WO2004032172A1

WO2004032172A1 - Cathode ray tube with improved image quality

Info

Publication number: WO2004032172A1
Application number: PCT/IB2003/004372
Authority: WO
Inventors: Boris Skoric; Antonius H. M. Holtslag; Marcellinus P . C. M. Krijn; Thomas C. Kraan
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2002-10-07
Filing date: 2003-09-30
Publication date: 2004-04-15
Also published as: AU2003265085A1

Abstract

A cathode ray tube device (1) comprises an elongated display screen (2) with a long axis (21) and a short axis (22), a neck portion (4) comprising an electron gun (5) for generating three in-line electron beams (6, 7, 8) oriented in an in-line plane, and a deflection system (11). The in-line plane is substantially perpendicular to the line scanning direction, (PinlinePscanning), the line scanning direction is substantially parallel to the long axis of the display screen (Pscanning21), the deflection system is self-convergent in the in-line direction (PscPinline). The neck portion has, in cross-sectional view, a long axis and a short axis, the long axis Al of the neck portion being substantially parallel to the short axis of the display screen, and the electron gun has two glass beads (41) which are situated in the in-line plane at either side of the electron gun.

Description

Cathode ray tube with improved image quality

BACKGROUND OF THE INVENTION

The invention relates to a picture display device comprising

- a cathode ray tube (CRT) comprising an elongated display screen with a long axis and a short axis comprising a phosphor pattern, a neck portion comprising an electron gun for generating three in-line electron beams oriented in an in-line plane, and

- a deflection system mounted on a cone portion for generating magnetic fields for deflecting said electron beam in a line scanning and frame scanning direction across the display screen.

Display devices as described above are known by the general term 'in-line CRT's'. In standard designs the three in-line electron beams are arranged in an in-line plane, which, in undeflected condition, is parallel to the line scanning direction, which line scanning direction is parallel to the long axis of the display screen. Given the fact that normally (although this is not to be construed as limiting the scope of the invention) the electron beams are scanned in horizontal lines, the line scanning direction usually coincides with a horizontal direction whereas the frame scanning direction is a vertical direction.

Although these standard design CRT's operate relatively satisfactorily, there is a trend towards ever allower CRT's of ever increasing size and an ever increasing quality of the image on the screen.

This is difficult to achieve, since the deformation of the electron beam spots on the display screen increases as the size of the CRT increases and the CRT is made shallower. In the corners of the screen the electron beam spots become very non-uniform, reducing strongly the image quality. In part this can be counteracted by implementation of dynamic voltages (dynamic focusing and/or astigmatism), but the required dynamic voltage swing or swings become ever greater.

SUMMARY OF THE INVENTION

It is an object of the invention to provide for an image improvement in relation to the above-mentioned problems. To this end the cathode ray tube is characterized in that the in-line plane is substantially perpendicular to the line scanning direction, the line scanning direction is substantially parallel to the long axis of the display screen, the deflection system is self- convergent in the in-line direction, the neck portion has, in cross-sectional view, a long axis and a short axis, the long axis of the neck portion being substantially parallel to the short axis of the display screen, and the electron gun has two glass beads which are situated in the inline plane at either side of the electron gun.

The in-line plane is oriented perpendicularly to the scanning direction, in contrast to the standard design in which the in-line plane and the line scanning direction are parallel.

The line scanning direction is parallel to the long axis, i.e. as in standard designs, which is usually the horizontal direction. The line scanning direction is that direction in which the image is scanned at a high frequency, i.e. considerably higher than in the other direction also called the frame scanning direction. The deflection unit is self-convergent in the in-line plane direction, which is hereinbelow also referred to as the 'vertical or y- direction'. As a consequence the electron beams will now be in focus along the y-direction, thereby greatly improving the electron beam spots. Dynamic focusing (if used, as is preferred) is only necessary in the x-direction and requires less dynamic swing. In addition the cathode ray tube in accordance with the invention has a neck portion within which the electron gun is situated, the neck portion having, in cross-sectional view, a long axis and a short axis, the long axis of the neck portion being substantially parallel to the short axis of the display screen, and the electron gun has two glass beads which are situated in the in-line plane at either side of the electron gun. This arrangement offers a very favourable energy dissipation. Due to the form of the neck and the positions of the glass beads (also called multiforms) the line coil may be positioned very close to the electron beams, which decreases the required energy and simultaneously increases the possibility of controlling the positions and deviations (in trajectory and in shape) of the electron beams by the coils of the deflection unit. It is noted that in US 5,170,102 a cathode ray tube is disclosed in which both the in-line plane and the scanning direction are rotated by 90 degrees in respect of the standard designs, i.e. the in-line plane as well as the scanning direction are parallel to each other and parallel to the short axis of the screen. The problems associated with self- convergence in the y-direction, i.e. perpendicular to the line scan, are briefly discussed but in a clearly discouraging manner, i.e. leading away from using self-convergence in the y- direction.

However, the inventors have realized that rotating the deflection plane by 90 degrees (as in US 5,170,102) requires an increase of the line frequency, and complicated electronics. By using horizontal scan combined with vertical self-convergence such problems are avoided. Modifying the design of the electron gun (i.e. modifying the position of the glass beads), and modifying the neck design (i.e. making the neck elongated in the in-line plane), enables the coils of the deflection unit to be brought closer to the electron beams, thereby reducing the energy dissipation. In a first embodiment of the invention, the cathode ray tube comprises a colour selection electrode (such as for instance a shadow mask, or a tensioned colour selection electrode), said colour selection electrode being provided with index elements on the side facing the electron gun, or forming itself an index means.

Preferably in said embodiment the colour selection electrode is provided with slits elongated in the scanning direction.

In an alternative embodiment the cathode ray tube does not comprise a colour selection electrode, and index elements are provided on the display screen. The invention is of particular importance to such embodiments, due to the fact that for embodiments not comprising a colour selection electrode, in known designs use of dynamic voltages in the electron gun is needed and in such standard design a good image is difficult to achieve unless different dynamic voltages are used for each electron beam, which severely complicates the design of the electron gun as well as the driving circuit. In the cathode ray tube device of the invention, spot uniformity over the screen is much improved, requiring a much smaller dynamic swing, but also differences between the beams are reduced and a single dynamic swing for the triplet of electron beams suffices.

Index elements may comprise, within the framework of the invention, index elements which, when hit by an electron beam, provide a luminous signal (which can be an IR or UN signal) which is perceived by an optical sensor, which optical sensor converts the optical signal to an electrical signal which is then used to correct the position of electron beams, or alternatively, or in addition, index elements are provided which, when hit by an electron beam, provide an electrical deflection correction signal, which is used to correct the position of the electron beams. The term deflection system is, within the framework of the present invention, to be broadly interpreted as encompassing deflection coils for influencing the deflection of the electron beams.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

In the drawings:

Fig. 1 is a sectional view of a display device. Fig. 2 is a cross-sectional view of the display screen.

Figs. 3A and 3B illustrate spot uniformity in prior art and invention.

Fig. 4 illustrates a neck part of a standard cathode ray tube.

Fig. 5 illustrates a neck part of an embodiment of the invention.

Fig. 6 illustrates index elements on or near the display screen and an electrical circuit for providing signals to the deflection system.

The figures are not drawn to scale. Generally, identical components are denoted by the same reference numerals in the figures. Fig.1 shows an index display device comprising a colour cathode ray tube 1 having a display window 2, a cone 3 and a neck 4. The neck 4 accommodates an electron gun 5 for generating three in-line electron beams 6,7 and 8 extending in one plane, the in-line plane. A deflection system 9 is mounted on the cone 3 for deflecting the electron beams 6,7,8 across the display window 2. A display screen 10 is situated on the inner side of the display window 2. Said display screen 10 comprises a plurality of red, green and blue-luminescing phosphor elements. Each group of (red, green or blue) phosphor elements forms a pattern. The display screen 10 may also comprise other patterns such as a black matrix (a black pattern) or colour filter patterns.

The device may have, and in this preferred embodiment has, a means for tracking the electron beams as they are scanned over the display screen. A measurement circuit 12 connected to an index means, for instance an index electrode on the screen, measures an index signal I_s and delivers measurement data which are in this example used by a control loop comprising a first control means 13 acting on the deflection system 9 in order to correct the trajectory of the electron beam 7 when it deviates from its nominal trajectory and/or comprising a second control means 14 acting on the formation of the electron beam 7 in order to correct the shape of the electron beam 7 when it deviates from its nominal shape. The shape of the electron beams may for instance be influenced by influencing the potentials supplied to the beam forming part of the electron gun (the Gl, G2 and or G3 electrodes). Alternatively, or in addition, the means 13 may also act on a device 16 separate from the deflection unit to control the relative position of the electron beams, for instance by means of a means comprising electromagnets for generating a multipole field in the neck portion. The relative position of the electron beams may also be influenced in the electron gun by means of dynamic convergence electrodes. The display device also comprises means 15 for imparting video information to the electron beams as they are scanned over the phosphor lines.

Fig. 2 shows a cross section of the display screen 2. The display screen 2 has an elongated shape with two perpendicular axes of symmetry: a long axis 21 and a short axis 22. The maximum deflection angle of the electron beams is indicated as being the angle 0 between the tube axis 7 and the electron beam orbit when the electron beam 6 is deflected so as to hit a corner of the screen. Next to the figure is schematically indicated that in the standard designs (upper part) the in-line plane P_ιni_ιne_. the scanning direction P_lanning and the direction in which the deflection system is +self-convergent P_sc are all parallel to each other and to the long axis 21. In the design described in US 5,170,102 a cathode ray tube is disclosed in which the in-line plane and the scanning direction, as well as the direction of self-convergence are rotated by 90 degrees with respect to the standard designs, i.e. the inline plane, the scanning direction and the direction in which the deflection system is self- convergent are all parallel to each other and parallel to the short axis of the screen. In figure 2 this is schematically indicated in the middle figure on the right hand side. Finally, on the right hand side, in the lower sub-figure, the configuration of a cathode ray tube in accordance with the invention is schematically illustrated. The in-line plane is 90 degrees rotated (along the short axis 22), the scanning direction is not changed (along the long axis 21), and the deflection system is self-convergent along the short axis (22). This is a combination of horizontal scan with vertical self-convergence, i.e. perpendicular to the line scan direction.

The invention is based on the following: In CRT's, it is common practice that the direction in which the deflection unit has self-convergent properties is the line-scan direction (also referred to as x-direction, being the long axis of the screen 21). This implies that in this direction, the electron beams are always in focus. In the direction of the frame- scan (hereafter also referred to as y-direction, being the short axis of the screen), this is not the case. Means for dynamic focusing of the beams in this direction are required.

For maskless CRT's self-convergence in the line-scan direction gives rise to the following problems: • The spot requirements in the direction of the frame-scan are very strict. As a result, focusing of the beams in the corners of the screen of slim CRT's becomes very difficult since it requires extremely large dynamic voltages in the gun.

• The red, green, and blue beams require different dynamic focus voltages in the corners of the screen. However, this requires a complicated design and a complicated driving circuit.

In case the beams do not land on their intended phosphor stripe, their position has to be corrected by means of correction coils. The sensitivity of these correction coils, when positioned in between gun and deflection unit, depends on the x-position on the screen: Towards the end of the line-scan, the sensitivity becomes smaller and can even change sign (reduced sensitivity means that the correction coils have a reduced influence on the position of the beams). The invention removes or at least reduces said problems by combining horizontal scan with vertical self-convergence (i.e. perpendicular to the scan direction). A better spot-uniformity is provided, less dynamic swing is needed, thus also reducing the need for different voltage swings for red, green and blue. Using horizontal scan, no complicated electronics is needed.

For CRT's having a shadow mask, self-convergence in the line-scan direction (or to be more precise, in the direction of the long axis of the screen) gives rise to the following difficulties:

• In the corners of wide-screen CRT's, the spot becomes very non-uniform: It acquires an elliptical shape with a large aspect ratio. Even more so for slim CRT's. The elongation is in the x-direction.

• Exposure of the phosphor and black matrix structure becomes difficult in the comers of wide-screen slim CRT's.

This invention reduces the above mentioned problems because the direction of self-convergence is changed from the x-direction to the y-direction. This is illustrated in

Figures 3A and 3B, showing on the left hand side spot-uniformity in regular CRT's i.e. when the deflection unit is self-convergent in the x-direction, and showing on the right-hand side spot-uniformity when the self-convergence is in the y-direction. Spot-uniformity is drastically improved.

The electron beams will now be in focus in the y-direction. Dynamic focusing is now required only in the x-direction. This requires a much smaller dynamic swing, enabling also to use a single dynamic voltage rather than three different dynamic voltages for each of the colors.

Thus, in all the different arrangements using horizontal scan in combination with vertical scan, self-convergence offers advantages.

Computer-simulations (with a 4:3 monitor with 90° deflection) have shown that, compared to the standard design, a better defined raster in the x-direction is achievable, combined with a better spot-uniformity, while achieving a comparable convergence and raster accuracy in the y-direction.

The second aspect of the invention relates to the design of the neck and the electron gun. Figure 4 schematically shows the standard design, the left hand part of the figure shows a cross section of the electron gun, showing schematically an electrode with three apertures and the position of the glass bead (also sometimes called multiforms) 41. The right hand side of the figure shows schematically a cross-section of the envelope at the deflection unit. Schematically shown are the line deflection coils 13 A. Figure 5 illustrates the design in accordance with the invention. The glass beads are arranged in the in-line direction, i.e. in this figure above and below the electrode. The neck design is of elongated form, with the long axis A] along the in-line plane, i.e. perpendicular to the scanning direction. Because of the shape of the neck, the line deflection coils 13A may be brought closer to the electron beams, reducing deflection energy because the deflection space is reduced. It is noted that the distance between the frame or field deflection coils may have to be increased (or is at least not decreased), which could lead to a small increase in deflection energy for the frame deflection coils. However, the line deflection coils account for a much larger part of the overall deflection energy, so a substantial net gain (reduction of power dissipation) is achieved because the line deflection coils can be brought closer to the beams and to each other.

Thus, the novel neck and gun design offers a reduction of the power dissipation, which is all the greater because the combination of horizontal scan with vertical self-convergence is used. Due to the gun and coil design, in combination with the horizontal scan and vertical self-convergence, the line coils (13 A) responsible for a major part of the power dissipation can be brought very close to electron beams 6, 7 and 8, because the glass beads 41 are positioned above and below the electrode and because Ai is perpendicular to the scanning direction. The invention is in particular advantageous for devices having line coils which extend further towards the electron gun than the frame coils. One can correct raster errors in the video domain. However, this requires extensive manipulation of data, which is expensive and may lead to additional costs and errors. Within the framework of a preferred embodiment of the invention, raster errors and other errors are counteracted via correction of the deflection and by using index elements (conducting and/or light emitting) extending along the line scanning direction. These patterns are provided with index elements extending along the long axis of the tube. Each index electrode may comprise a plurality of interconnected conducting elements. Fig.6 shows an index electrode 60, provided on the display screen, whose conducting elements 61 are interconnected by their joint side ends. When the electron beam 7 passes over a given conducting element 62 of the index electrode 60, an index signal I_s appears which is indicative of the position of the electron beam 7 with respect to the given conducting element 62 and/or of the shape of the electron beam 7. A measurement circuit 12 connected to the index electrode 60 measures this index signal I_s and delivers measurement data which are in this example used by a control loop comprising a first control means 13 acting on the deflection system 9 in order to correct the trajectory of the electron beam 7 when it deviates from its nominal trajectory and/or comprising a second control means 14 acting on the formation of the electron beam 7 in order to correct the shape of the electron beam 7 when it deviates from its nominal shape. The shape of the electron beams may for instance be influenced by influencing the potentials supplied to the beam forming part of the electron gun (the Gl, G2 and or G3 electrodes). Alternatively, or in addition, the means 13 may also act on a device 16 separate from the deflection unit to control the relative position of the electron beams, for instance by means of a means comprising electromagnets for generating a ultipole field in the neck portion; examples of such fields are illustrated in Figure 4. The relative position of the electron beams may also be influenced in the electron gun by means of dynamic convergence electrodes. The display device also comprises means 15 for imparting video information to the electron beams as they are scanned over the phosphor lines. The index system (Fig. 6) comprises two electrodes 61a, 61b having elements 62 extending along the phosphor lines, and in between which the phosphor lines (in this figure a green phosphor line G is shown) extend. The index elements in cathode ray tubes in accordance with the invention may also be formed by optical index elements on the screen or near the screen, for instance on a shadow mask. In such embodiments light, usually UN light, is generated when an electron beam 6, 7, 8 hits an index phosphor element. These elements are aligned along the scanning direction. Light emitted by the index phosphors is measured by an optical detector 25, which produces a tracking signal. The index signal produced by the index element extending in the line scanning direction (the x-direction, as seen for instance in Figure 6) allows a fast and highly accurate correction of raster errors using the index signals, without having to resort to manipulation of data in the video domain. Raster errors can be reduced to some extent by optimising the wire distribution of the deflection coils, however at the expense of residual East- West raster. In a device in accordance with the invention, raster errors and additional or residual errors are preferably corrected in the deflection unit directly or, most preferably, by additional means. To this end static and/or dynamic multipoles, either electromagnetic (by means of coils) or electrostatic can be used. In an embodiment use is made of a dynamic 6-pole coil, positioned at the screen-side of the deflection unit, of which the excitation current is proportional to x . y, where (x, y) is the screen position.

It is noted that the above examples show conductive index elements. Within the framework of the preferred embodiments of the invention, index elements may comprise index elements which, when hit by an electron beam, provide a luminous signal (which can be an IR or UN signal) which is perceived by an optical sensor, which optical sensor converts the optical signal to an electrical signal which is then used to correct the position of electron beams.

It is noted that within the framework of the invention the phrase 'deflection system is self-convergent in the in-line direction' encompasses embodiments in which the combination of the deflection unit and possible additional means to correct the raster errors has self-convergent properties in the y-direction. It is noted that within the framework of a preferred embodiment of the invention, the index elements and their signals are used to correct errors, but that this does not exclude embodiments in which a part of the raster errors (in a first order approximation) is corrected by means that do not operate with the index signals. In an alternative, preferred, embodiment of the invention, the cathode ray tube comprises a colour selection electrode (such as for instance a shadow mask, or a tensioned colour selection electrode), said colour selection electrode being provided with index elements on the side facing the electron gun, or forming itself an index means. Preferably in said embodiment the colour selection electrode is provided with slits elongated in the line scanning direction (x-direction).

While the invention has been described in connection with preferred embodiments, it will be understood that modifications thereof within the principles outlined above will be evident to those skilled in the art, and thus the invention is not limited to the preferred embodiments but is intended to encompass such modifications. Modifications include amongst others any and each combination of above described features and characteristics even if not explicitly described in the claims. Any reference signs do not limit the scope of the claims. The word "comprising" does not exclude the presence of elements other than those listed in a claim. The use of the word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

Claims

CLAIMS:

1. Cathode ray tube device (1) comprising an elongated display screen (2) with a long axis (21) and a short axis (22) comprising a phosphor pattern (10), a neck portion (4) comprising an electron gun (5) for generating three in-line electron beams (6, 7, 8) oriented in an in-line plane, and a deflection system (11) mounted on a cone portion for generating magnetic fields for deflecting said electron beam in a line scanning and scanning direction across the display screen, characterized in that the in-line plane is substantially perpendicular to the line scanning direction, (P_mim_e -¹ Pscan_nmg)_. the line scanning direction is substantially

parallel to the long axis of the display screen (P _scanning II 21) , the deflection system is self-

convergent in the in-line direction (P_sc || Pm_ϋne), and the neck portion has, in cross-sectional view, a long axis (A and a short axis, the long axis of the neck portion being substantially parallel to the short axis of the display screen, and the electron gun has two glass beads (41) which are situated in the in-line plane at either side of the electron gun.

2. Cathode ray tube device as claimed in claim 1, characterized in that the cathode ray tube comprises a colour selection electrode, said colour selection electrode being provided with index elements on the side facing the electron gun, or forming itself an index means.

3. Cathode ray tube device as claimed in claim 1, characterized in that the cathode ray tube does not comprise a colour selection electrode and on or near the display screen an index system is provided having index elements (21a, 21b) extending in the line scanning direction for generating index signals in relation to the position of electron beams vis-a-vis the index elements, and an electrical circuit (12, 13, 14) is provided for providing deflection correction signals corresponding to the index signal to the deflection system (13) for correcting the positions of the electron beams.