Patent GB-607779-A (GB)
Title: Electrical control circuits for scanning operation in cathode ray tubes, oscillographs, or the like
IPC: H04N1/047
Filed: 1945-08-30
Abstract: 607,779. Control circuits for facsimile receivers. STANDARD TELEPHONES & CABLES, Ltd. Aug. 30, 1945, No. 22404. Convention date, Jan. 4, 1943. [Class 40 (iii)] A circuit for generating two trains of sawtooth waves having different and periodically varying repetition frequencies for controlling respectively the line and frame scanning of the electron beam of a cathode-ray oscillograph facsimile signal reproducer comprises means for generating an operating potential, the magnitude of which varies cyclically with a period which is relatively large compared with the period of either train of saw-tooth waves and a separate generator for each of the trains of saw-tooth waves, each of which generators is controlled by said operating potential in such manner as to generate the corresponding sawtooth waves with variable repetition frequency but constant amplitude. The arrangement shown in Fig. 1 is employed for the reception of characters on a cathode-ray tube in a facsimile system in which the transmitter comprises a drum producing impulses corresponding to the shape of a character and rotated at a variable speed as described in Specification 606,138. The transmitter drum 52 at the receiving station is rotated at a variable rate corresponding to that at the transmitter station by a motor 30 and a variable-speed device 57 and has apertures 53, 55 associated with photo-electric cells 54, 56 respectively to produce frame and line synchronizing impulses which are applied to gas-filled valves 60, 61. An arm 63 coupled through reduction gearing to the shaft of the transmitter co-operates with a rheostat 62 to vary the rate of charging of a condenser 66 and the frame -impulse applied to the grid renders the valve 60 conducting and discharges the condenser to terminate the sawtooth voltage applied to the plates 45 of the cathode-ray tube 44. In a similar manner the varying charging voltage is applied to a condenser 73 associated with the valve 61 which is controlled by the line synchronizing impulses to discharge the condenser 73 and to produce the saw-tooth voltages for the plates 46. In a modification, Fig. 3, the long-period control voltage is derived from a valve 80 which during conduction charges a condenser 83, the latter being discharged through a resistor 84 when the valve becomes non-conducting owing to a rise in the cathode voltage. The varying anode potential of the valve 80 is applied to the normally positively biassed cathode of a valve 81 associated with a condenser 88 which is charged over a resistance 89 and is discharged when the valve 81 becomes conducting by reason of the relative fall in cathode potential. The voltage of the condenser 83 is applied through a resistance-capacity network 85, 86 to the cathode of a valve 82 controlling the horizontal deflection. When the valve becomes conducting, condenser 86 discharges, the period of the saw-tooth oscillation being determined by the voltage supplied from the condenser 83. These saw-tooth oscillations may be applied to a differentiating circuit 87 to provide impulses controlling the frequency of the supply to a synchronous motor driving the transmitting apparatus. A coupling condenser 91 between the output of the valve 81 and the positively biassed cathode of the valve 82 ensures that the frame scanning wave commences in phase with a line oscillation.