GB607315A - Television transmitting systems - Google Patents

Abstract

607,315. Television. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Dec. 21, 1945, No. 34781. Convention date, July 6, 1944. [Class 40 (iii)] [Also in Groups XX and XL (a)] In a television transmitter utilizing a tube of the type in which an electrostatic image of a subject for transmission is formed on a target electrode (e.g. Orthicon) from which secondary electrons are released in an amount proportionate to the intensity of the electrostatic image, an electrode is provided for collecting the released electrons to produce across an impedance connected thereto a D.C. potential of intensity determined by the intensity of the electrostatic image, said potential operating means for controlling the transmission of image signals produced in the tube in accordance with the average light intensity of the subject for transmission. Photo-electrons liberated by a light-responsive surface 12, Fig. 1, in accordance with the light intensity of an optical image formed thereon are accelerated by a ring electrode 20 towards a target surface 16 between which and the surface 12 is located a fine wire mesh screen 18. Secondary electrons from target 16 are collected by screen 18 and produce a P.D. across the impedance element 52, which varies in accordance with the average light intensity of the optical image, i.e. the D.C. correction voltage. This correction voltage is fed to valve 84, the cathode resistor 78 of which forms part of the grid circuit of the modulator amplifier 66, 68 which is thus supplied with a bias varying in accordance with the potential across the impedance 52. Instead of employing the latter potential to control the transmission level in the above manner it may be utilized for the electromagnetic operation of an iris diaphragm (see Group XX) to maintain the light intensity of the optical image substantially! uniform regardless of the light intensity of the object, Fig. 2 (not shown). For optimum operation of the pick-up tube, the average potential of the screen 18 should be altered by' a small amount in accordance with the intensity of the optical image focused on screen 12. This may be-effected manually by means of potentiometer 54 or may be made automatic by means of the circuit of Fig. 3 (not shown) in which the potentiometer is replaced by a high resistance connected to a source of biassing potential.