GB444074A - Improvements in or relating to television - Google Patents

Abstract

444,074. Television. BROWNE, C. O., 29, Monk's Drive, West Acton, London, and ELECTRIC & MUSICAL INDUSTRIES, Ltd., Blyth Road, Hayes, Middlesex. Sept. 12, 1934, No. 26249. Addition to 404,020. [Class 40 (iii)] Variations in the average or the peak strength or both, of picture signals, are automatically counteracted in a television transmitter of the type in which scanning is effected by an electron beam, the picture elements being stored either on a mosaic photo-electric screen, or on a mosaic screen associated with a separate photosensitive surface as described in Specification 442,666, or on a mica screen carrying photosensitive material. In a transmitter, Fig. 1 (not shown) in which the image is projected on to a photoelectric surface and signals are passed capacitatively to a conductive plate, the picture signals are applied to an output amplifier 11, in shunt with a valve 13 whose anode current governs a solenoid 24 which operates an iris diaphragm 28 to control the luminosity of the image projected on to the photo-electric screen. The arrangement is such that the grid current flows when the picture signals reach their maximum positive value, corresponding to the darkest parts of the picture, and as a result the diaphragm responds so as to maintain a steady mean brightness of the projection. Alternatively the diaphragm may respond to the maximum brightness represented by the signals, or to the mean brightness or to mean and peak brightness in combination; or the invention may be applied to systems in which a datum amplitudelevel is given to the signals during the periodical interruption of the scanning beam. In a modification, Fig. 3 (not shown), the diaphragm is controlled by a motor controlled by the valve. Specification 446,661 also is referred to.