BEAM INDEX DISPLAY TUBE AND DISPLAY SYSTEM INCLUDING THE BEAM INDEX DISPLAY TUBE

Abstract: A beam index display tube which has a screen structure enabling a circular spot to be used and has vertical beam indexing. The screen (18) comprises a plurality of transversely extending, substantially contiguous lines (36 to 46), each line being composed of a plurality of triplets of different colour emitting elements (R, G, B) and index elements (20) distributed along each line in accordance with a predetermined pattern. The phosphor elements in each triplet are spatially separated from each other, the intervening spaces being left empty, being occupied by an index element or being filled with a black matrix material. Additionally by staggering the triplets between adjacent lines then considered in a direction transverse to that of the spot scanning, no phosphor element is adjacent another phosphor element. Vertical indexing is made possible by the index elements (20) being offset heightwise relative to the direction of scanning. Alternate elements (20) are offset in alternate directions so that it is possible to determine if the scanning electron beam is too high or too low because additional indexing signals are generated.

Description: The present invention relates to a beam index display tube comprising an envelope in which there is provided an electron gun and a cathodoluminescent screen and means for detecting index signals produced in response to scanning the index elements by an electron beam produced by the electron gun and to a display system including such a beam index display tube. Beam index display tubes offer a number of advantages compared to the widely used shadow mask display tubes. These are efficient electron utilization due to the absence of a shadow mask; no convergence errors, no grey-scale tracking difficulties; the tube is less susceptible to magnetic interference and black and white pictures can be reproduced faultlessly. However colour selection is done with an electron beam generally narrower than one phosphor area and in consequence it is necessary for there to be time synchronisation between the electron spot and the colour information. A known proposal for a beam index cathode ray tube is disclosed in British Patent Specification GB-A-806510. Several different embodiments are disclosed of single and triple beam colour cathode ray tubes and in the interests of brevity it is not intended to enumerate each one here. The Patentees of Specification GB-A-806510 are concerned with (1) correlating the impingement of the spot on the screen with the colour information supplied to the electron gun(s) and (2) not using a secondary emitting material to produce an index signal. The screen comprises horizontally or vertically extending triplets of different colour emitting phosphor stripes. Index strips, comprising a material of which the specific conductivity varies upon electron impingement sandwiched between front and rear electrodes, are disposed either between the triplets or midway along the central strip of each triplet. In the case of a single electron beam cathode ray tube, height control can be applied in the case of horizontal triplets with the index strip extending midway along the centre phosphor strip by amplitude modulating (or wobbling) the electron beam spot in a sinusoidal manner and detecting the instants at which the beam spot crosses the index strip. If the time interval between successive index signals is the same then the beam is correctly adjusted heightwise. However if they are not equal then depending on the nature of the time intervals it can be determined if the electron beam spot is too high or too low. If the index strips lie between vertically arranged triplets, then an index signal can be derived as the electron beam scans in the horizontal direction, which signal can be used to synchronize the colour information supplied to the electron gun. It is however not possible to monitor the position of the electron spot both in the horizontal and vertical direction substantially simultaneously. An object of the present invention is to provide a beam index tube which enables monitoring and tracking of the electron spot both in the horizontal and vertical direction to be monitored substantially simultaneously. According to one aspect of the present invention there is provided a beam index display tube comprising an envelope in which there is provided an electron gun and a cathodoluminescent screen, and means for detecting index signals produced in response to scanning the index elements by an electron beam produced by the electron gun, characterized in that the cathodoluminescent screen comprises a plurality of transversely extending substantially contiguous lines, each line comprising a plurality of multiplets of different colour emitting elements and a pattern of index elements, said index elements being offset heightwise relative to each other to overlap partially their adjacent lines. Preferably successive index elements in each line are offset heightwise in opposite directions. Offsetting the index elements heightwise relative to each other and to the scanning beam path enables the horizontal tracking to be monitored continuously whilst enabling vertical indexing to be realised. Height correction is important otherwise the electron beam may excite wrong colour emitting elements. By constituting the pattern of index elements by a repetition of a subpattern of index elements monitoring is simplified. Advantageously the subpatternpitch is a multiple of the multiplet pitch. The position of each subpattern of the index elements with respect to the colour emitting elements is then known. By making the distance between each subpattern equal to the multipletpitch or a multiple of the multipletpitch the position of each index element with respect to adjacent colour emitting elements is then known. Arranging the index elements within a subpattern thus that the distance between successive index elements within a subpattern differs from the distance between successive index elements belonging to neightbouring subpatterns reduces the regularity of the pattern of index elements and renders the index elements more readily identifiable. The relative position of the pattern of index elements with respect to the multiplets is preferably the same in adjacent lines, which simplifies monotiring. By arranging for index elements to be present in say each of n out of every n + m successive multiplets then a good indication of the relative horizontal position of the electron beam is provided. A simple arrangment is for n = 2 and m = 1. Preferably the colour emitting elements constituting a line may be non-contiguous heightwise with the colour emitting elements constituting an adjoining line. In embodiments of the present invention the colour emitting elements constituting a line are offset by half a multiplet pitch with respect to the corresponding colour emitting elements constituting an adjoining line. By the screen comprising staggered multiplets, rather than vertical stripes, having spaces or black matrix material between adjacent phosphor elements, the multipletpitsch is increased and therefore the width of the spot can be greater and more symmetrical for example circular or substantially circular. More particularly the staggered triplets can have a pitch of 1.4 mm at a triplet height of 0.7 mm. Thus a spot having a width of approximately 500 µm and a height of 700 µm is then possible. Experience has shown that such a spot size can be achieved fairly easily for beam currents of the order of 2 mA. As the spot may be round or substantially round then it is possible to use an anastigmatic deflection coil as used in monochrome datagraphic display tubes instead of the type similar to a deflection coil used in an in-line tube. Spot rotation, a problem occurring in usual index-tubes, is not a problem with circular electron beams. Further deflection defocusing can be minimised by using dynamic focusing. It is preferably to index the vertical position of the electron beam if the multiplets comprise vertical non-continuous phosphor elements. According to another aspect of the present invention there is provided a beam index display system comprising a beam index display tube made in accordance with the present invention, means for scanning an electron beam produced by the electron gun over the screen, and circuit means coupled to the index signal detecting means and to the scanning means for producing beam height correcting signals in response to the detected index signals and for supplying the correcting signals to the scanning means for adjusting the vertical height of the electron beam relative to the centre of a line being scanned. An embodiment of a beam index display system according to the invention is characterized in that the scanning means comprises electro-magnetic deflection coils and a velocity modulation coil and in that the circuit means includes a video selector and means for supplying a scan speed signal to the velocity modulation coil for synchronizing the scanning electron beam with the video information from the video selector. The present invention will now be described, by way of example, with reference to the accompanying drawings, wherein: Figure 1 is a perspective view, partly broken away, of a beam index display tube system made in accordance with the present invention, Figure 2 is a vertical cross-sectional view through a portion of the faceplate and the screen, Figure 3 shows part of a cathodoluminescent screen of a display tube in accordance with the present invention, Figure 4A to 4H show various signals derived in operating the beam index display tube shown in Figure 3, Figure 5 is a block schematic diagram of the circuitry for processing the index signals derived from the screen shown in Figure 3 and Figure 6 shows a screen structure in which there are two neighboring index elements, seperated by a distance of 2/5 of a triplet pitch, to every two triplets of phosphor elements. Figure 7 shows another screen structure in which there are two index elements to every two triplets of phosphor elements. Figure 8 shows various signals derived from operating the beam index display tube with a screen structure as shown in figure 7. In the drawings corresponding reference numerals have been used to indicate the same features. The beam index display tube shown in Figures 1 and 2 comprises an envelope 10 formed by a faceplate 12, a cone 14 and a neck 16. On the inside of the faceplate 12 is provided a cathodoluminescent screen 18 comprising a plurality of transversely extending lines. The phosphor elements are arranged in triplets luminescing in red (R), green (G) and blue (B). The triplets are backed by an electron permeable aluminium layer 19. Fast decaying light emitting index elements 20, for example for ultra-violet emitting material, are also provided in accordance with a predetermined pattern and geometrical arrangement on the aluminium layer 19. Example of arrangements of phosphor and index elements will be described later with respect to Figures 3, 5 and 7. In the neck 16 of the tube is provided a single beam electron gun 22 which produces an electron beam 24 having a substantially circular cross section at the screen. The beam 24 is scanned across the screen 18 by means of a deflection coil system 26. In addition to the normal line and frame scanning coils, the system 26 includes a scan velocity modulation coil which modifies the rate of line scanning in response to control signals provided by a control circuit 28. Index signals 30 produced by the index elements 20 in response to electron beam impingement are detected by a sensor 32 disposed on envelop 10 in which a viewing window is made. The sensor may alternatively be disposed in the neck 16. The sensor 32 may comprise a semiconductor photo-electric sensor if the index signals are of a suitable amplitude for such sensors. Alternatively if the index signals have an amplitude which is too low for detection by semi-conductor sensors then the detector may comprise a photomultiplier tube optically coupled to the external surface of the cone. Figure 3 shows a portion of the screen 18. As already mentioned, the screen 18 comprises rows of triplets of phosphor elements R, G, B. The height of each element is substantially equal to the height of a raster line. A space equivalent to the width of a phosphor element, hereinafter termed an elemental space, is provided between adjacent phosphor elements and aligned with certain of these elemental spaces the index stripes 20 are provided on the aluminium layer (not shown) in accordance with a predetermined pattern. The remaining elemental spaces may be left empty or, preferably, filled with a black light absorbing material. A triplet may for instance be composed of phosphor elements of 300 µm width with black interspaces of 170 µm. The screen is produced by photographic techniques, known per se . The triplets of phosphor elements forming adjacent horizontally extending lines are offset or staggered with respect to each other so that corresponding colour emitting phosphor elements in adjacent lines are displaced by half the pitch of a triplet. However the phosphor elements in alternate lines are aligned vertically so that as far as interlaced scanning is concerned each field effectively comprises continuous vertical stripes. However by offsetting or staggering the phosphor elements in adjacent rows then an improved colour resolution for the same video modulation rate is obtained. In particular a relatively large circular beam spot can be used, which spot has a diameter greater than the width of a phosphor element, without causing colour distortion because the adjoining elemental spaces do not comprise phosphor. In Figure 3 the arrangement and disposition of the index elements 20 are such that there are index elements between, or in corresponding positions in, two successive triplets out of every three. The index elements in each line are arranged in a pattern constituted by a repetition of a suppattern, the suppattern being constituted of two index elements being separated by a distance of one triplet pitch, the subpattern pitch, i.e. the distance between corresponding index elements in successive subpatterns being three times the triplet pitch. There is an offset of half the pitch of a triplet between the index elements 20 of one line and the adjacent line(s). Since there is also an offset of half the pitch of a triplet between the colour emitting elements of one line and corresponding colour emitting elements of adjacent line(s) the relative position of the pattern of index elements with respect to the colour elements is the same in adjacent lines. Also the index elements are displaced heightwise so that alternate index elements in a line partially overlap the line above and the intervening index elements partially overlap the line below. With such an arrangement of index elements it is possible to obtain vertical indexing and thereby the possibility to adjust, if required, the path of the spot produced by the electron beam as it is scanned along a line. Figure 3 also illustrates by way of example electron beam spots 36 to 44 having a central trajectory (36); a trajectory which is slightly too high, (38), or slightly too low, (40), and a trajectory which is much too high, (42), or much too low, (44). The trajectories of the beam spots 36, 38, 40, 42 and 44 are indicated in figure 3 by the dashed lines Σ, ↑, ↑↑, ↓ and ↓↓ respectively. The manner and means by which these adjustments and corrections are realised will be described with reference to Figures 3, 4A to 4H and 5 of the accompanying drawings. It will be assumed that a line is being scanned. Figure 4A represents a 40.5 MHz master clock signal produced by a master clock 50 (Figure 5). Figure 4B represents a nominal 13.5 MHz index clock signal produced by an index clock signal generator 52 (Figure 5). Figure 4C shows the output of a triplet counter 54 (Figure 5) which has an output connected to a video selector 56 (Figure 5). Figure 4D shows schematically the index signal, designated Σ, for a spot being scanned centrally in line 46 along a trajectory Σ. Figures 4E and 4F respectively show schematically the index signals for a spot which trajectory ↑ respectively ↓ is slightly too high or too low with respect to trajectory Σ, as represented by the single arrows in the right hand margin of Figures 4E and 4F. 4G and 4H respectively show schematically the index signals for a spot which is trajectory ↑↑ respectively ↓↓ is much too high or much too low with respect to trajectory Σ, as represented by double arrows in the right hand margin of these figures. Before discussing the signal processing, Figures 4D to 4H will be described in greater detail. In Figure 4D as the spot passes centrally along the line 46 (Figure 3) index signals are produced having their leading edges in time slots T5, T11 and T23 corresponding to the index elements in that line. Now if the spot is slightly too high (Figure 4E) it will impinge upon not only the index elements in the line 46 but also those index elements in the line 48 which overlap the top of the line 46 so that additional signals are produced in the time slots T2 and T20. Conversely if the spot is slightly too low (Figure 4F), it will impinge upon not only the index elements in the line 46 but also those index elements in the line 49 which overlap the bottom edge of the line 46 so that an additional signal is produced in the time slot T14. A comparison of Figures 4E and 4F shows that the occurrence and positions of the additional signals distinguishes a signal representing a spot which is slightly too high from a signal representing a spot which is slightly too low. If the spot should be displaced much too high then it will impinge not only on the index elements in the line 46 and those elements in the line 48 which overlap the line 46 but also the other index elements in the line 48 which are displaced upwards and overlap the line above. Thus compared to Figure 4E an additional signal occurs in the time slot T8. In the converse situation shown in Figure 4H, compared to Figure 4F, an additional signal occurs in the time slot T8. Thus the number and relative disposition of the pulses in the respective signals provides an indication of a spot relative to the line being scanned. The fact the index signals overlap their immediately following time slot due to the effect of the persistence of the index ultra violet light is of minor consequence because index signal detection is based on detecting the leading edge. The circuit shown in Figure 5 comprises the index signal detector 32 which produces an electrical signal which is amplified in an amplifier 58. The amplified signal is applied to the block 60 in which it is differentiated and the differentiated signals are compared against a reference level. Those differentiated signals which exceed the reference level are shaped in a pulse shaped also included in the block 60. Thus noise signals and weak signals due to stray radiation are eliminated. In the case of modulating the scan speed so that it is in synchronism with video selector, an operation which may be termed the "X-correction", the 40.5 MHz master clock oscillator 50 has its output frequency divided by three in a frequency divider 62. The signal output from the divider 62 is applied to a phase comparator 64 in which the phase of the pulses from the pulse shaper in the block 60 is compared with the frequency divided reference signal. The phase difference between the leading edges of these signals is used to form an "error" or "correction" signal in an X-corrector 66 which has an output connected to the scan velocity modulation coil. Optionally and/or in addition as indicated in broken lines, the "error" signal may be used to adjust the video selector 56. In the case of correcting the position of the spot heightwise, that is, the "Y-correction", the pulses from the block 60 are stored in a shift register 68 so as to constitute a recent history of the index signals. These pulses are also supplied to another phase comparator in which that are compared with the clock signals produced by index clock signal generator 52. The "error" signal produced by the phase comparator 70 on a line 72 is used to adjust the frequency of the signal generator 52. The output from the signal generator 52 is applied to the triplet counter 54, the video selector 56 and to a delay circuit 74 having a time delay of the order of some nanoseconds. Such a short time delay is introduced to compensate for signal processing delays elsewhere in the circuit. The output of the delay circuit 74 is supplied to the shift register 68 as a shift clock signal. It is also supplied to a pulse generator 76 constituted by six flip-flops. The output from the pulse generator is applied to a group of five shift registers 78 to 86 which store predetermined patterns of pulses corresponding to spot on-line (Σ), register 78; spot slightly too high, register 80; spot slightly too low, register 82; spot much too high or much too low, register 84; and phase, register 86. These registers 78 to 86 are coupled to a comparator 88 to which the output of the shift register 68 is connected. In the comparator 88, the pulse sequence relating to recent history is compared with the contents of the registers 78 to 86 and a decision is made as to whether a "Y-correction" is necessary and, if so, what is the extent of the correction. In consequence an appropriate signal is sent to a "Y-corrector " 90. If phase recognition is achieved in the comparator 88 then a signal is sent on a line 92 to a reset input of the triplet counter 54. The reason for doing this is to synchronise the video selector 56 to the beginning of each triplet by determining the instant of scanning a particular phosphor stripe, say the green stripe G. Figure 6 shows another embodiment of a screen 18 formed by a plurality of substantially horizontal lines constituted by triplets of R, G, B phosphors. The main difference between this screen and that shown in Figure 3 is the number and disposition of the index elements 20. There are two index elements 20 to every two triplets and these are located in alternate spaces between the phosphor elements of the pairs of triplets. The distance between successive index elements within a suppattern is 2/3 of the triplet pitch; the distance between successive index elements belonging to neightbouring suppatterns is 4/3 of the triplet pitch. The advantage of having index elements at a distance of 2/3 of the triplet pitch, i.e. at a distance different from the triplet pitch is that the frequency of the index signal and the frequency of the video signal differ so that coupling between these signals, which may give rise to oscillatory behaviour of the video signal, is reduced. Successive index elements are offset heightwise in opposite directions so as to provide signals for height adjustment. Also in the screen 18 shown in Figure 6 the index elements are displaced laterally by one elemented position proceding from line to line. For the sake if completeness the relative spot positions for on-line, slightly too high and much too high have been shown, indicated in figure 6 by trajectories A, B and C respectively. Figure 7 shows yet another screen structure in which there are two index elements to every two triplets of phospor elements. The distance between index elements within a suppattern is 2/3 of the triplet pitch. Figure 7 differs from figure 6 in that the index elements are displaced half a triplet pitch proceeding from line to line. The advantage is that the relative position of the index elements with respect to the phosphor elements the same is in adjacent lines. For comparison the relative spot positions for on-line, slightly too high and slightly too low have been shown by spots E, D and F respectively. The signals derived for these spot positions are given schematically in figure 8. In processing the signals derived from the index elements using the circuitry shown in Figure 5 the shift registers 78 to 86 store predetermined patterns of pulses appropriate to the arrangement of the index elements. The present invention is applicable to other arrangements of index elements besides those disclosed with reference to Figures 3 and 6 but falling within the scope of the claims. From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the design, manufacture and use of display systems and beam index display tubes and component parts thereof and which may be used instead of or in additional features already described herein, without departing from the scope of the claims.

Claims: 1. A beam index display tube comprising an envelope in which there is provided an electron gun and a cathodoluminescent screen, and means for detecting index signals produced in response to scanning the index elements by an electron beam produced by the electron gun, characterized in that the cathodoluminescent screen comprises a plurality of transversely extending substantially contiguous lines, each line comprising a plurality of multiplets of different colour emitting elements and a pattern of index elements, said index elements being offset heightwise relative to each other to overlap partially their adjacent lines. 2. A display tube as claimed in claim 1, characterized in that successive index elements in each line are offset heightwise in opposite directions. 3. A display tube as claimed in claim 1 or 2, characterized in that the pattern of index elements is constituted by a repetition of a subpattern. 4. A display tube as claimed in claim 3, characterized in that the subpattern pitch is a multiple of the multiplet pitch. 5. A display tube as claimed in claims 3 or 4, characterized in that the distance between successive index elements within a subpattern differs from the distance between successive index elements belonging to neighbouring subpatterns. 6. A display tube as claimed in claim 4, characterized in that successive elements within a subpattern are separated by a distance equal to 2/3 of the multiplet pitch. 7. A display tube as claimed in claim 4, characterized in that successive index elements are separated by a distance equal to the multipletpitch or a multiple thereof. 8. A display tube as claimed in any of the claims 4 to 7, characterized in that there are index elements in each of n out of every n + m multiplets, where n and m are whole numbers. 9. A display tube as claimed in claim 8, characterized in that n is 2 and m is 1. 10. A display tube as claimed in any of the claims 1 to 9, characterized in that the relative position of the pattern of index elements with respect to the colour emitting elements is the same for each line. 11. A display tube as claimed in any of the claims 1 to 10, characterized in that adjacent colour emitting elements are separated from each other by a space having a width of the order of the width of a colour emitting element and wherein the major part of the height of each index element is aligned with its respective space. 12. A display tube as claimed in any one of claims 1 to 11, characterized in that the colour emitting elements constituting a line are non-contiguous heightwise with the colour emitting elements constituting an adjoining line. 13. A display tube as claimed in any one of claims 1 to 12, characterized in that the colour emitting elements constituting a line are offset by half a triplet pitch with respect to the corresponding colour emitting elements constituting an adjoining line. 14. A display tube as claimed in any one of claims 1 to 13, characterized in that the index elements comprise a material which emits non-visible radiation in response to electron beam impingment. 15. A beam index display system comprising a display tube as claimed in any one of claims 1 to 14, means for scanning an electron beam produced by the electron gun over the screen, and circuit means coupled to the scanning means for producing beam height correcting signals in response to the detected index signals and for supplying the correcting signals to the scanning means for adjusting the vertical height of the electron beam relative to the centre of a line being scanned. 16. A display system as claimed in claim 15, characterized in that the circuit means is able to determine, from the pattern of the index signals produced, the degree of misalignment of the scanning electron beam with reference to the centre of a line being scanned. 17. A display system as claimed in claim 15 or 16, characterized in that the scanning means comprises electro-magnetic deflection coils and a velocity modulation coil and in that the circuit means includes a video selector and means for supplying a scan speed signal to the velocity modulation coil for synchronising the scanning electron beam with the video information from the video selector. 18. A display system as claimed in claim 15, 16 or 17, characterized in that the electron gun produces a substantially circular spot at the screen.

Inventors: BARTEN, PIET GERARD JOSEPH
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1988
Main classification: H01J29
IPC codes: G09G1, H01J29, H04N9