Deflection yoke with a forked shunt

Abstract: A pair of E-shaped shunts (23a,23b) are disposed near the top and bottom, respectively, of a deflection yoke for reducing inner and outer horizontal trap errors. A central portion of the E-shaped shunt (23a,23b) reduces a ratio between magnitudes of inner and outer E-W pincushion raster distortions.

Description: The invention relates to a deflection apparatus in a color picture tube (CRT) display system as defined in the preamble of claim 1. Such a deflection apparatus is known, for example, from US-A-4,246,560. The electrons of each of the three electron beams of the CRT, R, G and B, will traverse a greater distance when deflected towards the edge of the viewing screen than when directed toward the center. Due to the separation of the electron guns, this may result in a separation of the landing points of the three electron beams when they are deflected towards the edges of the screen. This effect causes the light spots of the three beams at points on the viewing screen away from the center to be separated. This is known as misconvergence and results in color fringes about the edges of the displayed images. Misconvergence may be measured as a separation or distance of the ideally superimposed red and blue lines of a crosshatch pattern of lines appearing on the screen when an appropriate test signal is applied to the picture tube. Each of the three electron beams scans a raster which may be identified by its color. Thus, a green raster is ordinarily scanned by the center electron beam and the outside beam scan red and blue rasters, respectively. The crosshatch pattern is formed in each of the red, green and blue rasters. The crosshatch pattern outlines the raster with generally vertical and horizontal lines, and also includes other intermediate vertically and horizontally-directed lines. The field flux lines produced by the vertical deflection winding are made barrel-shaped at a portion of the yoke that is intermediate the gun end and the screen end of the yoke. Such field nonuniformity reduces misconvergence at the 12 o'clock point. To enhance the barrel-shaped vertical field nonuniformity, a pair of shunts near the top and bottom of the yoke, respectively, have been utilized (see the above mentioned patent US-A-4,246,560). An outer horizontal trap misconvergence error is defined as the separation between horizontal red and blue lines at the top or bottom of the screen of the CRT. An inner horizontal trap misconvergence error is defined at a region of the CRT screen midway between the top and center of the CRT screen, or midway between the bottom and center of the CRT screen. A CRT having an aspherical or flat faceplate may tend to introduce a significant difference between the outer and inner horizontal trap errors. It may be desirable to reduce the difference between the outer and inner horizontal trap errors. The reduction of such difference facilitates reduction of an overall outer/inner horizontal trap error. In a deflection apparatus of the invention, a pair of forked shunts are placed near the top and bottom, respectively, of the deflection yoke. Each forked shunt includes a pair of outer branch members. The angular position in the X-Y plane occupied by the shunt varies as a function of the coordinate Z along the longitudinal Z-axis of the CRT. Therefore, the forked shunt varies the vertical deflection field nonuniformity at different X-Y planes of the yoke that are perpendicular to the Z-axis of the yoke. As a result, the aforementioned difference between outer and inner horizontal trap error is reduced. Due to the flatness of the faceplate of the CRT, the magnitude of an East-West pincushion distortion may vary in a non-linear manner as a function of the coordinate X of the beam along the X-axis of the screen of the CRT. Such non-linear variation may hinder conventional East-West distortion correction circuitry from fully correcting the distortion. It may be desirable to utilize the forked shunts in such a way that the ratio between the magnitudes of inner East-West geometry distortion error and outer East-West geometry distortion error is greater than a predetermined value. By maintaining such ratio greater than a predetermined value, conventional East-West pincushion distortion correction circuits can be used for providing overall acceptable inner/outer East-West pincushion distortion correction. In accordance with an inventive feature, the forked shunt includes a center branch member between the outer branch portions. Advantageously, the center branch member increases the ratio between the magnitude of the inner East-West geometry distortion error and that of the outer East-West geometry distortion error. WO-A-91/18410 discloses a color picture tube system including H-shaped shunts for providing trilemma correction and T-shaped shunts for correcting corner trap convergence errors and A-zone convergence errors. A deflection apparatus of the invention includes a cathode ray tube of an in-line system including an evacuated glass envelope. A display screen is disposed at one end of the envelope. An electron gun assembly is disposed at a second end of the envelope. The electron gun assembly produces a plurality of electron beams that form corresponding rasters on the screen upon deflection. A deflection yoke is mounted around the envelope and includes a vertical deflection coil for producing a vertical deflection field in the cathode ray tube. A horizontal deflection coil produces a horizontal deflection field in the cathode ray tube. A core made of magnetically permeable material is magnetically coupled to the vertical and horizontal deflection coils. A pair of E-shaped shunts is disposed adjacent to one of the deflection coils and between opposite ends thereof for modifying one of the deflection fields. Each of said shunts includes a center leg and two outer legs spaced apart in a horizontal direction and a member connecting together the plurality of legs. FIGURE 1 illustrates a side view of a deflection yoke embodying an aspect of the invention that is mounted on a cathode ray tube; FIGURE 2 illustrates a front view of the deflection yoke of FIGURE 1 as seen from the display screen of the cathode ray tube; FIGURE 3 illustrates a side view of the yoke of FIGURE 1 in more detail; FIGURES 4a and 4b illustrate, each, a corresponding display pattern on a screen of a cathode ray tube for explaining corresponding beam landing errors; FIGURE 5a illustrates a shunt embodying an aspect of the invention that is used in the yoke of FIGURE 1; FIGURE 5b illustrates a shunt embodying an aspect of the invention that can be used in the yoke of FIGURE 1 instead of the shunt of FIGURE 5a; FIGURES 5c and 5d illustrate the angular positions in the corresponding X-Y plane occupied by the back and front portions, respectively, of the shunt of FIGURE 5b; and FIGURES 6a-6d illustrate field distribution functions of the yoke of FIGURE 1. In FIGURE 1, a CRT 10 includes a screen or faceplate 11 upon which are deposited repeating groups of red, green and blue phosphor trios. CRT 10 is of the type A68EET38X110 with a Super-Flat faceplate size 27V or 68 centimeter. The deflection angle is 108°. The distance from the yoke reference line to the inside of the screen at the screen center, referred to as the throw distance, is 275 millimeter. Faceplate 11 is much flatter than typical CRT's and sagittal heights are only half that of typical face contour. The contour of the inner surface of the faceplate 11 is defined by the following equation. Z c = A1 · X 2 + A2 · X 4 + A3 · X 6 + A4 · Y 2 + A5 · X 2 · Y 2 + A6 · X 4 · Y 2 + A7 · X 6 · Y 2 + A8 · Y 4 + A9 · X 2 · Y 4 + A10 · X 4 · Y 4 + A11 · X 6 · Y 4 + A12 · Y 6 + A13 · X 2 · Y 6 + A14 · X 4 · Y 6 + A15 · X 6 · Y 6 where: Z c is the distance from a plane tangent to the center of the inner surface contour. X and Y represent distances from the center, in the directions of the major and minor axes, respectively. A1 to A15 are coefficients that depend on the diagonal dimension of the faceplate. For a tube faceplate of CRT 10 with a viewing screen having a diagonal dimension of 68 cm, suitable coefficients A1 to A15 are shown in Table I. A CRT with the contour defined by these coefficients may benefit in convergence characteristics when using inventive features described below. The X and Y dimensions must be in millimeters to use the coefficients of the Table. A1 = 0.2380978 x 10 -03 A2 = 0.1221162 x 10 -09 A3 = 0.9464281 x 10 -14 A4 = 0.3996533 x 10 -03 A5 = -.3144822 x 10 -08 A6 = -.2969186 x 10 -14 A7 = 0.0000000 x 10 +00 A8 = 0.6663320 x 10 -09 A9 = 0.2935719 x 10 -14 A10 = -.3869349 x 10 -18 A11 = 0.0000000 x 10 +00 A12 = 0.1755161 x 10 -13 A13 = 0.9320407 x 10 -19 A14 = 0.7687528 x 10 -25 A15 = 0.2308889 x 10 -28 An electron gun assembly 15 is mounted in a neck portion 12 of the tube opposite the faceplate. Gun assembly 15 produces three horizontal in-line beams R, G and B. A deflection yoke assembly designated generally as 16 is mounted around the neck and flared portion of the tube by a suitable yoke mount or plastic liner 19. Yoke 16 also includes a flared ferrite core 17, a vertical deflection coil 18V and a horizontal deflection coil 18H. Deflection yoke 16 is of the self-convergence type. FIGURE 2 illustrates in greater detail deflection yoke 16, embodying an aspect of the invention. Similar symbols and numerals in FIGURES 1 and 2 indicate similar items or functions. In FIGURE 2 the yoke assembly is viewed from the electron-beam exit side. Plastic yoke mount 19 of FIGURE 2 serves to hold pair of saddle-type horizontal deflection coils 18H in proper orientation relative to flared ferrite core 17 around which vertical deflection winding 18V is wound. Thus, deflection yoke 16 is a saddle-toroid (ST) type. In the side view illustrated in FIGURE 3, a beam-exit end is on the right. Similar symbols and numerals in FIGURES 1, 2 and 3 indicate similar items or functions. A longitudinal or Z-axis of yoke 16 or CRT 10 of FIGURE 1 is defined in a conventional manner. In each plane of yoke 16 defined by a corresponding coordinate Z that is perpendicular to the Z-axis, a corresponding Y-axis is defined in parallel to a vertical or minor axis of screen 11. Similarly, a corresponding X-axis is defined in parallel to a horizontal or major axis of screen 11. The coordinate X=Y=0 in each plane of yoke 16 is located on the Z-axis. In the vicinity of a beam entrance end of yoke 16 of, for example, FIGURE 1, a vertical deflection field produced by coil 18V is preferably pincushioned-shaped for correcting vertical coma error. To reduce over-convergence at the 6 and 12 o'clock hour points, the vertical deflection field produced by vertical deflection coil 18V is made barrel-shaped at an intermediate portion of the yoke, between the beam entrance and exit ends of yoke 16 of FIGURE 1. It may be desirable to enhance the degree of barrel-shaped field nonuniformity over what can be obtained by arrangement of the winding distribution of the vertical deflection coil. Accordingly, a pair of field formers or E-shaped shunts 23a and 23b made of soft or permeable material are mounted near the top and bottom of the yoke in the intermediate portion of the yoke. Field formers 23a and 23b, embodying an inventive feature, increase the barrel-shaped field nonuniformity and are mounted on the side of plastic yoke mount or insulator 19 that faces vertical deflection winding 18V between vertical deflection winding 18V and the neck of CRT 10. FIGURE 4a, illustrates an example of a convergence pattern of horizontal blue and red lines displayed on screen 11 of FIGURE 1. Similar symbols and numerals in FIGURES 1-3, and 4a indicate similar items or functions. The convergence pattern example of FIGURE 4a may result when, instead of shunts 23a and 23b, conventional rectangular shunts, not shown, are installed. For example, a distance d between a red line RED1 and a blue line BLUE1 at the corner of screen 11, conventionally referred to as 2:00 o'clock hour point, is equal to -0.49 millimeter. Distance d is taken with respect to the red line. Thus, a negative value d indicates that the red line is above the blue line. Similarly, at the 2:30 o'clock hour point, the distance d is equal to +0.82 millimeter, at 1:00 o'clock hour point, the distance d is equal to -0.56 millimeter and at point A distance d is equal to +0.38 millimeter. Vertical misconvergence at the 2:00 o'clock hour point and at the 1:00 o'clock hour point is referred to as external or outer horizontal trap error. Whereas, vertical misconvergence at the 2:30 o'clock point and at the A point is referred to as internal or inner horizontal trap error. Varying the winding distribution of, for example, coil 18V affects both the convergence error d at, for example, the 2:00 o'clock hour point and at the 2:30 o'clock hour point in the same sense. For example, it may make both values more negative; alternatively, it may make both values more positive. A similar situation occurs with respect to coil 18H. It may be desirable to reduce the difference between the external and internal horizontal trap errors. When such difference becomes small, selecting an appropriate winding distribution can result in overall reduction of horizontal trap errors. Advantageously, E-shaped shunts 23a and 23b vary the vertical deflection field nonuniformity in different degrees when lines RED1 and BLUE1 of FIGURE 4a are formed on screen 11 and when lines RED2 and BLUE 2 are formed. In this way, the difference between external and internal horizontal trap errors is reduced. FIGURE 5a illustrates in detail and with the actual proportion shunt 23a or 23b, embodying an aspect of the invention. Similar symbols and numerals in FIGURES 1-3, 4a, 4b and 5a indicate similar items or functions. E-shunt 23a or 23b of FIGURE 5a is slightly curved to follow the curvature of liner 19 of FIGURE 3. The dimensions shown in FIGURE 5a are applicable to the shunt in its flat condition prior to forming the curvature by bending the shunt. E-shunt 23a or 23b of FIGURE 5a includes a connecting member or base 30 that is disposed closer to the beam entrance than to the beam exit side of yoke 16 of FIGURE 1. A longitudinal axis AX 30 of base 30 of FIGURE 5a is generally perpendicular to the Z-axis. A central leg or branch 31 has a longitudinal axis AX 31 that is generally parallel to the Z-axis. A front end 31c of central branch 31 is placed closer to the beam exit end than the rest of the shunt. A pair of arm-shaped, outer legs or branches 32a and 32b, are disposed on opposite sides of branch 31 and symmetrically with respect to axis AX 31 . Thus, branches 31, 32a and 32b are joined together at base 30 and extend from base 30 separately. Back ends 32ab and 32bb of outer branches 32a and 32b, respectively, and back end 31bb of central branch 31 are remote from the beam exit end and are joined to one another via base 30. Thus, each magnetic path with high permeability exists between each one of branches 32a, 32b and 31 and each of the other ones of branches 32a, 32b and 31 and passes through base 30. Back end 32ab of branch 32a, for example, is located closer to the beam entrance end than a front end 32af. The longitudinal axis AX 32a of branch 32a forms an angle α with axis AX 31 . Advantageously, the difference in angular position, not shown, with respect to the corresponding Y-axis in which back end 32ab is located in its X-Y plane and the angular position, not shown, with respect to the Y-axis in which front end 32af is located in the corresponding X-Y plane results in different degrees of deflection field nonuniformity modification of the barrel-shaped vertical deflection field at the different planes. Front end 32af of branch 32a that is closer to the beam exit end affects more outer horizontal trap error than back end 32ab. Conversely, back ends 32ab and 32bb of branches 32a and 32b, respectively, that are close to base 30, affect more inner horizontal trap error. By suitable selection of the dimensions associated with branches 32a, 32b and 31 and of base 30, the difference between outer and inner horizontal trap error can, advantageously, be reduced. With shunts 23a or 23b of FIGURE 5a, the vertical misconvergence error is equal at the 2:00 o'clock hour point to 0.47 millimeter, it is equal at the 1:00 o'clock hour point to -0.48 millimeter, it is equal at the 2:30 o'clock hour point to +0.34 millimeter and it is equal at the A point to +0.2 millimeter. The results indicate that the difference between external and internal horizontal trap errors is, advantageously, smaller than that obtained with the aforementioned prior art rectangular shunt. FIGURE 4b illustrates an example of uncorrected external or outer East-West pincushion distortion error and an example of uncorrected internal or inner East-West pincushion distortion error. Similar symbols and numerals in FIGURES 1-3, 4a, 4b and 5a indicate similar items or functions. Customarily, the extent of external East-West pincushion distortion error is determined by the expression: E r = 2x(d4+d5) + (d6+d7) and that of the internal East-West pincushion distortion error is determined by the expression: I r = 2x(d8+d9) = (d6+d7) The values d4 to d9 represent corresponding lengths in FIGURE 4b. It may be desirable to have the ratio, I r / E r , greater than, for example, 0.35 so that a conventional East-West pincushion distortion correction circuitry can be utilized to correct both errors to acceptable extents. In accordance with an inventive feature, central branch 31 of FIGURE 5a increases the ratio I r / E r between the magnitudes of internal and external East-West raster distortion errors relative to such ratio, obtained if branch 31 were removed For example, when central branch 31 is removed, the ratio, I r / E r , is equal to 0.34; whereas, with shunts 23a and 23b of FIGURE 5 that include central portion 31, the ratio, I r / E r , is equal to 0.368. In this way, conventional East-West raster distortion correction circuits, not shown, may be utilized. FIGURE 5b illustrates in detail a shunt 23a' embodying an inventive feature. Shunt 23a' of FIGURE 5b can be used instead of shunt 23a of FIGURE 5a. Shunt 23a' of FIGURE 5b may provide a greater degree of barrel shaping to the vertical deflection field than shunt 23a of FIGURE 5a. FIGURE 5c illustrates a cross-section at the back portion of shunt 23a' of FIGURE 5b in an X-Y plane 110. As shown in FIGURE 5c, the shunt occupies an angle in the range of 0°-18° in each quadrant. FIGURE 5d illustrates a front view of the profile of shunt 23a' in the X-Y plane as viewed from the screen. As shown in FIGURE 5d, shunt 23a' occupies angles between 0°-18° and between 38° and 47° in each quadrant. Similar symbols and numerals in FIGURES 5a-5d indicate similar items or functions. The angular difference between the front and back portions of the shunt provides the aforementioned required inner/outer horizontal trap error difference and inner/outer East-West pincushion distortion ratio. FIGURES 6a, 6b, 6c, 6d, 6e and 6f illustrate field distribution functions V 0 (Z), H 0 (Z), V 2 (Z), H 2 (Z), V 4 (Z) and H 4 (Z), respectively, of the deflection fields in yoke 16 of FIGURE 1. Similar symbols in FIGURES 1, 2, 3, 4a, 4b, 5a-5d and 6a-6f indicate similar items or functions. The strength or intensity of the magnetic field produced by the deflection coil 18H, for example, of FIGURES 2 and 3 can be measured with a suitable probe. Such measurement can be performed for a given coordinate Z=Z1 of FIGURE 3, for a coordinate Y=0 of FIGURE 2 and for a given coordinate X = X1. For the purpose of measurement, coordinate X1 varies in the direction of the X-axis, the horizontal deflection direction. The plane in which coordinate X = X1 varies separates the bottom edges of top saddle coil 18H of FIGURE 3 from those of bottom saddle coil 18H. The results of measuring the strength of the magnetic field as a function of coordinate X, for a constant coordinate Z = Z1 and for coordinate Y = 0, can be used for computing, in a well known manner, field distribution functions or coefficients H 0 (Z1), H 2 (Z1), H 4 (Z1) and other higher coefficients of a power series H(X) = H 0 (Z1) + H 2 (Z1)X 2 + H 4 (Z1)X 4 . The term H(X) represents the strength of the magnetic field as a function of the X coordinate, at the coordinates Z = Z1, Y = 0. A graph can then be plotted depicting the variation of each of the coefficients H 0 (Z), H 2 (Z), H 4 (Z), and other higher order coefficients, as a function of the coordinate Z. In an analogous manner, coefficients V 0 (Z), V 2 (Z), V 4 (Z) and other higher order coefficients can be evaluated as a function of the coordinate Z with respect to vertical deflection coil 18V. To obtain the functions shown in FIGURES 6a-6f, each of the coordinates X and Y are measured in millimeters. In FIGURES 6a-6f, field distribution function drawn in solid lines are obtained when E-shunts 23a and 23b of FIGURE 5a are installed in yoke 16 of FIGURE 2. Whereas, field distribution functions drawn in broken lines are obtained when E-shunts 23a and 23b, for illustration purposes only, are removed from yoke 16. The positions of horizontal coil 18H, shunts 23a or 23b and core 17 relative to the Z-axis are also shown in FIGURES 6a-6f. As shown in FIGURE 6e, field distribution function V 4 (Z) has a peak value at a coordinate Z that is closer to the beam entrance end of shunts 23a or 23b than to its beam exit end. FIGURE 6c shows the effect of shunt 23a or 23b on function V 2 (Z). As a result of shunt 23a or 23b, the peak of function V 2 (Z) attains a larger negative value. Such value corresponds to a barrel-shaped vertical deflection field. To correct N-S raster distortion, a pair of magnets 21a and 21b of FIGURES 2 and 3 are mounted near the top and bottom, respectively, of the yoke at the front or beam-exit portion of the yoke. Magnets 21a and 21b are affixed in recesses in mount 19 and are poled as indicated. Magnets 21a and 21b of FIGURE 2 that are disposed near the beam exit end of the yoke are used to correct external North-South (top-bottom) pincushion distortion. Magnets 21a and 21b may degrade the barreling of the vertical deflection field necessary to provide proper convergence. To restore in part the barreling of the vertical deflection field, a pair of magnets 22a and 22b is disposed adjacent the flared inner surface of the yoke at the top and bottom closer towards the beam-entrance end of the yoke. Magnets 22a and 22b are mounted to conform to the contour of coil 18H and disposed between coil 18H and the neck of CRT 10. Magnets 22a and 22b as well as E-shunts 23a and 23b compensate for the vertical convergence error that might be otherwise introduced by magnets 21a and 21b, respectively. The convergence error compensation is obtained because of the resulting increase of the barreling of the vertical deflection field in a region of the deflection field that is further away along the Z-axis from the screen of CRT 10 than magnets 21a and 21b. To correct internal N-S geometry distortion, a pair of permanent corner magnets 24a and 25a of FIGURE 2 are mounted on liner 19 at opposite sides of top magnet 21a. Magnet 24a is disposed approximately at angle  = +29° and magnet 25a is disposed approximately at angle  = -29° and symmetrically with respect to the corresponding Y-axis. Thus, each of magnets 24a and 25a is disposed closer to the Y-axis than to the X-axis because angle  is smaller than 45°. A pair of corner magnets 24b and 25b of FIGURE 2 are disposed symmetrically to magnets 24a and 25a, respectively, with respect to the X-axis. Corner magnets 24b and 25b are disposed at opposite sides of magnet 21b. Magnets 24a, 21a and 25a affect beam spot landing position mainly when the beam spot is above the vertical center of the screen of the CRT. In a similar manner, magnets 24b, 21b and 25b affect it mainly when the beam spot is below the vertical center.

Claims: A deflection apparatus, comprising: a cathode ray tube (10) of an in-line system including an evacuated glass envelope, a display screen (11) disposed at one end of said envelope, an electron gun assembly (15) disposed at a second end of said envelope, said electron gun assembly producing a plurality of electron beams (R,G,B) that form corresponding rasters on said screen upon deflection; a deflection yoke (16) mounted around said envelope, including: a vertical deflection coil (18V) and a horizontal deflection coil (18H) for producing a deflection field in said cathode ray tube; a core (17) made of magnetically permeable material magnetically coupled to said vertical and horizontal deflection coils; characterized by a pair of E-shaped shunts (23a, 23b) disposed adjacent to at least one of said deflection coils (18V) between a beam entrance end and a beam exit end thereof at a top end and a bottom end, respectively, of a vertical axis (Y) and symmetrically with respect to a horizontal axis (X) of said yoke, each of said shunts including a center leg (31) and two outer legs (32a, 32b) horizontally spaced apart, and a member (30) connecting together the plurality of legs. An apparatus according to Claim 1, characterized in that the angle between the longitudinal axis (AX 31 ) of said center leg (31) and the longitudinal axis (AX 32a , AX 32b ) of each of said outer legs (32a, 32b) is substantially smaller than 90°. An apparatus according to Claim 1, characterized in that the angular position with respect to said vertical axis (Y) of said yoke occupied by said outer legs (32a, 32b) in a vertical plane (XY) perpendicular to the longitudinal axis (Z) of the cathode ray tube varies as a function of the coordinate Z along said longitudinal axis for reducing the difference between the inner horizontal trap error and the outer horizontal trap error. An apparatus according to Claim 3, characterized in that said central leg (31) increases the ratio between the magnitudes of inner East-West pincushion distortion and outer East-West pincushion distortion. An apparatus according to Claim 1, characterized in that said connecting member (30) is disposed perpendicularly to the direction of the longitudinal axis (AX31) of said central leg (31) and to an electron beam axis of said yoke (18). An apparatus according to Claim 1, characterized in that each of said center (31) and outer legs (32a, 32b) runs generally in the direction of an electron beam.

Inventors: MAILLOT ANDRE, PERRON DAVID, MAILLOT, ANDRE, PERRON, DAVID, Maillot, André
Applicants: THOMSON TUBES & DISPLAYS, THOMSON TUBES & DISPLAYS S.A.
Application year: 1993
Main classification: H01J29
IPC codes: H01J29