Patent GB-611713-A (GB)
Title: Improvements in television transmitters
IPC: H04N5/228
Filed: 1945-01-18
Abstract: 611,713. Television transmitters. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Jan. 18, 1945, No. 1544. Convention date, May 13, 1943. [Class 40 (iii)] In. a television transmitter, in order to remove " dark-spot " signals causing, at the receiver, an uneven shading of the reproduced image, shading-control potentials at line and frame frequency are generated, the line-frequency signals being mixed with the video signals, whereby the horizontal component of the shading in the reproduced image is removed, and the frame-frequency signals being combined with the resultant image signals by means of an electronic switch which is rendered conducting during the blanking intervals, so as to set the blanking signal level at the level of the frame frequency signals whereby the vertical component of shading is also removed. The apparatus of Figs. 1a, 1b will be considered first with the arms of both banks of the switch 51 connected to the second switch contacts, the first contacts being used, as shown, only when the video signal output from the iconoscope 10 contains no " dark spot " signals. A lamp 20 is placed behind the iconoscope; this illuminates the walls of the tube so that a signal corresponding to picture white, termed the " white level " signal, is produced during the fly-back periods when the cathode ray to the mosaic is cut off by the signals 34. To prevent light from the scene being televised from falling on to the walls of the tube and thus causing the amplitude of the " white level " signal to change with change of scene a mask 19 is provided. By means of the back lighting and the mask, the dark spot signal may be reduced considerably and also made independent of the scene being televised. The iconoscope output is amplified at 28 and 30 and then mixed with blanking signals 48 from circuit 46 which is so adjusted that the " white level " signals are suppressed to the black level. The combined signals are amplified at 42 and applied to cathode follower 50, the output of which is combined with horizontal shading signals from the circuit 160. The horizontal shading signals comprise saw-tooth and parabolic wave-forms suitably combined to remove that component of the dark spot signals producing variations in amplitude in any one scanning line. The partially corrected signal is applied through volume control resistor 58 to amplifiers 62 and 64 at the output of which it is combined with further blanking signals 90 which ensure that the signal during the blanking intervals is driven a predetermined amount into the blacker-than-black region. The combined signal is applied through switch S1 to the grid of clipper valve 92 to which a bias voltage is applied through an electronic switch circuit. As described in greater detail below, this bias voltage is so varied in accordance with vertical shading signals from circuit 172 that the vertical components of the dark spot signal are removed. Synchronizing signals 126, 128 are mixed in double triode 122 and combined with the output of valve 92 which is then applied to output stage 138. The circuit 172 supplies saw-tooth and parabolic wave-forms which are suitably combined and applied to potentiometer 228 which is ganged to potentiometer 58 so that the correction signal varies with the amplitude of the image signal. Thence it is applied to one end of the electronic switch circuit comprising diodes 188, 190, 192, 194. Switching potentials synchronized with the blanking signals are applied to the switch from triode 200 so that during each blanking interval the grid of valve 92 is set at the potential of the slider 116, which supplies a steady adjustable bias, plus the vertical shading potential then obtaining. The slider 116 is so set that in the absence of dark spot signals the valve 92 would cut off the negative portions of the image signals at the blanking level of the output of amplifier 64. The actual clipping level varies with the vertical shading signal so that the dark spot signals are removed at the grid of the valve 92. The switch S2 in the cathode circuit of the valve 92 serves as a gamma control; with the switch closed the valve has a square law characteristic and with the switch open its characteristic is linear. In an alternative manner of carrying out the invention, the arms of the switch S1 are set on the third switch contacts. The vertical shading signals are, in this case, applied to the circuit 46 to modulate the amplitude of the blanking signals 48. Since the vertical shading signals are not applied to the electronic switch, the valve 92 cuts off at the blanking level of the output of 64. This level, however, varies in accordance with the vertical' shading signals. Thus the dark spot signal is removed as before at the grid of valve 92. The Specification as open to inspection under Sect. 91 of the Acts describes a further manner of achieving the object of the invention, viz. the removal of " dark spot " signals, wherein the vertical shading signals from circuit 172 are applied to an electrode 240 of the camera tube to modulate its efficiency and thus effectively prevent the production of the vertical component of the " dark spot " signals. The horizontal shading signals are inserted as before. This subject-matter does not appear in the Specification as accepted.