GB782929A - Improvements in and relating to television apparatus - Google Patents

Abstract

782,929. Television. FERNSEH GES. Feb. 14, 1955 [Feb. 12, 1954], No. 4296/55. Class 40 (3). A clamp circuit for controlling the direct current component of a " negative modulation " television signal comprises a pair of diodes 3, 4 to which the signal is supplied with the synchronizing pulses extending in the positive direction and which are rendered non-conducting during retrace intervals by means of pulses supplied to the cathodes so that the grid of amplifier 6 is clamped at black level (i.e. earth potential) during the occurrence signals representative of the darkest point in the image which occurs during the picture trace periods but not during the occurrence of the black level during the retrace periods. If this point is not a " black " point the average level of the signal would be depressed so that white would be reproduced as grey and to prevent this a further diode 7, connected across the clamp 3, 4 and the cathode of which is positively biased, conducts when, due to the absence of a true black point during the picture trace period, the signal level drops below the level determined by the positive cathode bias such conduction preventing any further depression of the signal in the black direction. In a modification of the invention a single diode only (such as diode 7), which is not biased is employed and it is arranged (by adjustment at the transmitter) that the video signal potential during retrace periods is at a lower, adjustable, potential than that corresponding to picture black. With this arrangement the darkest image point occurring during picture trace periods is clamped to the black level if the potential representative of such point is higher than the potential occurring during retrace periods. When, in the extreme case of a white image area, the potential representative of the darkest image point is lower than that occurring during retrace periods the former potential can only rise until the latter potential becomes equal to earth potential. In a further arrangement only the diodes 3, 4 are employed and some of the lines of the picture (e.g. at the upper or lower margin) are scanned so that they have an adjustable " grey " value during the occurrence of which the diodes conduct. During the transmission of a " white " picture this " grey " level is clamped to the black level and black picture points, if any, are clamped to the black level as previously described.