GB781912A - Improvements in and relating to cathode-ray tube systems - Google Patents

Abstract

781,912. Television. PHILCO CORPORATION. Oct. 7, 1953 [Oct. 10, 1952], No. 27549/53. Class 40 (3). In a colour-television receiver in which synchronization of the scanning of the colour reproducing elements in the picture reproducing tube screen with received colour signals is controlled by means of " indexing " signals derived by scanning " indexing " stripes included in the tube screen with a carrier modulated electron beam, desaturation of the colours by the latter beam is minimized by deriving a control signal from the " indexing " signals and employing it to modify the operating potentials of the picture tube. As shown in Fig. 1, the " indexing " signals, which are developed across load resistor 46 and employed in known manner to control via elements 60, 80 the application of colour signals from terminals 62, 64, 66 to the picture beam control electrode 20 of cathode-ray tube 10, are generated by means of a separate " indexing " beam which is amplitude-modulated at a carrier frequency by means of a pilot oscillator 52 connected to the " indexing " beam control electrode 18 and scanned over the screen 34 synchronously with the picture producing beam. In accordance with the present invention the " indexing " signals are also employed to modify the operating potentials of the picture tube so as to minimize the desaturating effect of the " indexing " beam on the colours produced by the picture beam and this is achieved by means of a control circuit 82 comprising a cathode-follower stage 84 supplied with the "indexing" signals from amplifier 56 and feeding a rectifier 86 which produces a D.C. potential proportional to the amplitude of the " indexing " signal across condenser 110, this potential being added to that of an adjustable bias source 50 so that the D.C. potential between the indexing beam control electrode 18 and the cathode 16 is equal to the sum of these two potentials. In operation, this potential is arranged so that, having regard to the amplitude of the carrier wave from oscillator 52 and the cut-off voltage of electrode 18, an optimum ratio of peak/average current of the indexing beam results such that a clearly defined " indexing " signal is produced without producing undesirable background lighting of the picture (i.e. colour desaturation). Due to ageing of the tube, however, the cut-off characteristic of electrode 18 may change, resulting in a change of the ratio of peak/ average beam current from the optimum value and this change is compensated by the change in the potential of electrode 18 due to the change in the potential across condenser 110 which results from the variation in amplitude of the " indexing " signals when the peak value of the beam current varies. Specifications 745,353 and 781,911, [Group XL (a)], are referred to.