GB919128A - Improvements in television-receiving circuit arrangements - Google Patents

Abstract

919,128. Television. PHILIPS ELEC TRICAL INDUSTRIES Ltd. June 12, 1959 [June 14, 1958], No. 20211/59. Class 40 (3) In a television receiver, Fig. 1, variation in background brightness of the reproduced image due to variation in the working point of the video amplifier 4 (to which the cathode of the C.R. tube 7 is directly coupled) consequent upon variation of the screen voltage to adjust image contrast, is compensated by means of a variable-resistor arrangement which supplies two oppositely varying voltages U and V, the first to control the contrast and the second for application to the grid of the C.R. tube to compensate for the shift in cathode voltage. In Fig. 1 the network comprises two ganged potentiometers 8, 12 and ancillary resistors 9, 10, 13, 14. A desired relationship between background brightness and contrast may be obtained by: (1) relative adjustment of potentiometer coupling, (2) adjustment of the ancillary resistors, (3) connection of resistors from the potentiometers sliding contacts to one side of the power supply and (4) connection of a resistor between the two sliding contacts. A second embodiment, Fig. 2, employs a single variable resistor r connected between fixed resistors W and R. Desired relationships between the derived voltages may be obtained by the addition of voltage dividers, as shown, for example, in dashed lines. A third embodiment is shown in Fig. 3 and a fourth in Fig. 4 where resistors 30 and 32 are small compared with 31. The Specification discusses the design of the network shown in Fig. 2 and includes typical component values for each of the networks shown in Figs. 2, 3 and 4. Specification 475,728 is referred to.