GB2227911A - CRT brightness enhanced by multiple beams - Google Patents

Description

k CATHODE RAY TUBE VISUAL DISPLAY ARRANGEMENT ED 694/JH lhis invention

relates to display arrangements employing cathode ray tubes and in particular to arrangements for increasing the maximum screen luminance beyond that already achievable.

Cathode ray tubes (CRTs) are employed increasingly in aircraft cockpits for producing colour displays and where there is a dual requirement for ruggedness and for high screen brightness in sunlight beam index tubes are often employed in preference to the better known shadow mask tubes.

Shadow mask CRTs provide several limitations to operation due to the fragility of the shadow mask structure and its absorption of considerable electron beam energy from each of the three beams employed to define each colour pixel, dissipation by way of the mask of this energy input at a single point surrounding an aperture through which the beams instantly pass determining the maximum beam current useful in any configuration and thus the screen brightness.

Such shadow mask CRTs can be made to exhibit adequate light output when operated in the so-called cursive writing mode wherein line-figures are drawn that occupy any small proportion of the screen area and surrounded by unaddressed regions that permit the shadow mask to dissipate energy absorbed locally where the figures are drawn through the remainder of the shadow mask.

However, with display information increasingly being generated by computer and often taking graphical form it is required that many display CRTs operate in the raster-scanned mode, wherein a 'spot' (which may be the adjacent spots from each of a triplet of phosphors) is scanned repetitively across the screen in one direction as a line that is displaced for each scan in an orthogonal direction to define a field, or raster, of scan lines. A modified form of shadow mask tube employs a mask comprising slits which extend orthogonal to the line scan direction and the phosphor triplets.

-4 I& - 2 ED 694/JH Clearly a larger proportion of the screen is written to and the electron beams must deflect at such speed that they are on each pixel phosphor for a shorter time. Restoration of local screen brightness by increasing the energies of the electron beams which are superimposed at each point on the shadow mask is inhibited by dissipating the energy through the shadow mask when all points are writtent to in turn.

The beam index CRT on the other hand has no such energy-absorbing mask, permitting a higher intensity beam to strike the screen directly.

It comprises usually an electron gun assembly that produces a single electron beam focussed onto a screen coated with stripes of electron beam excitable phosphors that emit different primary colours arrayed in a repetitive sequence of triplets according to the colours emitted, there being also provided at intervals in the sequence indexing stripes arrayed in the same direction that respond to the electron beam to emit detectable radiation through the tube to a detector.

The beam index tube is operated in the raster scan mode, electron beam modulation being multiplexed between channels of an electrical colour signal associated with the three primary colours of the phosphors of the triplet stripes sequentially scanned in each line, the emission of radiation from the index stripes serving to synchronise the colour signal multiplexing with the impinging of the beam on the correct phosphor stripe.

In order to get sufficient image resolution in the line scan direction the phosphor stripes are narrow, and possibly separated by guard stripes to prevent colour impurities, with the result that there is also a limitation on the permitted spot dimension in the line scan direction.

There is, however, difficulty in focussing a high intensity beam to a spot of small dimensions and even with guard stripes between adjacent phosphors a limitation is placed upon beam current and therefore screen brightness. This is mitigated to some extent by focussing the beam to give a 'spot' which is elongated orthogonally to the direction of line scan, A A ED 694/JH that is, along the stripe in the field scan direction, but this is limited by the separation of adjacent scan lines.

It is an object of the present invention to provide a CRT display arrangement and method of operating such arrangement which permits brighter raster scanned images to be displayed.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which:- Figure 1 is a cross sectional view through a beam index CRT with associated drive circuit of a first form of display arrangement according to the present invention, in which two electron beams in the CRT are superimposed at the screen, Figure 2 is a view. similar to Figure 1, of a second form of display arrangement according to the invention in which the electron beams in the CRT are displaced at the screen and delay means is associated with one of the CRT modulation grids, Figure 3 is a view, similar to Figure 2,of a third form of display arrangement according to the invention showing a different form of modulation signal delay means.

Referring to Figure 1, a visual display device 10 for displaying multiple, that is full, colour raster scan television-type images comprises a beam index (BI) CRT 11 and associated driving and controlling circuitry 12 both of which are in many detail respects well known and only described here in outline.

The CRT comprises an evacuated envelope 13 containing not one but a pair of electron guns 14 1 and 14 2 forming a pair of cathodes, each having associated therewith a control grid 14 1 ' and 14 2 ' and common electron accelerating and beam focussing electrodes (not shown) by which a pair of electron beams BmI and Bm2 are formed between the cathode and a final anode 15 carried adjacent a screen portion 16 on which is formed stripes of red, green and blue phosphors 17 M 17 G3 17 B respectively, separated from each other by guard stripes 18.

The colour-emitting phosphor stripes are grouped in a repetitive sequence or array of triplets each comprising one phosphor of each colour. Reference in the specification to a phosphor by

ED 694/JH colour is intended to refer to the colour of the visible radiation principally emitted by that phosphor when excited by an electron beam, accelerated between the electron gun and anode, impinging upon it.

The beams are focussed so that they impinge on the screen as circular or elliptical spots dimensioned in the direction of stripe width slightly larger than the stripe width, thereby producing the maximum amount of excitation of the phosphor of a stripe on which they impinge whilst the colour purity of the emitted light is preserved by the guard bands separating adjacent stripes of different colour phosphors.

At intervals across the screen indexing stripes 19 are formed of a phosphor or other material which responds to excitation by the electron beams to emit radiation, which may be invisible, or effect secondary electron emission back into the tube envelope and such indexing stripes may be carried on the guard stripes 18 or assume a part of the sequence of colour stripes.

The control circuitry 12 includes a power supply unit (PSU) 20 for producing a potential difference between the anode 15 and cathodes by which each electron beam energy is defined and thus the light output of the CRT or brightness or luminance of the displayed image. In the embodiment shown in Figure 1 the cathodes are aligned with respect to each other such that both beams are brought to a focus at the same spot on the screen and combine to define a light emissive spot whose brightness is determined by both beams. Considering for the moment the operation of one beam, say beam Bml, the image is formed, that is, given contrast, by modulation of the beam current by component 'red', 'green' and 'blue' video signals from some source (not shown). such as a camera or computer graphics generator, at video multiplexer 21 which, in accordance with colour sync. signals from a formatting circuit 22, applies each of the 'colour' signals in turn to a video amplifier 23 1 and thence to the control grid 14 1 ' of the CRT to modulate the electron beam current. The formatting circuit 22 may also -c t A ED 694/JH receive synchronising signals with the video signals from their source.

The circuitry 12 includes a beam deflection generator and amplifier 24, also synchronised with the formatting circuit 22, and deflection coil means 25 which causes the electron beam to effect a repetitive line scan across the phosphor stripes perpendicular to their length, that is, within the plane of the Figure. Orthogonal field scanning circuits also exist but are not shown to avoid complexity.

A photodetector or, if appropriate, a secondary emission detector, 26 is disposed to receive emissions of the indexing stripes 19 and an indexing signal generated therein and applied by way of amplifier 27 to the formatting circuit 22 represents the crossing of the indexing stripes by a line scanned electron beam.

The fixed spatial relationships between the colour phosphor stripes and the indexing stripes enables the formatting means to provide a signal synchronised with the electron beam scanning the red, green and blue phosphor stripes whereby the multiplexer can gate 'red', 'green' and 'blue' video signals in synchronism with the beam crossing the respective phosphor and produce the colour image by varying the electron beam current with those video signals.

The grid 14 2 ' which controls beam Bm.2 is connected to receive modulation signals from the output of the video multiplexer 21 by way of drive amplifier 23 2 so that during each scan both beams are modulated with the same information and the superimposed spots at the screen appear as one.

It will be appreciated that the beam energy impinging on any particular phosphor stripe can be made greater than from any single beam without recourse to an individual beam current which prohibits focussing down to a similar sized spot that avoids excitation of adjacent phosphor stripes.

Thus by the simple expedient of splitting the electron beam energy between two beams and modulating the beams with the same colour signal information a greater beam current and screen brightness of emission can be realised.

0 ED 694/JH In any raster scanned CRT the observed image results from a very brief excitation of any partictilar part of the screen phosphor repeated at field scan intervals, or double field scan intervals if interlacing is used, followed by a relatively short phosphor emission decay interval and a relatively long non-emissive interval which is unappreciated as the observer's vision integrates these repeated emissions into an apparently consistent image.

It will be understood that within.any field scan this image may be reinforced and appear brighter by increasing the light output from the screen by delaying one beam with respect to the other so that the different beams impinge upon the same portion of the screen in turn, delayed in time but modulated with the same image information equally delayed in time.

Referring now to Figure 2 this shows a second embodiment 30 of display arrangement according to the present invention as a cross sectional view through'a beam index CRT and drive circuitry similar to that shown in Figure 1, so that corresponding parts are given the same reference numerals, but differing in that within tube ll' the electron gun/cathodes 14 33 14 4 are aligned with respect to each other and the screen so that the beams M1 and Bm2 impinge on the screen a predetermined distance d apart in the direction of line scan and, in controlling circuitry 12', the provision of a modulation signal delay means 31 connected between the colour signal multiplexer 21 and grid drive amplifier 23 2 The distance d at the screen between the impingement points of the beams is chosen such that they are at corresponding points with respect to the phosphor stripes 17 R$ 17 G3 17 B and the index stripes 18.

In the screen construction shown the index stripes 18 appear at a pitch of two phosphor stripes which, of course, repeat in triplets, so that corresponding points appear at a pitch of six phosphor stripes. Similar consideration will apply to other screen layouts.

The distance d is thus a multiple of six times the phosphor stripe pitch and depending upon the beam line scanning c 1 X 4 ED 694/JH rate defines a delay between the trailing beam Bm2 impinging upon the same point on the screen as beam Bml. The delay means 31 is set to produce such a delay interval into the-modulation signal to grid 14 2 ' so that the same piece of video information modulation both of the beams M1 and Bm2 as they impinge upon any particular phosphor part of the screen, that is, any index stripe.

The delay may be more than one line scan period, but less than the field period, such that the displaced points of impingement are on different scan lines of the raster and in one convenient form the delay may be an exact multiple of the line scan period so that the two beams impinge upon the same stripe but in different raster lines.

The delay means 31 may take any suitable form for providing the delay interval, from a few microseconds when the beams impinge on the same raster line to hundreds of microseconds when they impinge on different raster lines.

For short delays it is convenient to use arrangements which delay the analog modulation signals, such as charge coupled devices, but for longer delays it is preferable to use digital storage techniques, such as shift registers or addressable stores such as random access memory (RAM).

Such digital storage is of most use when the video signals which feed the display are generated digitally in some equipment or computer and the delay means may conveniently be introduced before the digital signals are converted into analog form for application as modulation signals to a control grid.

Referring to Figure 3, this shows a third embodiment of display arrangement 40 according to the present invention as a cross sectional view through a beam index CRT ll' and drive circuitry 12" similar to those shown in Figure 2, so that corresponding parts are given the same reference intervals, but differing in respect of the application of video signals.

The video signals of the three primary colours (RGB) are produced in digital generation circuitry 41, which may be a computer or special video generating circuit card, and are applied by data lines 42 to a RGB multiplexer and digital-to- X 4 ED 694/JH analog converter (DAC) 43, corresponding to the signal multiplexer 21 of arrangements 10 and 30, which receives the synchronising signal from image formatting means 22 to derive and supply to drive amplifier 23 1 analog grid control signals of the colour corresponding to the leading beam Gm2) impingement position determined from the indexing system.

The data lines 42 also supply a delay sore 44 which is controlled through read/write input/output means 441 by adaress means 45, itself controlled or synchronised by the image formatting means 22 so as to store each of the digital video signal words for a predetermined delay interval before applying them to a RGB multiplexer and DAC 46 similar to that 42, but controlled by the address means 45 rather than formatting means 22.

The analog output of DAC 46 is applied to drive amplifier 23 2 and grid 14 2.' with the appropriate delay.

It will be appreciated that the digital delay circuitry and digital-toanalog conversion may take other suitable forms and may, for example, perform the multiplexing as part of the writing to the delay store so that the only signal data stored is that which will be applied to the grid 14 2 Again the duration of the delay is immaterial and as it is a function merely of the interval between writing the video data into, and reading it from, a store, it is readily changeable, even in operation and continuously.

It will be appreciated that it is not unusual to apply corrections to the scanning of a CRT electron beam to correct for various distortions caused by the geometry of the tube and interactions of the deflection fields, that is, the beam deflection forces vary in accordance with the position of the beam in the tube envelope in order to ensure that the point at which it impinges on the screen follows a uniform raster pattern.

It will also be appreciated that when there is more than one electron eam and they are intentionally displaced, as in accordance with the present invention, but subjected to the same deflection forces, the influence of the scan correction forces on the leading beam may cause any trailing beam to be i -t 411 4 ED 694/JH displaced slightly from its nominal scan path to such an extent that it impinges on a 'wrong colour' phosphor stripe or overlaps two phosphor stripes.

The arrangement 40 of Figure 3 may be modified to inchide a correction store 47 under the control of read/write address means 45 whereby the delay applied to the video signal can be varied as a function of the position of the leading beam in the field scan. Conveniently the variable delay may comprise a map of trailing beam variations from its nominal beam position with respect to the leading beam for each position of the leading beam in the raster, that is, for each index stripe or each phosphor stripe triplet crossed, when subjected to scan correction forces.

The variations so mapped may include positional variations which upon reading are related to the scanning rate to define variations in delay interval by suitable processing circuitry (such as computer at 41) and then added to the nominal delay interval or may be stored directly as variations in nominal delay interval. Most simply, however, the variations are stored as the delay intervals each combined with the nominal delay interval, that is, as the actual delay interval to be used for each position of the beam and so signalled by the image formatting means 22.

It will be appreciated that although the invention has been described with reference to a two-beam beam index CRT it is not limited as to the number of beams, how the beams are formed or even CRT type provided the beams all follow the same path in each scan.

For instance, the CRT illustrated has individual electron gun/cathodes to provide the individual beams. A cathode construction may be provided which produces a beam elongated along one axis, usually the direction of stripe extensions, and subject at different regions thereof to individual control grids in the manner of 14 1 ' and 14 2 In such a case the regions of the beam impinge upon the screen to form in effect two separately modulatable spots separated in the field scan direction and operate with a delay interval of

R,z te ED 694/JH one scan line period in a manner corresponding to that described above where two separate beams are so displaced.

Furthermore, the display arrangement of the invention may incorporate a shadow mask CRT in which a group of three beams directed via an aperture of shadow mask at a triplet of phosphors at the screen correspond to the normal single beam of a beam index tube. A shadow mask CRT in accordance with the invention may be provided with two or more groups of three beams, each group being directed to different parts of the shadow mask such that each does not input too much energy at any one point of the shadow mask and provides for improved dissipation through the mask.

Alternatively, a single group of three beams may be aligned such that they are directed to different but close apertures of the shadow mask whereby each beam falls on a different portion of the screen as covered by the rjaster scan and can be of increased intensity due to the time- separated impinging of all three beams on the same part of the screen, which provides all of the visual information for that part with little corruption of colour priority due to excitation of the phosphors at slightly different times.

The form taken by the embodiment 10 of Figure 1 in which the plural beams are superimposed is not, of course, applicable to the shadow mask tube in respect of superimposed groups of beams, for the reasons of heat absorption problems discussed above.

It will be understood that within the context of this specification the reference to the same modulation being applied to more than one beam means the same instantaneous part of a modulation signal that moves as a function of time is applied to both beams not necessarily that modulating one of equal signal level.

I z 1.

ED 694/JH

Claims (17)

1. Claims

   A visual display arrangement comprising a raster scanned cathode ray tube having a screen carrying an array of colour emitting phosphor triplets and operable to produce a plurality of Individually modulatable electron beams disposed with respect to each other to Impinge upon corresponding phosphors of the triplet arrays, scanning means operable to scan the beams repetitively producing for each beam an Impingement with the screen that forms a raster of line scans displaced from each other in an orthogonal direction to form a field such that the fields for all beams are spatially superimposed and modulation means operable to modulate the intensity of each electron beam with an electrical modulation signal representative of a visual image to be generated at the screen such that the same modulation is applied to the different beams as they impinge upon phosphors at the same portion of the superimposed fields to reinforce the excitation of phosphors thereat.
2. 2. An arrangement as claimed in claim 1 in which the beams are directed at the screen so as to Impinge thereon at different positions of the screen over which all beams are scanned as leading and trailing beams and the modulation means includes delay means operable to delay the modulation signal to each beam which trails the leading beam in each scanned raster by a time interval related to the separation of the impingement point of that beam from the leading beam in the direction of beam scan.
3. 3. An arrangement as claimed In claim 2 in which each time interval is less than one field scan interval.
4. 4. An arrangement as claimed in claim 2 or claim 3 in which the cathode ray tube is of the beam-index type in which the screen comprises a plurality of phosphor stripe triplets and index stripes, each stripe extending in the field scan direction, arrayed in the raster line scan direction, and in which the beams are directed to impinge upon corresponding portions of the phosphor stripe triplet and index stripe arrays.
5. 5. An arrangement as claimed in claim 4 in which the beams are directed to impinge upon the same stripes displaced in the ED 694/JH field scan direction by an integral number of line scans.
6. 6. An arrangement as claimed In any one of claims 2 to 5 In which the delay means comprises a digital delay line operable to delay the modulation signal In digital form.
7. 7. An arrangement as claimed in claim 6 in which the delay means Includes a RAM.
8. 8. An arrangement as claimed in any one of claims 2 to 7 including correction means operable to vary the modulation signal delay to each trailing beam throughout the raster in accordance with variations of the Instantaneous relative positions between said trailing and the leading beam due to scanning non- linearities and scanning corrections applied to cause the leading beam to describe a uniform raster.
9. 9. An arrangement as claimed in claim 8 In which the correction means includes a correction store into which is mapped for each trailing beam a function related to the actual separation between that beam and the leading beam in the direction of scan for each addressable position at which the leading beam impinges on the screen and reading means operable in accordance with the instantaneous position of the leading beam throughout the field scan to read said stored function for each trailing beam and effect a delay to the modulation signal in accordance therewith.
10. 10. An arrangement as claimed in claim 9 In which the function mapped into the correction store is the instantaneous value of trailing beam modulation signal delay interval.
11. 11. An arrangement as claimed in claim 9 in which the function mapped into the memory is an offset in nominal delay interval or nominal separation between the trailing beam and leading beam.
12. 12. A visual display arrangement substantially as herein described with reference to, and as shown in, any one of Figures 1 to 3 of the accompanying drawings.
13. 13. A method of forming a visual display comprising providing a colour cathode ray tube with a plurality of individually modulatable electron beams, causing the beams to be focussed onto a screen so as to excite radiation emissive n, a i ED 694/JH phosphors thereat, scanning each of the beams repetitively in a raster pattern of scan lines, the lines being displaced from each other In an orthogonal direction to form a field of scan lines, such that the fields are spatially superimposed and modulating the intensities of each of the scanned beams with electrical modulation signals representative of a visual image to be generated at the screen such that the same modulation Is applied to the different beams as they Impinge upon the same portions of the superimposed fields to reinforce emissions from said portions.
14. 14. A method as claimed in claim 13 comprising causing the beams to impinge upon the same portions of the screen defining the lines of the raster as leading and trailing beams displaced in time whereby said beams instantaneously impinge upon physically displaced portions of the screen and delaying the modulation signals to each trailing beam by a delay Interval comprising said time displacement.
15. is. A method as claimed in Claim 14 applied to a beam index cathode ray tube, in which the radiation emissive phosphors comprise triplets of dif f erent -colour light emitting phosphor stripes and beam index stripes that extend substantially orthogonally to the line scan direction, in which said time displacement Is a function of scan rate and separation between beam index stripes and phosphor stripes of the same colour emission.
16. 16. A method as claimed in claim 14 or claim 15 comprising varying the delay Interval between modulating the leading beam and each trailing beam in accordance with variations in displacement between them due to variations In scan deflection forces applied to the leading beam throughout the field scan to define a uniform raster.
17. 17. A method of forming a visual display substantially as herein described with reference to any of the accompanying drawings.

    Publ 1990 at The Patent 0Met.Stat House. 86f71 Hith H01born, London WC1 R 47P.Purther copies maybe obuinod&om Mes branch. at Mary Cray. Orpuigwn. Kent BR5 3RD Printed by Itultipl#x techniques It& at Mary Cray. Kent. Con lie?