GB434496A - Improvements in or relating to television and like receiving systems - Google Patents

Abstract

434,496. Television; valve amplifying circuits. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., Electra House, Victoria Embankment, London.-(Assignees of Holmes, R. S.; 164, Merion Avenue, Haddonfield, New Jersey, U.S.A.) Feb. 28, 1934, No. 6510. Convention date, Feb. 28, 1933. [Classes 40 (iii) and 40 (v)] In a television system employing a cathode-ray receiver and in which synchronizing signals and picture signals are transmitted over the same channel, the synchronizing signals being of opposite sense to the picture signals, the average illumination or background brilliancy is controlled by means which separates from the combined signal that portion which lies on the same side of the " zero " line thereof as do the synchronizing signals, the said portion being rectified and utilized to control the intensity of the spot. Picture signals 34 as indicated in Fig. 2 are combined with synchronizing impulses reversed in phase to produce a signal wave of the form illustrated in Fig. 4 which is received at 40 and applied to an alternating current amplifier 42, 44, 46, The output from the valve 46 is fed to the grid 26 of the cathoderay tube 10, the picture signals controlling the intensity of the beam and the synchronizing impulses cutting off the beam during its return movement. The combined signal wave is also fed to a valve 50 having its grid normally at zero potential and controlled by resistances 51, 53 such that the picture signals produce no plate current whilst the synchronizing impulses produce impulses corresponding to their amplitude in relation to the "zero" axis 47, Fig. 4, dependent upon the general level of the picture signals. These impulses which control the generators 22, 24 of saw-tooth wave forms for the line and picture scanning are rectified by the diode part of a diode-triode valve 52. When the amplitude x of these impulses increases, the plate current of the valve 52 is reduced, with reduction in the potential drop across resistance 60, so that the bias on the grid 26 becomes less negative with respect to its cathode 28. In an alternative arrangement, Fig. 5, the valve 46 is connected to the output of the valve 42 and the valve 44 is operated with a high negative bias derived from the plate current of the valve 42 and a resistance 63 so that it operates as an anode bend rectifier. The synchronizing impulses are arranged to be positive on the grid 64 of the valve 44 of which the plate current varies with the effective amplitude of the synchronizing impulses. The potential at the point 66 is directly applied to the control grid 26 of the cathode-ray tube. In an alternative arrangement, Fig. 6, (not shown), the output from the valve 42 is fed to a grid-bias rectifier valve biassed to a point near its cut-off so that with increase of the effective amplitude of the synchronizing impulses the plate current increases and makes more negative the cathode 28 of the tube 10 connected to the plate circuit of the valve. In a further arrangement, Fig. 7, (not shown), the output of the valve 42 is fed to a valve arranged to operate as a grid-leak rectifier, the output circuit of which is connected to the grid of the cathode-ray tube which becomes more positive with increase of the effective amplitude of the synchronizing impulses. Specification 395,499 is referred to.