at the anode of the valve 13 is taken off via potentiometer 20, 21, the voltage across resistor 21 constituting the A.G.C. voltage. Contrast is controlled by varying the screen-grid potential of video amplifier 5, by means of potentiometer 22. In such an arrangement, due to the A.G.C. operating on the peaks of the sync. pulses and maintaining their level constant, variation of the contrast control causes an undesirable shift in the black level. According to the invention, this is compensated by feeding part of the variable contrast control voltage into the cathode circuit of the A.G.C. valve, via resistor 24, so as to also vary the A.G.C. delay. In a second embodiment (Fig. 2, not shown) contrast is controlled by a variable resistor in the cathode circuit of the video amplifier and the compensatory variable delay voltage has to be applied to the grid of the A.G.C. valve. The Specification also refers to the problem of adjusting the contrast to compensate for variations of incident extraneous light on the picture tube. In this case the black level requires to be varied in the same direction as the white level and so that the ratio of white level to twice black level remains constant. According to an alternative embodiment of the invention this may also be done by varying the A.G.C. delay when adjusting contrast, but in the opposite sense to that used in the previous embodiments. |
