Patent GB-708016-A (GB)
Title: Improvements in or relating to apparatus for translating electric signals into legible characters
IPC: G09G1/18
Filed: 1951-08-23
Abstract: 708,016. Local copying or viewing by scanning. EASTMAN KODAK CO. Aug. 23, 1951 [Aug. 24, 1950], No. 19856/51. Class 40 (3) Apparatus for visibly displaying a plurality of characters represented or selected by a series of respective signal voltages comprises means for scanning a replica of a selected character in response to appropriate voltages and generating video signals which are applied to a cathode-ray tube to reproduce the character and means for progressively centring the displayed characters on the cathoderay tube in spatial relationship to one another in the order in which their video signals were generated. The reproduced characters which may be in a line or in a rectangular pattern, may be photographed. By repeated scanning of the selected characters, the visual reproduction on the screen of the cathode-ray tube may be prolonged. In the diagrammatic arrangement, Fig. 1, the characters which can be transmitted are formed of material having low secondary emission qualities on a screen 11 of high secondary emission in a tube 10 and a reproducing tube 12 has a fluorescent screen 13. Horizontal deflection circuits 30, 31 and vertical deflection circuits 33, 34 are controlled by horizontal and vertical sweep generators 32, 35. The video signals are amplified at 36 and a blanking generator 37 is controlled from the vertical sweep generator 35. Appropriate voltages for selecting characters are applied to keying circuits 38, and keying control circuits 39 control a step generator 40 for spacing the characters reproduced. A synchronizing pulse mixer inverter 41 provides pulses to synchronize the reproduction of the successive characters on the screen 13 of the tube 12. The control grid 22 of the tube 12 has a grid return resistor 42 shunted by a diode rectifier 43 to maintain the background level constant. Detailed circuit arrangements.-Voltages represetative of the characters to be selected are applied to terminals A ... E of triodes 44 ... 48 normally non-conducting whilst the left hand sections of five double triodes 54 . . . 58 are conducting. The right-hand sides of the valves 54 ... 58 are normally non-conducting, and a pulse applied from a generator comprising a valve 70 fed with 60 cycle A.C. makes the righthand section of valve 54 conductive, the lefthand side cutting off and remaining in that condition for about 1/300 sec. During the time the left-hand side of the valve 54 is cut off, valve 44 conducts and produces a voltage over cathode resistor 53 proportional to the input voltage at A. When the left-hand side of valve 54 again conducts, the right-hand side cuts off and passes a pulse to the right-hand side of valve 55 which conducts, and by making valve 45 conductive produces across cathode resistor 53 a voltage proportional to the input voltage B. The step generator comprises two double triode valves 76, 77 having a common cathode output resistor 78 across which appear stepped voltages occurring simultaneously with the cutting off of the left-hand sections of the valves 55 ... 58, and of magnitudes dependent on the resistance ratios in four divided potentiometers such as 79, 80 connected in the respective grid inputs of the four sections of the two valves 76, 77. A pulse simultaneous with that applied to the valve 54 is fed to the grid of a valve 81 of the synchronizing mixer and four pulses are applied from the resistance 78 to the grid of the valve 82 providing in all five control pulses for the horizontal sweep generator 32 and also for the vertical sweep generator 35. The input to the generator 32 includes a diode 89 which provides clamping at the grid of the tube 90 and with the use of a larger synchronizing signal, permits a reduction in the return time of the horizontal sweep. The five pulses are also applied to the blanking generator 37. The output of the horizontal sweep generator 32 is applied via a clamping circuit 97, 98 to a double triode valve 95, and outputs taken from cathode resistors 99, 100 are fed respectively to the horizontal deflector circuit 31 of the tube 12 and to the horizontal deflection circuit 30 of the transmitter tube 10. Potentials for the various electrodes for intensity and focusing control of the tubes 10, 12 are taken from potentiometers 113, 114 and 115, 116. Spacing between characters produced on the screen 13 may be varied by means of the adjustable tapping on resistance 78, but this results in displacement of the characters on each side of the centre line of the face of the tube. A circuit for correct justification, i.e. leaving the position of a terminal, e.g. extreme left-hand, character unaltered, is described in connection with Fig. 7 (not shown) in which the step voltage and sawtooth voltage are combined in a triode, and fed from plate and cathode to the grids of two valves whose outputs are 180 degrees out of phase. These outputs are fed to the grids of a double triode and the cathode ouputs are fed to the horizontal deflecting plates of the tube 12 and inversely or oppositely to the electrodes of a double diode which in effect changes the voltage reference point from the average value of the step wave to its base, and maintains the position of the left-hand character.