Patent GB-1057478-A (GB)
Title: A picture control circuit for a television receiver
IPC: H04N5/59
Filed: 1964-10-27
Abstract: 1,057,478. Television receivers. HAZELTINE CORPORATION. Oct. 27, 1964 [Nov. 20, 1963], No. 43745/64. Heading H4F. In a television receiver in which black level stabilization is achieved by direct-current (D.C.) restoration or by D.C. coupling, overloading of the high-voltage power supply 18 applied to anode 21 of cathode-ray tube 15 due to high beam current on scenes having a high average brightness value is prevented by deriving a signal representative of the average value (D.C. level), which varies from scene to scene, of the video signal and using it to vary the potential of the screen grid of tube 15 to limit the amount of beam current flowing. In a conventional television receiver, Fig. 1, a picture control circuit 14 is included in which the detected video signal, the synchronizing pulses extending in a negative direction from the blanking level, is A.C. coupled to the control grid of tube 15 via capacitor 23 the D.C. component (black level) being restored by D.C. restorer circuit 24 keyed into operation by flyback pulses from horizontal deflection circuit 18 supplied through winding 26. The average value or D.C. level of the D.C. restored video signal is derived at the grid of triode 29 via resistor 28, the A.C. components of the signal being shorted to ground through capacitor 30. Triodes 29 and 31 and resistors 32 and 33 comprise a D.C. amplifying circuit 34 responsive to the derived average value of the video signal in such a way that the potential on the screen grid with respect to the cathode of tube 15 is varied in the same sense as variations in the average brightness value of the transmitted scene. Thus as the average brightness value increases the potential applied to the screen grid increases thus decreasing the transconductance g m  of the tube and reducing the beam current. If the cathode potential of the tube 15 is initially set at a value to correctly reproduce black for a medium average brightness scene i.e. the black level corresponds to cut-off potential of the tube, an increase in screen brightness which causes an increase in potential of the screen grid increases the cut-off potential causing black to be reproduced as a dirty grey. This effect is counteracted by coupling a portion of the variation in screen grid potential to the cathode of tube 15 through resistance divider network 37, 38 to offset the increase in cut-off potential and maintain correct black level operation. Correct black level operation is mtaintained when the video signal represents scenes of low average brightness, there being no overload problem, since the increase in voltage at the cathode offsets the decrease in cut-off potential of tube 15 which would otherwise occur due to the decrease in screen potential. In a further embodiment, Fig. 2 (not shown), the video signal, the synchronizing pulses extending in a positive direction from the black level, is D.C. coupled to the cathode of tube 15 and an increase to screen brightness produces a change in the negative direction in the D.C. level i.e. average level. This negative increase in the D.C. voltage is caused to increase the potential applied to the screen grid, to limit the amount of beam current flowing, and to decrease the potential applied to the control grid to offset the increase in cut-off potential of tube 15 and thus to maintain correct black level operation. In a further embodiment, Fig. 3, the video signal, the synchronizing pulses extending in a positive direction from the blanking level, is A.C. coupled to the cathode of the tube 15 through capacitor 61 being D.C. restored in circuit 62 (similar to circuit 24 in Fig. 1). As the average brightness value of the transmitted scene varies, the level at which the video signal black level is made to appear at the cathode of tube 15 also varies due to the A.C. coupling through capacitor 63. If the average brightness value of the scene increases the D.C. component at the grid of triode 68 changes in a negative direction thus increasing the potential applied to the screen grid of tube 15 which varies the cut-off potential of the tube in such a manner as to track the variations in black level at the cathode caused by A.C. coupling through capacitor 63. In addition to producing this D.C. insertion effect the increase in screen grid potential reduces the average beam current flowing so that overloading is prevented on scenes of high average brightness. In this embodiment the video signal may be A.C. coupled to the control grid instead of the cathode of tube 15.