GB783185A - Improvements in or relating to cathode ray tube apparatus - Google Patents

Abstract

783,185. Automatic control systems for cathode-ray tubes. CINEMA-TELEVISION, Ltd. Oct. 7, 1955 [Oct. 7, 1954], No. 28914/54. Class 38 (4). [Also in Groups XL (a) and XL (c)] In a cathode-ray tube circuit wherein the target is divided up into a number of minor areas 1-9, Fig. 1, and in which the beam can be centred by steady deflecting fields on any one of the minor areas, registration errors in the scanning master are corrected by prescanning the beam, after selection of a desired minor area, to scan a small area 12 of the minor area centred on a register mark 11, any deviation from this centring being converted into an error signal which is used to correct the beam scan of the whole minor area which follows the predeflection. The target may be of an information storage type, as shown, or a tube in which the optical image from a luminescent screen is projected on to a target having symbols of varying translucence or reflectance so as to give output signals in an associated photosensitive member. The beam is centred on any desired minor target area by suitable D.C. deflecting currents and scanning currents are then superimposed for effecting the prescanning and scanning. The register marks are rectangular, e.g. square, dots and are at least as big as one picture element, one corner of each dot coinciding with the centre of its minor target area. As the beam is prescanned it develops a signal as it passes over the register mark and the phase of this signal indicates the error of registration of the prescan, this signal being fed to a comparator 40 and a resulting registration correcting signal being fed to line and frame deflection amplifiers 29, 29<SP>1</SP> to centre the beam on one corner of the registration mark. The output signal from the registration mark during the pre-scanning is fed via amplifier 27 and gate 39, to terminal 47, Fig. 3, of a comparator circuit 40, 30, Fig. 2, comprising all the circuit shown in the Figure. This signal renders valve 49 operative to transmit negative pulse to diode clamp circuit 42-45 while a positive pulse is similarly applied to the clamp circuit from valve 50. These pulses cause the diode clamp circuit to operate to bring the grid of valve 46 to the potential of terminal 41. The signal applied to 41 has a saw-tooth potential wave balanced to ground from the scan generator 33, Fig. 2, and is zero when the reference mark signal is generated if the pre-scanned area 12 is properly centred about the reference mark ; if the registration is not correct a positive or negative potential is applied to the grid of tube 46 equal to and due to the potential applied to 41 and a potential drop proportional to the registration error is produced across resistor 58. The correcting potential can thus be taken from terminal 67 and is applied to the line or frame deflection amplifiers 29 or 29<SP>1</SP> to centre the beam on one corern of the registration mark condenser 56 ensures that the grid of valve 46 remains at the same potential while the beam is scanning the minor area, since then the diode clamp circuit is inoperative. The grid of an additional valve 59 can be biased to any of several potentials by switch 63 and potentiometers 60, 61 or 62, or by a suitable pulseoperated circuit. Registration correction is effected for both line and frame scan directions, as indicated in Fig. 2, where the control signal giving information as to which minor area to be scanned is applied at 37<SP>1</SP> to circuit 37 which controls gates 31, 32, 35. Circuits 33, 34 are scanning generators for the pre-scanning and scanning respectively and control device 38 transmits gating signals to the correcting signal generators 30 fed by comparators 40 to ensure application of the registration correction signals at the right times to line and frame deflection amplifiers 29, 291. Since an output signal in the main load circuit of the tube 21 is not desired during the pre-scanning of area 12, gate 35 is only opened during the scanning ; gates 31, 32, 35, 39 are synchronously operated. The dimensions of a register mark may be less than those of a picture point so that signals from the register mark are of too high frequency for the external amplifier circuits during the scanning of the whole minor area. The output of deflection amplifiers 29, 29<SP>1</SP> is preferably proportional to the algebraic sum of their inputs as described in Specification 761,461, [Group XL (c)]. The tube 21 is a monoscope and is scanned by electromagnetic means. Electrostatic scanning may alternatively be used. For asymmetrical deflection each register mark is near one corner of its minor target area.