GB542820A - Improvements in or relating to television systems - Google Patents

Abstract

542,820. Television contrast amplifying systems. BAIRD TELEVISION, Ltd. Aug. 28, 1940, No. 13573. Convention date, Aug. 29, 1939. [Classes 40 (iii) and 40 (v)] A contrast amplifier for television reception for expanding the contrast range of an image on the screen of a cathode-ray tube is adjusted to produce a linear overall response with a γ greater than unity. In one arrangement, Fig. 3, negative bias is applied to the grid of a triode 1 through a D.C. restoring device D, and television signals in which positive represents white are fed to the input terminal 3. The bias may be chosen so that, in addition to the desired contrast amplification being obtained, the negative synchronizing pulses are cut off. In another arrangement, Fig. 4, two valves 5, 6 are connected in series, resistances 11, 12 are inserted in their cathode circuits, the anode of valve 5 is connected by a condenser 13 to the grid of valve 6 which is connected by a resistance 14 to its cathode, and a diode 7 with a resistance 10 is inserted between the cathodes. Input of positive polarity with its D.C. component is supplied to terminal 8 of grid of valve 5 and output is taken from terminal 9 of anode of valve 6. For weak inputs the diode is non- conductive and negative feed back keeps the amplification low. When the signal increases sufficiently to make the diode conductive, the effective resistance in the cathode circuit of valve 5 and therefore negative feed back is reduced and amplification increased. The resistance 10 controls the change in amplification without affecting the transition level. The input circuit of valve 5 may have the biasing arrangement of Fig. 3.