Patent GB-753442-A (GB)
Title: Improvements in or relating to cathode-ray tubes having beam position indicating means
IPC: H04N9/24
Filed: 1951-11-29
Abstract: 753,442. Colour television. PHILCO CORPORATION. Nov. 29, 1951 [Dec. 1, 1950], No. 28030/51. Class 40 (3). [Also in Groups XL (a) and XL (c)] A cathode-ray tube, preferably a tri-colour receiver, has beam position or " indexing " means forming part of or positioned closely adjacent to the screen and having a secondary emission ratio substantially different from that of other parts of the screen, the differences in secondary emission ratio being utilised to produce an output signal which is indicative of the position of impingement of the beam on the screen. In one form, Fig. 1, the tube has video grid 4, first anode 3, second anode 5 formed as wall coating, screen 8, focusing coil 6, deflecting yoke 7. The screen 8 comprises groups of three red, blue and green phosphor strips 22, 23, 24, Fig. 3, formed on a glass plate 27 which may be the envelope wall, and are coated with a thin conductive electron permeable film 28, indexing strips 29, 30 being deposited on film 28 over the gaps between the groups of phosphor strips. The film 28 is formed by depositing a lacquer film on the phosphors, settling the conductive film over the lacquer film and baking out the lacquer, and the indexing strips 29, 30 are formed by evaporation of metal through a mask. The indexing strips may be of gold and the film of aluminium, or vice versa, to give substantially different secondary emission ratios. Alternatively the indexing strips may be of a mixture of caesium, caesium oxide and silver. The indexing strips and film may each have a plurality of different secondary emission ratios provided they lie in mutually exclusive or alternating ranges. The input is a dot sequential colour signal. In a modification, the phosphor strips may be dispensed with. In another form, Fig. 4 (not shown), the indexing members are constituted by a grid spaced from the front surface of the screen, in which case conductive film 28 is dispensed with. The glass base-plate has a conductive inner surface, e.g. by evaporating a thin film of tungsten thereon, to allow an external potential to be applied to the screen. The indexing grid wires are of gold, platinum or tungsten or coated with secondary emissive material. In another form, Fig. 5, the screen has glass plate 27, conductive layer 31, groups of red, blue and green glass filter strips 33, 34, 35 and white fluorescing layer 36, metal strips 37 on edge and of secondary emissive material or coated therewith serving as indexing elements. The secondaries emitted by screen 8 in any of the preceding forms are collected by second anode 5. The output voltage is fed to an amplitude discriminator such as diode 40 biased to beyond cut-off by adjustable tap 41 on potentiometer 43 so that the diode only passes the indexing signals which have larger amplitude than signals from the remainder of the screen. If the indexing strips have relatively small secondary emission, impingement of the beam thereon results in no output across resistor 41. Other forms of discriminator, or two of the above discriminators connected in parallel may be used in particular cases. The utilization device 54 may supply signals to deflection generator 19 to render the beam sweep linear as described in U.S.A. Specification 2,476,698. In this case the corrected generator 19 may supply deflection voltages to a second similar cathode-ray tube in which event the phosphor strips may be dispensed with in the first tube. Alternatively the system may be similar to that described in U.S.A. Specification 2,457,911 in which the first cathode-ray tube has a constant intensity beam, and the second tube a variable intensity beam. In another form, Fig. 6 (not shown), the output indexing signals are fed via an amplifier to a delay line made up of filter sections and terminated non- reflectively, from spaced points on which signals with different delays render conductive three respective pentagrid valves so that red, blue and green video signals applied to the third grid of the respective valves are fed to the control grid 4 at the correct moments. The screen, Fig. 8, is similar to that of Fig. 3 except that the indexing strips 29 are broader than the phosphor strips 22, 23, 24 and spaces are left between phosphor strips of each group. The initial beam intensity is reduced by the colour signals so that the indexing signals are always of greatest amplitude (as the indexing strips have greater secondary emissivity than the remainder of the screen). The amplifier may amplify the indexing pulses without distortion, or alternatively only amplifies the fundamental frequency thereof in which case the output is a sine wave. The sine wave may or may not be converted to pulses before applying to the pentagrid valves. In another form, Fig. 11, the indexing strips 29, 30 are electron permeable and are located in front of the green phosphor strips 24. This screen is used in the circuit arrangement of Fig. 10 (not shown) in which a ring electrode separate from second anode 5 collects the secondaries from screen 8. The ring may be of the same material as second anode 5 or of silver. When no colour signals are received the beam is of minimum intensity and a light attenuating glass plate outside the tube wall reduces the light visible to an observer to zero. The indexing signals are amplified, limited and passed through a phase-shifter to control red, blue, and green signal sampling circuits which may be similar to those of Fig. 6. An additional phase inverter is inserted between the sampling circuits and video grid, so that the beam current is increased by the video signals. The gain of the green signal sampler may be greater than that of the other samplers to compensate for the loss in intensity of green light due to the beam passing through indexing strips 29, 30 before impinging on the green strips. It is shown that by properly proportioning resistors in the circuits connected to the screen and secondary electron collecting ring the output voltage can be made to vanish when the beam impinges on conductive layer 28, Fig. 11, despite variations in beam current representative of video signals. Thus the output signal is only indexing pulses. The amplifier and limiter passes only the fundamental component of the indexing pulses, i.e. a sine wave, and renders the amplitude substantially uniform. The indexing strips need not be vertically disposed, transversely scanned as particularly described, but may be shaped and arranged in other ways. The beam need not be intensity-modulated, and the indexing signals may be used for other purposes, i.e. generally in beam position indicating systems. Specifications 753,441, 753,444 and 753,555, [Group XL (a)], also are referred to.