Deflection unit for a colour cathode ray apparatus

Abstract: In a color cathode-ray apparatus, electron beams emitted from an electron gun assembly (10) are defelect­ ed along horizontal and vertical axes by horizontal and vertical deflecting units. The horizontal deflecting unit (16, 17) includes first and second horizontal deflecting yokes (16, 17) which are located such that the center of the second horizontal deflecting yoke (17) is defined between the center of the first horizontal deflecting yoke (16) and the electron gun assembly (10). First and second horizontal deflection currents are supplied to the first and second horizontal deflecting yokes (16, 17) and are changed in different manners, in accordance with a degree to which the electron beams are to be deflected, and the first and second deflection yokes generate pincushion and barrel magnetic fields, respectively.

Description: The present invention relates to a color cathode-ray tube and, more particularly, to a color cathode-ray tube which has a deflection unit improved such that three electron beams are converged accurately on a screen and neither a negative cross pattern nor a positive cross pattern will appear on any part of the screen. Of color cathode-ray tubes having a shadow mask, the most prevailing one is the type which comprises an in-line electron gun assembly having three electron guns, and a shadow mask having slot-like apertures. The color cathode-ray tube of this type has a deflection yoke which is designed to generate such horizontal and vertical deflection magnetic fields that the red, green and blue electron beams pass through an aperture at the same time. These deflection magnetic fields are known, as is disclosed in, for example, Japanese Patent Disclosure No. 57-1857. More specifically, the horizontal deflection magnetic field is shaped like a pin-cushion as is shown in Fig. 1A, and the vertical deflection magnetic field is shaped like a barrel as is illustrated in Fig. 1B. When the cathode-ray tube has a relatively small deflection angle, these magnetic fields deflect the three electron beams correctly, thus achieving good beam-convergence on any part of the screen. Therefore, the tube need not have a device for correcting the beam-convergence. When the cathode-ray tube has a great deflection angle, e.g., 100° or 110°, however, a convergence error, which is generally called "crossing of vertical lines," will occur. It is regarded as impossible to correct this convergence error even if the horizontal and vertical deflection magnetic fields are changed in characteristic, as is disclosed in Iwasaki et al. "SST Deflection Yoke for Use in an In-Line High-Precision Color" in the technical report of Journal of Television Society of Japan, ED619, 1982. More precisely, when a saddle-and-toroidal deflection yoke is incorporated in a color cathode-ray tube of a great deflection angle, a positive anisotropic astigmatism will occur, inevitably forming a positive cross pattern (Fig. 2A) on the screen. When a saddle-and-saddle deflection yoke is incorporated in a color cathode-ray tube of a large deflection angle, a negative anisotropic astigmatism will occur, inevitably forming a negative cross pattern (Fig. 2B) on the screen. Either anisotropic astigmatism is mainly attributed to the fact that the centers of deflection of the magnetic fields, which have been generated by the horizontal and vertical deflection coils, respectively, take different positions. It is generally considered the most desirable that when a saddle-and-toroidal deflection yoke is used, the saddle coil for deflecting the electron beams horizontally have a center of deflection located closer to the electron guns than the center of deflection of the toroidal coil for deflecting the beams vertically. If the centers of deflections of both coils take the same position, a convergence error, generally known as a positive cross pattern (Fig. 2A), will be made. The positive cross pattern is corrected into such a neutral one as is shown in Fig. 2C when the center of deflection of the saddle coil is deviated toward the electron guns from the center of deflection of the toroidal coil. When the center of deflection of the saddle coil is further deviated toward the electron guns, however, a negative cross pattern will appear on the screen as is shown in Fig. 2B. Therefore, it is generally considered possible to change a positive or negative cross pattern to a neutral cross pattern by adjusting the positional relation between the horizontal deflection coil and the vertical deflection coil. Indeed, neither a positive cross pattern nor a negative cross pattern appear on the horizontal axis, at point P0 in the region near this axis, or at points P2 in the corner regions when the positional relation of the horizontal deflection coil and the vertical deflection coil is properly adjusted. However, a positive cross pattern and a negative cross pattern are formed at other points, such as a point P1 between the horizontal axis and any corner region, or a point P3 located at the outer edge of any corner, even if the positional relation of the coils is adjusted. Besides, the cross patterns appearing at point P1 and the outer edge of the corner are inverted. This inversion of cross patterns is prominent in proportion to the deflection angle or the size of the screen. Therefore, in the conventional color cathode-ray, a magnetic plate or the like is attached to the deflection yoke in order to reduce the chance of forming positive and negative cross patterns. The use of the magnetic plate, however, does not reduce said chance sufficiently. Document EP-A-0 283 904 (state of the art in accordance with Article 54(3) EPC) discloses a color cathode ray tube apparatus comprising an evacuated envelope having a tube axis Z, a phosphor screen, an electron gun assembly and deflection magnetic field generating means. The phosphor screen is formed in the envelope , wherein the tube axis Z passes through a center of the phosphor screen, and has horizontal and vertical axes X and Y orthogonal to the tube axis Z. The electron gun assembly is an in-line type electron gun assembly and arranged in the evacuated envelope, for emitting a central beam, and side beams, toward the phosphor screen, the electron beams being landed on said phosphor screen to cause the phosphor screen to emit light rays. The deflection magnetic field generating means are arranged outside the evacuated envelope, for generating horizontal and vertical deflection magnetic fields inside the envelope so as to horizontally and vertically deflect the electron beams and scan the phosphor screen with the electron beams. The horizontal deflection magnetic field includes a main deflection magnetic field having a barrel-shaped distribution and composed of a vertical component formed in the envelope so as to be symmetrical about a Y-Z plane including Y-and Z-axes and extend along the Y-axis, and an auxiliary deflection magnetic field substantially antisymmetric about the Y-Z plane including the Y- and Z-axes and mainly composed of a vertical component. The object of the present invention is to provide a color cathode-ray apparatus which has a good convergence characteristic. To solve this object the present invention provides a color cathode-ray apparatus as specified in anyone of claims 1, 7 and 8. Preferred embodiments of this color cathode-ray apparatus are described in claims 2 to 6. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Figs. 1A and 1B are diagrams showing the distribution of a horizontal deflection magnetic field of a pincushion type, and the distribution of a vertical deflection magnetic field of a barrel type; Figs. 2A, 2B, and 2C show a positive cross pattern, a negative cross pattern, and a neutral cross pattern, respectively; Fig. 3 is a diagram explaining the cross pattern appearing at the outer edge of each corner of the screen, and also the cross pattern appearing between the horizontal axis of the screen and the corner thereof; Fig. 4 is a sectional view of a color cathode-ray tube which is one embodiment of the present invention; Fig. 5 is an enlarged view showing a part of the cathode-ray tube shown in Fig. 4; Figs. 6A and 6B are diagrams representing how the deflection magnetic field generated by the deflection unit (Figs. 4 and 5) is distributed along the axis of the cathode-ray tube; Fig. 7 is a diagram explaining the convergence characteristic of a color cathode-ray apparatus according to the invention; Figs. 8A, 8B and 8C show the waveforms of the AC currents which can be supplied to the first and second horizontal deflection coils both shown in Fig. 5; Fig. 9 is a circuit diagram showing the deflection current source shown in Fig. 5; Fig. 10 is a sectional view showing a modification of the deflection unit shown in Fig. 5; Fig. 11 is a sectional view illustrating another modification of the deflection unit shown in Fig. 5; Fig. 12 shows the waveform of an AC current which is supplied to the first and second vertical deflection coils incorporated in the deflection unit shown in Fig. 11; and Fig. 13 is a circuit diagram showing the deflection current source shown in Fig. 11. Figs. 4 and 5 schematically illustrate a color cathode-ray tube according to the present invention. As is shown in Fig. 4, the color cathode-ray tube comprises an envelope, in-line electron gun assembly 10 incorporated in the envelope, and shadow mask 3 provided within the envelope. The envelope of this tube has a face plate 1 and a phosphor screen 2 formed on the inner surface of the plate 1. The screen 2 consists of dot-shaped or stripe-shaped phosphor layers which are arranged orderly. These phosphor layers emit red, green and blue light rays when the electron beams emitted from the electron gun assembly are applied to them. Several panel pins 5 protrude from the inner periphery of a skirt 1-1 of the face plate 1. Springs 6 connect a mask frame 4 to pins 5. Hence, the mask frame 4 is secured to the face plate 1. A mask frame 4 supports a shadow mask 3, forming a predetermined gap between the phosphor screen 2 and the shadow mask 3. The shadow mask 3 has a number of apertures 12. An inner shield 14 is fastened to the mask frame 4. A funnel 7, which is a part of the envelope, is connected to the skirt 1-1 of the face plate 1. A neck 9, which is also a part of the envelope, extends from the funnel 7. An electron gun assembly 10 is arranged within the neck 9. A deflection unit 8 is mounted on the neck 9. More specifically, the deflection unit 8 is fastened to the neck 9 by means of a support 8-1 and steadily mounted on the funnel 7 by means of wedges 8-2. A ring-shaped magnet 11 is also mounted on the neck 9. The electron gun assembly 10 has three electron guns for emitting a red electron beam, a green electron beam, and a red electron beam, respectively, toward the phosphor screen 2. The three electron guns are arranged side by side in horizontal direction, thus forming an in-line gun assembly which is predominantly used in color cathode-ray tubes generally called "three-beam, shadow mask tubes." The gun assembly 10 is designed to apply the red, green and blue beams onto the same point on the phosphor screen 2. The deflection unit 8 is designed to generate a magnetic field which causes the three electron beams to pass through the same aperture of the shadow mask 3. As is understood from Fig. 5, the deflection unit 8 (i.e., a deflection yoke) comprises one toroidal coil 15 and two saddle coils 16 and 17. The toroidal coil 15, which functions as a vertical deflection coil, is formed of ferrite core 18 and a wire wound around core 18. It generates a magnetic field whose intensity is distributed as indicated by a curve I in Fig. 6A and 6B. The saddle coils 16 and 17, both function as horizontal deflection coils, generate magnetic fields whose intensities are distributed as indicated by a curve II in Fig. 6A and a curve III in Fig 6B, respectively. The saddle coil 17 is mounted partly on the funnel 7 and partly on the neck 9; the saddle coil 16 is mounted on the coil 17; and the toroidal coil 15 is mounted on the coil 16. The first saddle coil 16 is so designed that a combination of this coil 16 and the toroidal coil 15 has a positive cross convergence characteristic. More specifically, the saddle coil 16 generates a pincushion magnetic field. When this magnetic field interacts with the magnetic field generated by the toroidal coil 15, the positive cross convergence characteristic shown in Fig. 2A will be obtained. In addition, as is shown in Fig. 6A, the deflection center C1 of the horizontal deflection magnetic field (curve II in Fig. 6A) generated by the first saddle coil 16 is located further to the electron gun assembly 10 than the deflection center C0 of the vertical deflection magnetic field (curve I in Figs. 6A and 6B) generated by the toroidal coil 15. The second saddle coil 17 is so designed that a combination of this coil 17 and the toroidal coil 15 has a negative cross convergence characteristic. More specifically, the saddle coil 17 generates a barrel magnetic field. When this magnetic field interacts with the magnetic field generated by the toroidal coil 15, the negative cross convergence characteristic shown in Fig. 2B will be obtained. In addition, as is shown in Fig. 6B, the deflection center C2 of the horizontal deflection magnetic field (curve III in Fig. 6A) generated by the second saddle coil 17 is located closer to the electron gun assembly 10 than the deflection center C1 of the horizontal deflection magnetic field generated by the first saddle coil 16, with respect to the center C0 of the vertical deflection magnetic field (curve I in Figs. 6A and 6B) generated by the toroidal coil 15. The deflection unit 8, which is constituted by the toroidal coil 15 (i.e., a vertical deflection coil), the first saddle coil 16 (i.e., a horizontal deflection coil), and the second saddle coil 17 (i.e., a horizontal deflection coil), has the specific cross convergence characteristic shown in Fig. 3, when the same deflection current is supplied to both saddle coils 16 and 17. The first saddle coil 16 generates a pincushion magnetic field, and the second saddle coil 17 generates a barrel magnetic field. These magnetic fields are combined, thus eliminating the convergence error on the horizontal axis of the phosphor screen 2, as is illustrated in Fig. 7. Moreover, in order to eliminate the convergence error on the entire surface of the phosphor screen 2, deflection currents I B and I C , whose waveforms are shown in Figs. 8A, 8B and 8C, are supplied from a deflection current source 20 to the first and second saddle coils 16 and 17, respectively. More specifically, to deflect the electrons beams vertically and thereby apply them to a point P1 located between the horizontal axis of the screen 2 and a corner of the screen 2 (Fig. 7), currents I B and I C are applied to the coils 16 and 17, wherein |I B | < |I C |. To deflect the electrons beams vertically and thereby apply them to a point P2 in the corner of screen 2, currents I B and I C are supplied to the coils 16 and 17, where I B = I C . Further, in order to deflect the beams vertically and thereby apply them to a point P3 at the outer edge of the screen corner, currents I B and I C are supplied to the coils 16 and 17, where |I B | > |I C |. When the relation between horizontal deflection currents I B and I C is changed in this way, the cross convergence over the entire screen will be improved. Fig. 9 is a circuit diagram of the deflection current source 20. Vertical deflection current I A is supplied to input terminals 26 and 27. Saturable reactors 24 and 25 are provided whose reactance can be adjusted and whose saturation points can be adjusted by permanent magnets 28 and 29. The primary ends of these reactors 24 and 25 are connected between input terminals 26 and 27, on the one hand, and the vertical deflection coil 15, on the other. Horizontal deflection current I₀ is supplied to input terminals 22 and 23. The horizontal deflection coil 16, which consists of two coils, is connected in parallel to the input terminals 22 and 23. The horizontal deflection coil 17, also consisting of two coils, is coupled in parallel to the input terminals 22 and 23. The secondary ends of the reactors 24 and 25 are connected between the horizontal deflection coils 16 and 17, on the one hand, and the input terminal 22, on the other hand. Therefore, current I₀ is modulated by vertical deflection current I A , and currents I B and I C (Figs. 8B and 8C) are supplied to the horizontal deflection coils 16 and 17, respectively. Currents I B and I C are changed in accordance with the degree of vertical deflection of the electron beams, and the convergence error is corrected in accordance with the positions on the screen 2 where the beams are applied. As a result, neither a positive cross pattern nor a negative cross pattern is formed on the screen 2. The circuit shown in Fig. 9 can be replaced by a similar circuit having saturable reactors. Further, saturable reactors 24 and 25 can be modulated by the magnetic flux leaking from the vertical deflection coil 15, not by the vertical deflection current. It is also possible that two horizontal deflection currents I B and I C can be supplied from two independent current sources. Moreover, various circuit elements can be utilized to adjust the reactances of the saturable reactors 24 and 25. As is shown in Fig. 5, the first saddle coil 16 is wrapped around almost the entire second saddle coil 17. Alternatively, as is illustrated in Fig. 10, the coil 16 can be positioned so that only its rear end portion surrounds the second saddle coil 17. When the coil 16 is so arranged, it will generate a magnetic field partly overlapping the magnetic field generated by the second saddle coil 17. Further, the vertical deflection coil 15, which is a toroidal coil, can be replaced by a saddle coil. Fig. 11 shows another deflection unit 8 according to the present invention. This deflection unit comprises one horizontal deflection coil 119 and two vertical deflection coils 120 and 121. These coils 119, 120 and 121 are saddle coils. The coils 120 and 121 generate vertical deflection magnetic fields which have different centers of deflection. The deflection center of the coil 120 is defined between the coil 121 and the electron gun assembly 10. The combination of the coils 120 and 119 exhibits a positive cross convergence characteristic shown in Fig. 2A, whereas the combination of the coil 121 and the coil 119 presents the negative cross convergence characteristic shown in Fig. 2B. Further, the combination of the coils 119, 120 and 121 will have the cross convergence characteristic shown in Fig. 3 when deflection currents I D and I E supplied to the saddle coils 120 and 121 are equal. The horizontal deflection coil 119 generates a magnetic field which is either uniform or a slightly pincushion-shaped. The first vertical deflection coil 120 generates a pincushion magnetic field. The second vertical deflection coil 121 generates a barrel magnetic field. In the deflection unit 8 shown in Fig. 11, a deflection current source 130 supplies deflection currents I F , I D and I E to the horizontal deflection coil 119, the first vertical deflection coil 120, and the second vertical deflection coil 121, respectively. Currents I D and I E have such waveforms as shown in Fig. 12. To deflect the electron beams vertically to a relatively small degree, current I D is smaller than current I E , whereby the positive cross pattern is eliminated. On the other hand, to deflect the beams vertically to a relatively large degree, current I E is smaller than current I D , whereby the negative cross pattern is eliminated. As a result, a good convergence characteristic is achieved all over the screen 2. Fig. 13 shows a modification of the deflection current source 130 (Fig. 11). The deflection current supplied to input terminals 26 and 27 is converted into currents I D and I E having the waveforms illustrated in Fig. 12, due to the non-linear current-voltage characteristic of diodes D1 to D4. Deflection currents I D and I E are supplied to the first vertical coil 120 and the second vertical coil 121, respectively. Currents I E and I D can be adjusted by changing the resistances of variable resistors R1 to R4. In the embodiments described above, the centers of deflection of the deflection magnetic fields are shifted, and the distribution of intensity of these fields is changed. The centers of deflection and the distribution of intensity are determined by the desired loci of the electron beams. The centers of deflections are identical with the centers of gravity of the coils, or with the positions where the magnetic fields are most intense, in most color cathode-ray tubes. The embodiment described above uses a combination of coils which are different both in intensity distribution and in center of deflection. According to the invention, a combination of coils which are different in only intensity distribution or only in magnetic field shape such as barrel and pincushion can be used, though these coils can correct a cross convergence error less effectively than that described up to here. As has been described, since the first and second horizontal deflection coils generate a pincushion magnetic field and a barrel magnetic field, the changes in the horizontal deflection currents flowing in these coils are minimized during the vertical deflection period. Hence, these current can be modulated easily. In addition, since the distance between the center of deflection of the horizontal deflection coils is short, it is easy to manufacture the deflection unit (i.e., the deflection yoke). The "center of deflection" of any coil used in the invention may be regarded as either the center of gravity of the coil or the point where the deflection magnetic field generated by the coil is most intense. The color cathode-ray apparatus according to the present invention has a good convergence characteristic all over the phosphor screen, and it is greatly useful from an industrial point of view.

Claims: A color cathode-ray apparatus comprising: an evacuated envelope (1, 7, 9) having a tube axis and horizontal and vertical axes intersecting with the tube axis at right angles and intersecting with each other at right angles; a phosphor screen (2) incorporated within said evacuated envelope; an in-line electron gun assembly (10) for emitting three electron beams toward said phosphor screen (2); horizontal deflection means (119) for generating a horizontal deflection magnetic field thereby to deflect the electron beams along said horizontal axis; vertical deflection means (120, 121) for generating a vertical deflection magnetic field thereby to deflect the electron beams along said vertical axis; and current-supplying means (130) for supplying horizontal and vertical deflection currents to said horizontal and vertical deflecting means (119, 120, 121), thereby to energize said horizontal and vertical deflecting means (119, 120, 121); characterized in that said vertical deflection means (120, 121) includes a first deflection coil (120) having a first deflection center, for generating a first magnetic field, and a second deflection coil (121) having a second deflection center, for generating a second magnetic field, said first and second deflection centers are located at different points on the tube axis, said first deflection center is defined between said second deflection center and said electron gun assembly, said current-supplying means (130) supplies a first vertical deflection current and a second vertical deflection current to said first deflection coil (120) and said second deflection coil (121), respectively, and said first and second vertical deflection currents are changed in The color cathode-ray apparatus according to claim 1, characterized in that said phosphor screen (2) has corner regions each having an outer edge, and intermediate regions between said horizontal axis and said corner regions, and said first and second vertical deflection currents are equal in order to apply the electron beams to a specified point in any corner region. The color cathode-ray apparatus according to claim 2, characterized in that said first vertical deflection current has an absolute value smaller than that of said second vertical deflection current in order to apply the electron beams to a specified point in any intermediate region. The color cathode-ray apparatus according to claim 3, characterized in that said first vertical deflection current has an absolute value greater than that of said second vertical deflection current in order to apply the electron beams to the outer edge of any corner region. The color cathode-ray apparatus according to claim 3, characterized in that said horizontal deflection means (119) generates a magnetic field which is either uniform or slightly pincushion-shaped. The color cathode-ray apparatus according to claim 1, characterized in that said first magnetic field is a pincushion type and said second magnetic field is a barrel type. different manners, in accordance with a degree to which the electron beams are to be deflected vertically. A color cathode-ray apparatus comprising: an evacuated envelope (1, 7, 9) having a tube axis and horizontal and vertical axes intersecting with the tube axis at right angels and intersecting with each other at right angles; a phosphor screen (2) incorporated within said evacuated envelope; an in-line electron gun assembly (10) for emitting three electron beams toward said phosphor screen (2); vertical deflection means (15) for generating a vertical deflection magnetic field thereby to deflect the electron beams along said vertical axis; horizontal deflection means (16, 17) for generating a horizontal deflection magnetic field thereby to deflect the electron beams along said horizontal axis; and current-supplying means (20) for supplying vertical and horizontal deflection currents, to said vertical and horizontal deflecting means (15, 16, 17), thereby to energize said vertical and horizontal deflecting means (15, 16, 17), wherein; said horizontal deflection means (16, 17) includes a first deflection coil (16) having a first deflection center, for generating a first magnetic field, and a second deflection coil (17) having a second deflection center, for generating a second magnetic field, said first and second deflection centers being located at different points on the tube axis and said second deflection center being defined between said first deflection center and said electron gun assembly; said current-supplying means (20) supplies a first horizontal deflection current I B and a second horizontal deflection current I C to said first deflection coil (16) and said second deflection coil (17), respectively, and said first and second horizontal deflection currents (I B , I C ) are changed in different manners, in accordance with a degree to which the electron beams are to be deflected horizontally, and said phosphor screen (2) has corner regions each having an outer edge, and intermediate regions between said horizontal axis and said corner regions, characterized in that said first and second horizontal deflection currents (I B , I C ) are equal in order to apply the electron beams to a specified point in any corner region, said first horizontal deflection current (I B ) has an absolute value less than that of said second horizontal deflection current (I C ) in order to apply the electron beams to a specified point in any intermediate region, said first horizontal deflection current (I B ) has an absolute value greater than that of said second horizontal deflection current (I C ) in order to apply the electron beams to the outer edge of any corner region, said vertical deflection means (15) generates a magnetic field which is either uniform or slightly pincushion-shaped, and said first magnetic field is a pincushion type magnetic field and said second magnetic field is a barrel type magnetic field. A color cathode-ray apparatus comprising: an evacuated envelope (1, 7, 9) having a tube axis and horizontal and vertical axes intersecting with the tube axis at right angles and intersecting with each other at right angles; a phosphor screen (2) incorporated within said evacuated envelope; an in-line electron gun assembly (10) for emitting three electron beams toward said phosphor screen (2); vertical deflection means (15, 120, 121) for generating a vertical deflection magnetic field thereby to deflect the electron beams along said vertical axis; horizontal deflection means (16, 17, 119) for generating a horizontal deflection magnetic field thereby to deflect the electron beams along said horizontal axis; and current-supplying means (20) for supplying vertical and horizontal deflection currents to said vertical and horizontal deflection means (15, 16, 17, 119, 120, 121), thereby to energize said vertical and horizontal deflection means (15, 16, 17, 119, 120, 121); characterized in that one of said vertical deflection means (15, 120, 121) and said horizontal deflection means (16, 17, 119) includes a first deflection coil (16, 120) for generating a barrel magnetic field, and a second deflection coil (17, 121) for generating a pincushion magnetic field, said current supplying means (20) supplies first and second deflection currents to said first and second deflecting means (16, 17, 120, 121), and said deflection currents are changed in different manners, in accordance with a degree to which the electron beams are to be deflected.

Inventors: MOROHASHI KATSUEI C O PATENT D, SHIMOKOBE JIRO C O PATENT DIVI, SHIMOMA TAKETOSHI C O PATENT D, MOROHASHI,KATSUEI C/O PATENT DIVISION, SHIMOKOBE,JIRO C/O PATENT DIVISION, SHIMOMA,TAKETOSHI C/O PATENT DIVISION
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1988
Main classification: H01J29
IPC codes: H04N9, H01J29