GB2213029A - Beam position control in a flat crt display system - Google Patents

Description

1.

1 1 PHB3340221%.,029 DESCRIPTION

CATHODE RAY TUBE DISPLAY SYSTEM AND ITS METHOD OF OPERATION This invention reLates to a cathode ray tube dispLay system comprising an enveLope incLuding a substantiaLLy fLat facepLate carrying a phosphor screen, means for producing an eLectron beam and directing the beam substantiaLLy paraLLeL to the facepLate through a first region towards a reversing Lens which turris the beam so that it traveLs in substantiaLLy the opposite direction through a second region, first clefLection means in the first region for scanning the beam substantiaLLy in a pLane paraLLeL to the facepLate, second clefLection means in the second region for clefLecting the scanning eLectron beam toward the screen, the first and second clefLection means serving to scan the beam over the screen in raster fashion, and beam position controL means incLuding further beam clefLection means operabLe to adjust the position of the scanning beam in the first region reLative to the reversing Lens in accordance with outputs from beam position sensing means situated between the first clefLection means and the reversing Lens.

The invention reLates aLso to a method of operating such a dispLay system.

A cathode ray tube dispLay system of this kind is described in British Patent Specification 2101396B. In this system, an eLectron gun in the rear region of the enveLope produces a Low energy eLectron beam which is clefLected to effect Line scanning by an adjacent eLectrostatic clefLection arrangement before entering the reversing Lens. After having been reversed in direction through degrees, the beam undergoes fieLd scanning by means of an array of seLectiveLy energised eLectrocles arranged in a pLane paraLLeL with a fLat facepLate in the front region of the enveLope and is clefLected thereby towards a phosphor screen carried on the facepLate onto the input side of a channeL eLectron muLtipLier disposed paraLLeL to, and spaced from.. the screen. Thus, the Line and fieLd scanned beam provides a raster scanning eLectron input to the eLectron muLtipLier.Having undergone current muLtipLication 2 PHB33400 within the eLectron muLtipLier, the emanating raster scanning beam is acceLerated onto the phosphor screen by means of a high voLtage fieLd estabLished between a backing eLectrode on the screen and the output side of the eLectron muLtipLier to produce, for exampLe, a TV picture. An advantage in using an eLectron muLtipLier in this manner is that the muLtipLier in effect separates the scanning function of the eLectron beam from the Light-generating process. The eLectron beam, prior to reaching the muLtipLier, need onLy be of Low energy so that the beam forming and raster scanning section of the tube operates at Low voLtage and current compared with the high voLtage, higher current screen output section. The term "Low energy" used herein is intended to signify an eLectron beam of Less than 2.5KeV and typicaLLy severaL hundred eLectron voLts. For exampLe, a Low voLtage, Low current beam having an acceLeration voLtage of around 400V may be used. The eLectron muLtipLier ampLifies the beam current and the ampLified-current beam is acceLerated across a short gap onto the screen to produce the power necessary to generate the Light output. 20 As a resuLt of the use of a Low energy eLectron beam in the beam forming and raster scanning section of the tube, the tube is more sensitive to ambient magnetic fieLds, for exampLe the Earth's magnetic fieLd, than a conventionaL clispLay tube using a high voLtage beam. In particuLar, a magnetic fieLd penetrating the first region of the tube and having a direction paraLLeL to the facepLate and generaLLy transverseLy to the beam direction can cause clefLection of the beam in a direction perpendicuLar to the facepLate before it reaches the reversing Lens, thereby producing a deviation from the intended path of the beam through the reversing Lens. If the beam faiLs to enter the reversing Lens within the Lens' acceptance window a totaL Loss of picture can resuLt.

An externaL magnetic shieLd comprising a box of mumetaL materiaL couLd be fitted around the tube's enveLope to aLLeviate the effects of extraneous magnetic fieLds, but this is expensive and adds to the buLkiness and weight of the tube.

Q 1 3 PHB33400 It is an object of the present invention to provide a more convenient and reliable way of reducing or eliminating the effects of external magnetic fields on the operation of a cathode ray tube display system of the kind mentioned in the opening paragraph, and particularly, but not exclusively, of ensuring that the electron beam acceptably enters the reversing lens within its design tolerances.

It is briefly mentioned in British Patent Specifitation 2101396B that, in order to counter any misalignment of the gun with the reversing lens and ensure that the beam enters the central part of the reversing lens, one or more electrodes could be used to sense the position of the beam as it scans across the reversing lens and control further, corrector, electrodes acting on the beam as it leaves the gun to deflect it in a plane perpendicular to the screen. It is intended that this arrangement be used during manufacture of the tube to sense errors in the positioning of the beam relative to the reversing lens resulting from gun misalignment during assembly and to enable those errors to be corrected at the final assembly stage by suitably adjusting manually a constant voltage applied to the corrector electrodes. This fixed voltage is then applied constantly during subsequent operation of the tube. As such, the arrangement would be unable to correct for example, deflection caused by changing ambient magnetic field conditions in subsequent operation of the tube.

According to one aspect of the present invention, a cathode ray tube display system of the kind mentioned in the opening paragraph is characterised in that the beam position control means is arranged to sense periodically during operation of the tube the position of the scanning beam and to control the further deflection means in accordance therewith so as to maintain the scanning beam substantially at a predetermined position with respect to the reversing lens.

Besides being capable of adjusting the position of the scanning beam relative to the reversing lens to correct for any positional error arising from misalignment of the electron beam 4 PHB33400 producing means during tube assembly, the display system according to the invention is also able to correct automatically deviation in the position of the scanning beam from the desired position relative to the reversing lens as a result of an ambient magnetic field penetrating the first region of the tube and influencing the beam during normal operation of the tube. Because the position of the scanning beam is monitored periodically, and operation of the further deflecting means varied appropriately to constrain the scanning beam to a predetermined path, the beam position is, if necessary, adjusted periodically to maintain its desired position regardless of, for example, changes in ambient magnetic fields or changes in the position or orientation of the tube in use.

Preferably, the beam position control means is arranged to sense the position of the scanning beam during field blanking (field flyback) periods. Position sensing may take place, for example, during each successive field blanking period or alternatively during every nth field blanking period where n is greater than one. By sensing position during field blanking periods, the possibility of interference with the display picture is minimised. The position of the scanning beam may, however, be sensed in addition, or alternatively, during line blanking periods.

In a preferred embodiment, the sensing means is positioned in the first region so as to sense deviation of the scanning beam from a plane substantially parallel to the faceplate. Any deviation in this sense is most critical to the successful operation of the reversing lens. To be reversed satisfactorily the scanning beam should enter the reversing lens within the lens' acceptance window. A deflection of the scanning beam towards or away from the screen, and thus out of this plane, could therefore, depending on the magnitude of deflection, result in the beam arriving at the reversing lens outside its acceptance window. If this happens the beam may not be reversed in the intended manner with the result that the beam may impinge upon the screen at the wrong place, thereby giving rise to a picture defect, or, in extreme cases, causing failure of beam reversal leading to severe picture 1; a PHB33400 defects. Other possibLe deviations of the scanning beam as a resuLt of the infLuence of, for exampLe, ambient magnetic fieLds in a direction orthogonaL to the screen are Less criticaL in this respect as in these cases the resuLting beam clefLection wiLL be within the intended pLane of the scanning beam so that it stiLL enters the reversing Lens in the desired manner.

The further clefLection means preferabLy comprises an eLectrostatic clefLection arrangement, aLthough magnetic'clefLection means couLd aLternativeLy be used. For simpLicity and convenience the eLectrocle arrangement may comprise a pair of eLectrocles disposed on opposite sides of the beam path, and situated between the eLectron beam producing means and the first clefLection means.

The sensing means may comprise sensor devices disposed above and beLow the predetermined pLane substantiaLLy paraLLeL to the facepLate which are responsive to the eLectron beam impinging thereon to produce respective eLectricaL outputs.

PreferabLy, the sensing means is Located outside the normaL clefLection extremities of the scanning beam and the beam position controL means is effective to controL the first clefLection means to clefLect perioclicaLLy, for exampLe during fieLd bLanking intervaLs, the beam beyond its normaL scanning range and towards the sensing means. The normaL scanning range is that required for the production of a raster-scanned dispLay on the screen so that further defLection outside this range produces no visibLe effect at the screen. By arranging that the sensing means is Located outside the normaL beam scanning range, there is no interference with the picture-producing raster-scanning beam.

The sensor means may comprise a set of two sensor devices Located near one extremity of the normaL scanning range of the beam with the two sensor devices of the set situated on opposite sides respectiveLy of a predetermined pLane substantiaLLy paraLLeL to the facepLate, and corresponding substantiaLLy to the desired pLane of the scanning beam. The sensor devices are arranged in this manner so that when the beam is clefLected periodicaLLy beyond its normaL scanning range it impinges on one or other, or both, of the sensor 6 PHS33400 devices of the set, depending on the position of the scanning beam, and its deviation, if any, from th e desired pLane, to produce appropriate response signaL outputs from the sensor device or devices affected, thereby indicating the position of the scanning beam reLative to the sensor devices.

PreferabLy, the two sensor devices are positioned symmetricaLLy with respect to the predetermined pLane and spaced apart by a distance Less than the diameter of the beam. Thus, when the beam is overscanned and is in the desired pLane, it impinges on both sensor devices producing equaL responses therefrom.

Operation of the controL means in controLLing the further defLection means is determined on the basis of these output signaLs from the sensor devices. In the event of any dispLacement of the scanning beam away from the desired pLane the controL means responds to the indicative sensor device outputs to energise the further clefLection means upstream of the sensing means to return the scanning beam back to the desired position with respect to the sensing means, thus ensuring that the scanning beam properLy enters the acceptance window of the reversing Lens.

In an embodiment of the invention the set of sensor devices comprises a pair of eLectrocles insuLated from one another and providing respective eLectricaL outputs. Impingement of the eLectron beam on these eLectrodes causes a current fLow and the reLative current fLow from their outputs is therefore dependent on the position of the beam in reLation to the pair of eLectrodes.

When the beam is cleftected out of its desired pLane in a direction towards or away from the screen one or the other of the two eLectrodes wiLL produce a higher current output.

The two outputs are subtracted eLectricaLLy in a subtractor circuit to provide a difference signaL in accordance with which energisation of the further clefLection means by the controL means is determined.

The performance of this system of sensor eLectrodes is dependent on the potentiaL of the surrounding tube structure.

7 PHB33400 PreferabLy, therefore, these eLectrodes are energised with a smaLL positive bias, around a few voLts, whereby secondary eLectrons generated at the eLectrodes surfaces by the impinging eLectron beam are returned to the eLectrodes. 5 Sensing means comprising a set of two sensor devices as described above is sufficient to provide basic information on dispLacement of the scanning beam out of the desired pLane, particuLarLy as regards clispLacement of the scanning beam as a whoLe out of the desired pLane. A further, simiLar, set of two sensor devices may be Located near the other extremity of the normaL scanning range of the beam, with the two sensor devices of this set aLso being situated on opposite sides respectiveLy of the predetermined pLane. This arrangement has the advantage that additionaL information concerning departure of the scanning beam from its desired pLane can be ascertained. More particuLarLy, the provision of two sets of sensor devices to either side of the normaL beam scanning range enabLes twisting of the scanning beam, that is, a rotation of the scanning beam about a centraL axis extending paraLLeL to the facepLate, to be sensed. In addition such an arrangement can be used to sense a defLection of the scanning beam within the pLane of the scanning beam LateraLLy of a desired centraL axis extending between the eLectron beam producing means and the reversing Lens as a resuLt of the infLuence of a magnetic fieLd in a direction perpendicuLar to the facepLate, i.e. a shifting of the scanning beam in the direction ofLinescan so that the extremities of the scanning beam are no Longer arranged symmetricaLLy with respect to the axis of the beam originating from the eLectron beam producing means. Such shifting is removed by the controL means appLying a correction input to the first (Line scanning) defLection means. ALthough defLection of the scanning beam in directions towards or away from the screen by a magnetic fieLd as described previousLy causes more serious probLems through the inabiLity of the reversing Lens aLways to reverse the beam successfuLLy, shifting of the scanning beam in a pLane paraLLeL to the facepLate can aLso give 1 8 PHB33400 rise to picture defects through the raster scanning beam impinging on the screen being shifted with respect to the screen in consequence.

In order to maximise the signaL to noise ratio obtained from the sensor devices' outputs, the eLectron beam is preferabLy turned hard on, for exampLe by means of the appLication of a video "bright up" puLse to the eLectron beam producing means, when the beam is defLected beyond its normaL scanning range towards the 'sensor means.

According to another aspect of the present invention a method of operating a cathode ray tube dispLay system of a kind comprising an enveLope incLuding a substantiaLLy fLat facepLate carrying a phosphor screen, means for producing an eLectron beam and directing the beam substantiaLLy paraLLeL to the facepLate through a first region-towards a reversing Lens which turns the beam so that it traveLs in substantiaLLy the opposite direction through a second region, first defLection means in the first region for scanning the beam substantiaLLy in a pLane paraLLeL to the facepLate, second defLection means in the second region for defLecting the scanning eLectron beam toward the screen, the first and second defLection means serving to scan the beam over the screen in raster fashion, is characterised by the steps of sensing periodicaLLy during operation of the tube deviation in the position of the scanning beam from a predetermined position reLative to the reversing Lens and adjusting the position of the scanning beam in accordance with the sensed position so as to return the scanning beam to the predetermined position.

The sensed deviation may be deviation from a predetermined pLane substantiaLLy paraLLeL to the facepLate, twisting of the scanning beam about an axis substantiaLLy paraLLeL with the facepLate, or dispLacement of the scanning beam within the pLane of the scanning beam, as described above.

In a preferred embodiment of the method, the sensing is accompLished duri. ng the fieLd bLanking period and comprises operating the first defLector means so as to defLect the eLectron c 1 9 PHS33400 beam beyond the normal scanning range and towards sensing means located between the first deflection means and the reversing lens and outside the normal scanning range of the beam.

Cathode ray tube display systems and methods for their operation in accordance with the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a diagrammatic cross-section through an embodiment of the tube of the present invention; Figure 2 is a diagrammatic, part-sectionaL, plan view of the rear region of the tube of Figure 1 illustrating typical electron beam paths and sensing means Located in that region, forming part of the electron beam position control means; Figure 3 is a diagrammatic part sectional view along the Line 111-111 of Figure 2, showing the disposition of the sensing means relative to other components; Figure 4 is a perspective, part sectional, view of a portion of the tube of Figure 1 at the region of the reversing Lens; Figure 5 illustrates a waveform applied to first deflector electrodes in the rear region of the tube for Line scanning the electron beam in that region; Figure 6 illustrates graphically the relationship between the position of the electron beam in the rear region of the tube and the output obtained as the difference between signals from a pair of sensor devices of the sensing means; Figure 7 shows schematically the circuit of the electron beam position control means of the system for correcting the position of the scanning electron beam in the rear region of the tube when deflected out of a predetermined plane substantially parallel with the faceplate; Figure 8 illustrates certain waveforms appearing in the circuit of Figure 7; and Figures 9 and 10 show schematically modified parts of the beam position control means circuit for providing correction of positional deviations of the beam from the predetermined plane in PHB33400 other directions.

Referring to Figures 1, 2 and 3, the cathode ray dispLay tube comprises a generaLLy fLat-waLLed rectanguLar enveLope 12 incLuding a substantiaLLy fLat, gLass facepLate 14, the remaining waLLs being formed of metaL, for exampLe, miLd steeL. Carried on the inside surface of the facepLate 14, there is a screen comprising a Layer 16 of phosphor materiaL covered by an aLuminium backing eLectrode 18.

The interior of the enveLope 12 is divided in a pLane paraLLeL to the facepLate 14 by an internaL partition 20 to form a front region 22 and a rear region 24. The partition 20, which comprises an insuLator such as gLass, extends over a major part of the height of the tube.

A pLanar eLectrode 26 is provided on a rear side of the partition 20. This eLectrode 26 extends over the Lower, exposed, edge of the partition and continues for a short distance up its front side. Carried on the inside of the rear waLL of the enveLope is a pLanar eLectrode 28 corresponding to the eLectrode 26. The eLectrode 28 may be insuLated from the rear waLL.

Means for producing a Low-energy eLectron beam is situated in the rear region 24. The eLectron beam producing means is arranged to direct an eLectron beam 32 upwardLy of the tube paraLLeL to the facepLate 14 and comprises an eLectron gun 30 having a heated cathode, an apertured grid eLectrode, an object forming apertured grid eLectrocle, an acceLeration eLectrode, a focussing eLectrode and a finaL acceLeration (anode) eLectrocle.

A upwardLy directed eLectrostatic.Line defLector 34 in the form of two eLectrodes is spaced by a short distance from the finaL anode of the eLectron gun and is arranged coaxiaLLy therewith. In 30 operation, the Line defLector 34 is energised to clefLect the beam 32 in a pLane paraLLeL with the facepLate 14 to effect Line scanning. Situated between the finaL anode (not shown) of the gun and the Line clefLector 34 is a corrector eLectrocle arrangement 37 consisting of a pair of eLectrocles, one on each side of the beam 35 path.

1 1 11 PHB33400 At the upper end of the envelope 12, there is a reversing (mirror) tens 36 comprising a trough-like electrode 38 which is spaced above, and disposed symmetrically with respect to, the upper edge of the partition 20 and insulated from the envelope 12. By maintaining a potential difference between the electrodes 26 and 38 the electron beam 32 is reversed through 180 degrees so as to travel in the opposite direction in the front region 22 over the front side of the partition 20 whilst continuing along'the same angular path from the line deflector 34.

On the front side of the partition 20 there is provided a planar deflection electrode arrangement. This arrangement comprises a plurality of laterally elongate, vertically spaced electrodes of which the uppermost electrode 40 may be narrower and acts as a correction electrode as will be described. The other electrodes, 42, are selectively energised to provide frame deflection of the electron beam 32 onto the input side of a channel plate electron multiplier 44 extending parallel to, and spaced from, the screen 16. The multiplier 44 has a matrix of channels of, say, 0.5mm pitch and comprises a laminated metal dynode electron multiplier. Other forms of channel plate electron muttipiers, such as a glass micro-channel plate electron multiplier, may be used instead. The electron beam, having undergone current multiplication within the multiplier 44, is then accelerated onto the phosphor screen 16 to produce a display by means of a high voltage accelerating field established between the screen electrode 18 and the output surface of the multiplier 44.

In operation of the display tube, the following voltages are, for example, applied with respect to the gun cathode potential of OV. The final anode of the electron gun is held at 40OV giving an electron beam acceleration voltage of 40OV. The electrodes 26 and 28 in the rear region 24 are also held at 40OV whilst line deflection is accomplished by applying in regular fashion potential changes of about 60V around a mean of 40OV to the plates of the line deflector 34. The trough-like electrode 38 of the reversing lens is at OV, compared to the 40OV of the extension of the 12 PHB33400 eLectrocle 26 over the bottom edge of the partition 20, to refLect the beam 32 through 180 degrees. The input surface of the muLtipLier 44 is at 400 V whiLst at the beginning of each frame scan the eLectrodes 42 are at 40OV, but are subsequentLy ramped down to OV in turn, so that the eLectron beam 32 in the front region 22 is initiaLLy clefLected into the uppermost channeLs of the muLtipLier 44 and then progressiveLy moves downwardLy over the muLtipLier, the point of clefLection being determined by the next eLectrocle 42 in the array to be at OV. Using overLapping waveforms appLied to the eLectrocles 42, verticaL, frame, scanning is achieved smoothLy.

The voLtage across the muLtipLier is typicaLLy about 150OV.

The screen eLectrocle 18 is typicaLLy at about l2kV to provide the necessary acceLeration for the beam from the muLtipLier output onto the phosphor screen 16.

It is seen therefore that the Line clefLector 34 and clefLection eLectrodes 42 are responsibLe for scanning the Low energy beam from the eLectron gun over the input surface of the muLtipLier 44 in raster fashion. In order to carry out a rectanguLar raster scan, it is necesary to provide a trapezium correction to the Linescan by appLying clynamicaLLy a correction to the Line clefLector 34.

The eLectrocle 38 is at a suitabLe distance from the partition's edge so that the beam, having been defLected through degrees remains substantiaLLy paraLLeL to the facepLate in the front region 22. As a precaution against misaLignment however, which wouLd Lead to the beam 32 not emerging paraLLeL to the pLane of the screen, a correction voLtage can be appLied to the correction eLectrocle 40 to adjust the exit angLe.

The Line scanning beam in the region between the Line defLector 34 and the reversing Lens 36 icleaLLy shouLd Lie in a pLane paraLLeL with the screen 16 and intersecting the entrance of the reversing Lens within its acceptance window, which is around midway between the eLectrode 26 at the upper end of the partition and the clownwardLy extending side waLL of the eLectrocle 38.

The corrector eLectrode arrangement 37 can be used to clefLect 4.

13 PHB33400 the path of the beam 32 in a pLane perpenclicuLar to the screen as it Leaves the eLectron gun in order to counteract any misaLignment between the gun and the reversing Lens so that, in icleaL operating conditions, the beam path is substantiaLLy paraLLeL with the screen 16 and enters the reversing Lens at the optimum height.

Referring particuLarLy to Figure 2, the beam is scanned symmetricaLLy with respect to a centraL axis 33 coinciding with the direction of the beam emanating from the gun 30. Through the aforementioned trapezium correction, the angLe of scan varies in accordance with the position of the Line in the fieLd. Thus, for the normaL raster scanned dispLay the wider scan angLe, denoted by in Figure 2,'is produced for a Line at the top of the screen 16, defining the extremities of the scanning beam transverseLy of the axis 33 in the rear region of the tube for normaL dispLay producing raster scan over the area of the screen 16.

The dispLay tube described thus far is simiLar in many respects to that described in British Patent Specification No.

2101396B, cletaiLs of which are incorporated herein by reference.

For a more detaiLed description of the operation of the tube, reference is invited to this specification.

In the event of the tube being subjected to ambient magnetic fieLds, unwanted defLection of the eLectron beam 32 can occur in the rear region 24. To simpLify the foLLowing description it is assumed that the magnetic fieLds can have components Hx, Hy and Hz in x, y and z directions where x, y and z are, as shown in Figures 3 and 4, mutuaLLy orthogonaL axes extending respectiveLy paraLLeL to the Line cleftection direction and the screen 16, perpenclicuLar to the pLane of the screen 16, and paraLLeL to the axis of the eLectron gun.

It is important for the successfuL operation of the tube that on entry to the reversing Lens the eLectron beam does not faLL outside the centraL 15-20% region of the width of the gap between the partition 20 and the pLane of the siclewaLL of the eLectrode 38 adjacent to the rear waLL of the enveLope 12. If it does, then severe clefocussing or even Loss of picture can occur.In this 14 PHB33400 respect, interaction between the Hx component and the z-veLocity component of the eLectron beam tends to defLect it in the y-direction, that is, across the width of the aforementioned gap.

Referring to Figure 4, a soLid Line denotes an exampLe of the desired trajectory of the eLectron beam 32 through the reversing Lens and a dotted Line an unwanted defLected trajectory caused by the action of an Hx magnetic component.

In order to suppress the effects particuLarLy of an Hx magnetic fieLd component and eLiminate, or at Least reduce significantLy, the amount ofshift to the beam in the y direction from the desired pLane, the dispLay tube is, in accordance with the invention, provided with a means for controLLing the position of the scanning beam in the rear region 24 of the tube to maintain the position of the scanning beam in that region in a pLane substahtiaLLy paraLLeL to the facepLate 14, this predetermined pLane being indicated by the dotted Line in Figure 3.

The principLe of operation of this position controL means wiLL be expLained with reference to Figures 1 to 3. Two sensor assembLies 50 and 51 are mounted on the back of the partition 20 in the rear region of the tube between the Line defLector 34 and reversing Lens 36 just beyond respective extremities of the normaL range of the scanning beam needed to raster scan the dispLay area of the screen 16.

By appLying appropriate signaLs to the Line defLector 34, as wiLL be described, the beam can be made to overscan and, depending on the position of the beam, to strike these sensor assembLies during the fieLd bLanking period. As can be seen from Figure 3, each sensor assembLy consists of a pair of sensor eLectrodes 50A and 51A, and 50B and 518 respectiveLy, the eLectrodes of each assembLy being separated from one another by an insuLative Layer. The sensor eLectrodes of each assembLy are situated above and beLow, and symmetricaLLy with respect to, a predetermined pLane substantiaLLy paraLLeL with the screen 16 and corresponding with the desired pLanq of the scanning beam for correct entry into the 35 reversing Lens 36. The spacing between the pair of sensor t PHB33400 electrodes and each sensor assemblyo determined by the insulative layer. is less than the width of the electron beam, for example, around one quarter of the beam diameter. Thus, when the beam lies in the predetermined planeo it is centred on the sensor assemblies and produces substantially identical currents from both sensor electrodes of each sensor assembly. these currents being indicated by the outputs referenced LB, LF, RS and RF in Figure 3.

The surfaces of the sensor electrodes facing the electron gun 30 onto which the electron beam impinges are cup-shaped and roughened so as to trap electrons.

Figure 5 shows a typical line scan waveform applied to the line deflector 34 during a succession of field periods, here shown as being of 20ms duration. The normal line scan waveform is indicated at LS and the additional overscan voltage pulse signals applied to the deflector causing the beam to be deflected towards the sensor assemblies 50 and 51, in turn, are indicated at SR and SL respectively which occur during each conventional field blanking period. V1. represents the peak to peak amplitude of the overscan pulse signals.

The overscan beam suffers in a similar fashion from magnetic fields to the normal line scanning beam and deflection suffered by the overscan beam is therefore indicative of deflection suffered by the line scanning beam.

Separate electrical connections are made to each sensor electrode. When the electron beam is deflected beyond its normal range to strike a sensor assembly, the relative currents reaching the front and back sensor electrodes, e.g. 508 and 50A respectively, vary in accordance with the y-position of the electron beam. The performance of the system depends on the potential of the sensor electrodes relative to the surrounding tube structure. A small positive bias, typically around 5 to 20V, provides acceptable results, causing secondary electrons generated at the sensor electrode surfaces to be returned to the sensor electrodes. The electrical currents produced from the two sensor electrodes of this sensor assembly are subtracted electrically to 16 PHB33400 give a difference signal., Vy. This voltage signal Vy is _proportional to the difference in currents produced by the two sensor electrodes. A plot of Vy against y position of the beam measured in an experimental system is shown in Figure 6.

The point C at Vy equal to OV is obtained when the beam is centred on a sensor assembly so that both electrodes thereof produce equal currents. The regions M and W also at Vy equal to Ov occur when the beam covers completely misses the sensor assembly. The width of the sensor electrodes is chosen to be considerably greater than the electron beam diameter so that significant deflection of the beam is required for this to happen.

The difference signals obtained from the sensor assemblies, assuming some deviation is present, are then used to control a variable voltage signal applied to the corrector electrode arrangement 37 which operates on the beam emanating from the electron gun 30 so as to return the scanning beam to its correct position, that is, in the predetermined plane.

One embodiment of a circuit for achieving this is shown schematically in Figure 7 in which components already described are designated with the same reference number. A conventional line scan waveform, denoted LS, from a line scan generator (not shown) is applied to an input of an electronic switch 70 controlled by an output of a timing circuit 71 and thence to a drive amplifier 72 for application to the plates of the line deflector 34 to line scan the beam 32. The electronic switch 70 is also connected to a pulse generator 74 which generates the overscan pulse signals, OP, under the control of the timing circuit 71. -The timing circuit is supplied with a conventional field synchronisation signals FS and during field blanking periods operates the electronic switch 70 so that the overscan pulse signals from generator 74 are supplied to the line deflector 34 via the drive amplifier 72 causing the beam to overscan toward the sensor assemblies 50 and 51.

Because the beam is not directed towards the two sensor assemblies 50 and 51 simultaneously but rather one after the other, respective sample and hold circuits 76 are connected to the outputs 17 PHB33400 from the individual sensor electrodes of the assemblies via associated amplifiers. The sample and hold circuits 76 are controlled by sample pulse waveforms S1 and S2 from the timing circuit 71.

During the period when the beam is situated, for example, in the vicinity of the sensor assembly 51 the signals from the sensor electrodes 51A and 51B, that is, R 9 and RF, are sampled and held in the associated circuits 76. Thereafter, the beam is deflected onto the sensor assembly 50 and the process repeated.

In the simple form of circuit shown in Figure 7, the signals from the sample and hold circuits associated with sensor electrodes 50A and 51A, and 50B and 519 respectively are added together by adders 77 and 78 whose outputs are then subtracted from one another in subtractor 79 to provide the difference, or error, signal Vy.

By closing the feedback loop a control voltage is derived by means of drive amplifier 80 supplied with the output from subtractor 79, which is first filtered by a low pass filter 81, and supplied to the electrodes of electrode arrangement 37 to correct the position of the scanning beam relative to the sensor assemblies, and hence the reversing lens, and return it by movement in the y direction to its desired position in the predetermined plane should it have been deflected away from this position by a magnetic field component Hx.

In addition to the features described above, a video "bright-up" pulse, VB, is used to turn the electron beam hard on for the periods it is directed towards the sensor assemblies 50 and 51 so as to maximise the signal to noise ratio of the system.

Figure 8 illustrates the timing between the beam overscan waveform, the sample and hold circuits control pulse waveforms., and the "bright-up" waveform appearing in the circuit of Figure 7 in relation to the field blanking period, denoted by FB.

It is envisaged that in a modified, and simpler, form of the system, only one sensor assembly, e.g. 50, may be used to correct for unwanted deflection of the scanning beam in the y-direction. The error signal Vy in this case would be derived by subtracting the outputs from the sample and hold circuits associated with the 0 18 PHB33400 sensor electrodes SOA and 50B. However, the effectiveness of this simplified system presupposes that the scanning beam is uniformly deflected over the extent of its scanning direction, that is both extremities and all intermediate points along a line transversely of the axis 33 are displaced by a similar amount from the desired scanning beam plane. The system shown in Figure 7 using two sensor assemblies is considered to be more preferable as combining the outputs from the two sensor assemblies in the manner described enables correction of the position of a non-uniformly deflected scanning beam to a be accommodated to a certain extent.

Moreover, whilst the system described above using two sensor assemblies allows correction for scanning beam movements entirely in the y-direction, the beam position control circuit can be adapted to obtain correction signals for shifts of the scanning beam in the x direction and also for any rotation of the plane of the scanning beam around an axis parallel to the z-axis, for example, the axis 33, caused by other components of magnetic fields.

Referring to Figure 9, there is shown schematically a modified part of the circuit of the beam position control system of Figure 7 in which output signals from the sensor assemblies 50 and 51 are processed in combination to provide error signals for x-direction and y-direction shifts and also for twist, that is, rotation about an axis parallel to the z-axis, indicated by angle S. Components corresponding with those of the circuit of Figure 7 have been designated with the same reference numbers.

As with the circuit of Figure 7, the outputs from respective pairs of the four sample and hold circuit 76 are added together and then the sums subtracted from one another to providethe y-direction error signal Vy, which is then used to control the deflector electrode arrangement 37 as previously.

The x-direction error signal, Vx, is derived by subtracting, in subtractor 82, the sum of the currents for both sensor electrodes of sensor assembly 50 obtained from adder 83 from the sum of the currents for both sensor electrodes of the sensor 19 PHB33400 assembly 51 obtained from adder 84. For the x-direction deflection sensing to operate correctly, the peak to peak amplitude of the overscan pulse signal, OP, supplied to the line deflector 34 during field blanking, indicated at V1. in Figure 5, must be adjusted so that any x-direction shift of the scanning beam causes the current in one or other of the sensor assemblies to fall if it is to be sensed.

Having derived an x-direction shift error signal Vx, correction for this shift may be accomplished in the beam position control system in two ways. The Vx signal may simply be fed into a shift input of the linescan drive amplifier 72 shown in Figure.7.

This has the effect of biassing one of the plates of the line deflector 34 relative to the other by an amount which varies in accordance with the level of the error signal Vx so that the scanning beam downstream of the line deflector 34 is displaced in the x-direction and returned to-the correct position where it is again symmetrical with respect to both the axis 33 and the sensor assemblies 50 and 51. However the magnetic field causing this x-direction shift will also likely cause a further shift on the scanning beam during the remainder of its trajectory downstream of the sensor assemblies and where it travels in the front region 22 of the tube parallel to the screen. This additional shifting will vary from top to bottom of the screen because of the different trajectory lengths involved so that the observed effect, provided the magnetic field is not too large,, is both a shift and a parallelogram distortion of the displayed picture. The aforementioned technique can be used satisfactorily to reduce, but not eliminate completely, x-direction shift.

A much improved correction for x-direction shift can be obtained using an alternative technique, as shown in Figure 10 which is a schematic representation of a modified form of part of the circuit of the beam position control system of Figures 7 and 9. The x-direction shift error signal Vx is derived in the manner described with reference to Figure 9. This signal is supplied to a low pass filter 90 whose output, here designated V1, is used to PHB33400 produce a shift in the switching voltages applied during the sensing period, (the field blanking period).

Two further voltages, V2 and V3, are derived from V1 by function generators 91 and 92 respectively. Voltage V2 provides a d.c. shift of the line scan signal so that the centre of the picture remains in the correct position while V3 is multiplied by the field modulation signal in the analog multiplier 94 used to modulate the line scan amplitude so as to provide corredtion for the parallelogram distortion mentioned above. In Figure 10, Vf indicates the field ramp voltage waveform, V1 indicates the line ramp voltage waveform, VS is equal to (V3+V1).Vf.K where K is a constant, and FB indicates the field blanking signal. The transfer functions used in the function generators 91 and 92 to derive V2 and V3 from V1 can be selected to suit the design of a particular tube and any magnetic shielding that may be present.

Compensation for twisting of the scanning beam in a tube of this kind can be applied by taking a small fraction of the line scan waveform supplied to the line deflector 34 and feeding it, either directly or after inversion, as the case may be depending on the sense of twist, to the two electrodes of the corrector electrode arrangement 37. The amount of twist correction applied depends in this case on the amplitude of this signal. Automatic correction of twist can alternatively be achieved by using the twist error voltage signal, Vt in Figure 9, derived from the sensor assemblies by adding the outputs of sensors 50B and 51A, and 50A and 51B respectively in adders 85 and 86 and then subtracting the outputs therefrom in subtractor 87, to control the amplitude of the drive voltage applied to the two electrodes of the correcting electrode arrangement 37. The d.c. potential across these electrodes is still derived from the y-direction error signal, Vy.

The above described embodiments of the scanning beam position control system have been represented in terms of analog components. However, it is envisaged that digital techniques may alternatively be-employed. In practice there are considerable advantages gained in digitising the signals at the outputs of the 1 t 21 PHB33400 sample and hold circuits 76 associated with the sensor assemblies and performing all signal processing digitally. The system circuit functions for which this approach is particularly attractive and beneficial are:

1) additions/subtractions needed to generate Vx, Vy and Vt, 2) the filtering of these error signals to provide optimum performance of the closed loop in terms of speed of response and stability, 3) the derivation of the voltage V2 and V3 (with reference to the version shown in Figure 10) from the filtered x-direction error signal.

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 cathode ray tube display systems and which may be used instead of or in addition to features already described herein. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present application also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not mitigates any or all of the same technical problems as does the present invention. The applicants hereby give notice that new claims may be formulated to such features andlor combinations of such features during the prosecution of the present application or of any further application derived therefrom.

22 PHB33400 CLAWS) 1. A cathode ray tube dispLay system comprising an enveLope incLuding a substantiaLLy fLat facepLate carrying a phosphor screen, means for producing an eLectron beam and directing the beam substantiaLLy paraLLeL to the facepLate through a first region towards a reversing Lens which turns the beam so that it traveLs in substantiaLLy the opposite direction through a second region, first defLection means in the first region for scanning the beam substantiaLLy in a pLane paraLLeL to the facepLate, second defLection means in the second region for defLecting the scanning eLectron beam toward the screen, the first and second defLection means serving to scan the beam over the screen in raster fashion, and beam position controL means incLuding further beam defLection means operabLe to adjust the position of the scanning beam in the first region reLative to the reversing Lens in accordance with outputs from beam position sensing means situated between the first defLection means and the reversing Lens, characterised in that the beam position controL means is arranged to sense periodicaLLy during operation of the tube the position of the scanning beam and to controL the further defLection means in accordance therewith so as to maintain the scanning beam substantiaLLy at a predetermined position with respect to the reversing Lens.

2. A cathode ray tube dispLay system according to CLaim 1, characterised in that the sensing means is positioned in the first region so as to sense deviation of the scanning beam from a pLane substantiaLLy paraLLeL to the facepLate.

3. A cathode ray tube dispLay sy'stem according to CLaim 2, characterised in that the sensing means comprises sensor devices disposed above and beLow said pLane substantiaLLy paraLLeL to the facepLate which are responsive to the eLectron beam impinging thereon to produce respective eLectricaL outputs.

4. A cathode ray tube dispLay system according to CLaim 2, characterised in that the sensing means is Located outside the normaL defLection extremities of the scanning beam and in that the beam position controL means operates to controL the first z S A 23 PHB33400 deflection means so as to scan periodically the electron beam beyond its normal scanning range and towards the sensing means.

Claims (1)

1. 5. A cathode ray tube display system according to Claim 4, characterised

   in that the sensing means comprises a set of two sensor devices located near one extremity of the normal scanning range of the electron beam with the two sensor devices being situated on opposite sides respectively of a predetermined plane substantially parallel to the faceplate corresponding to the desired plane of the scanning beam and being responsive to the electron beam impinging thereon to produce respective 10 electrical outputs.

   6. A cathode ray tube display system according to Claim 5, characterised in that the sensing means includes a further set of two sensor devices, located near the opposite extremity of the normal scanning range of the beam, the two sensor devices of the further set being situated on opposite sides respectively of the predetermined planes and being responsive to the electron beam impinging thereon to produce respective electrical outputs.

   7. A cathode ray tube display system according to Claim 6, characterised in that the beam position control means is responsive to outputs from the sets of sensor devices indicative of a shift of the scanning beam within the plane of the scanning beam laterally of the desired central axis extending between the electron beam producing means and the reversing lens to apply a correction input to the first deflection means so as to remove said shift.

   8. A cathode ray tube display system according to any one of Ciaims 5 to 7, characterised in that the two tensor devices of the or each set are positioned symmetrically with respect to the predetermined plane and are spaced apart from one another by a distance less than the diameter of the electron beam.

   9. A cathode ray tube display system according to any one of Claims 5 to 8, characterised in that the two sensor devices of the or each set comprise a pair of mutually insulated electrodes producing respective output currents in response to the electron beam impinging thereon.

   24 PHB33400 10. A cathode ray tube display system according to Claim 9, characterised in that the sensor devices are energised with a positive bias sufficient to collect secondary electrons emitted therefrom.

   11. A cathode ray tube display system according to any one of Claims 5 to 10, characterised in that the beam position control means includes a subtracting circuit for subtracting outputs from the two sensor devices of the or each set and means for controlling energisation of the further deflection means in accordance with the difference signal produced thereby.

   12. A cathode ray tube display system according to any one of Claims 4 to 11, characterised in that the electron beam position control means is connected to the electron beam producing means and operable to turn the beam hard on during scanning of the beam beyond its normal scanning range.

   13. A cathode ray tube display system according to any one of Claims 1 to 12, characterised in that the beam position control means is arranged to sense the position of the scanning beam during field blanking periods.

   14. A cathode ray tube display system according to any one of Claims 1 to 13, characterised in that the further deflection means comprises an electrostatic deflector arrangement.

   15. A cathode ray tube display system according to Claim 14, characterised in that the electrostatic deflector arrangement comprises a pair of electrodes situated between the electron beam produced means and the first deflection means and disposed on opposite sides of the beam path.

   16. A method of operating a cathode ray tube display system of a kind comprising an envelope including a substantially flat faceplate carrying a phosphor screen, means for producing an electron beam and directing the beam substantially parallel to the faceplate through a first region towards a reversing lens which turns the beam so that it travels in substantially the opposite direction through a second region, first deflection means in the first region for scanning the beam substantially in a plane C 1 PHB33400 parallel to the faceplate, second deflection means in the second region for deflecting the scanning electron beam toward the screen, the first and second deflection means serving to scan the beam over the screen in raster fashion, characterised by the steps of sensing periodically during operation of the tube deviation in the position of the scanning beam from a predetermined position relative to the reversing lens and adjusting the position of the scanning beam in accordance with the sensed position so as to return the scanning beam to the predetermined position.

   17. A method according to Claim 16, characterised in that the step of sensing deviation in the scanning beam comprises sensing deviation of the position of the scanning beam from a predetermined plane substantially parallel to the faceplate.

   18. A method according to Claim 17, characterised in that the step of sensing deviatidn in the position of the scanning beam further comprises sensing twisting of the scanning beam about an axis substantially parallel with the faceplate and extending between the electron beam producing means and the reversing lens.

   19. A method according to any one of Claims 16 to 18, characterised in that the position of the scanning beam is adjusted by means of an electrostatic deflection arrangement situated between the electrode beam producing means and the first deflection means.

   20. A method according to Claim 17 or Claim 18, characterised by the steps of sensing deviation in the position of the scanning beam further includes sensing shift of the scanning beam within the plane of the scanning beam laterally of a desired central axis extending between the electron beam producing means and the reversing lens and applying a correction input to the first deflection means so as to remove said shift.

   21. A method according to any one of Claims 16 to 20, characterised in that the step of sensing deviation in the position of the scanning beam comprises periodically-deflecting the beam ' beyond its normal scanning range and towards sensing means arranged in the first region between the first deflection means and A p 1 26 PHB33400 the reversing lens and dutside the extremities of the normal scanning range.

   22. A cathode ray tube display system substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

   23. A method of operating a cathode ray tube display system substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

   Published 1989 atThe PatentOffice, State House,68,71 High Holborn. London WCIR 4TP. FMher copies maybe obtained from The Patent Offte. Wes Branch, St Mary Cray, Orpington, Kent BR5 3F.D. Printed by Multiplex techniques ltd, St Mary Cray, Kent, Con. 1/87