Title:
Deflection coil system for color television
United States Patent 3906288


Abstract:
A deflection coil system for television, comprising two saddle-shaped horizontal deflection coils. So as to reduce landing errors, the angle distance between the ends of the foremost coil heads of these coils differs. This angle distance is preferably smaller for the upper coil than for the lower coil.



Inventors:
Duistermaat, Jan Hendrik (Emmasingel, Eindhoven, NL)
Panis, Constantius Johannes Waltherus (Emmasingel, Eindhoven, NL)
Application Number:
05/399859
Publication Date:
09/16/1975
Filing Date:
09/24/1973
Assignee:
U.S. PHILIPS CORPORATION
Primary Class:
Other Classes:
315/370, 335/213
International Classes:
H01J29/76; (IPC1-7): H01J29/50
Field of Search:
315/27XY,27GD,13C,370,371 335
View Patent Images:
US Patent References:
3714500COLOR TELEVISION DISPLAY DEVICE1973-01-30Kaashoek
3195025Magnetic deflection yoke1965-07-13Marley
3165677Television tube deflection coil assembly with separable yoke sections1965-01-12Gostyn et al.
2850678Color television receiver1958-09-02O'Fallon
2616056Unsymmetrical deflection yoke1952-10-28Thalner



Primary Examiner:
Farley, Richard A.
Assistant Examiner:
Montone G. E.
Attorney, Agent or Firm:
Trifari I, Frank Steckler Henry R.
Claims:
What is claimed is

1. A deflection coil system which is adapted to be arranged about a color television display tube at the area where a rearmost cylindrical part of said display tube which comprises electron guns, changes over into a flared foremost part which is provided with a display screen, the said deflection coil system comprising an upper and a lower saddle-shaped horizontal deflection coil, each coil comprising at least one conductor which extends about a window in a plurality of turns, each of the horizontal deflection coils comprising two active parts situated on both sides of the window where the conductors extend in the longitudinal direction of the display tube along the surface of the display tube, and a foremost and a rearmost coil head where the conductors extend substantially perpendicular to the conductors in the active parts, the said coil heads having approximately the shape of a part of a flat ring which is perpendicular to the axis of the display tube, the ends of said ring being arranged at a given angle distance from each other, and means for correcting landing errors comprising the value of the said angle distance at the foremost coil heads is different for the two horizontal deflection coils and said coils being asymmetrical about a horizontal plane and symmetrical about a vertical plane.

2. A deflection coil system as claimed in claim 1, wherein the angle distance at the upper horizontal deflection coil is smaller than at the lower horizontal deflection coil.

Description:
The invention relates to a deflection coil system which is adapted to be arranged about a color television display tube at the area where a rearmost cylindrical part of said display tube which comprises electron guns changes over into a flared foremost part which is provided with a display screen. The said deflection coil system comprising an upper and a lower saddle-shaped deflection coil, each of which consists of one or more conductors which extend about a window in a number of turns. The said deflection coils serving for deflection in the horizontal direction of the electron beams generated in the display tube. Each of these horizontal deflection coils comprises two active parts, situated on both sides of the window, where the conductors extend in the longitudinal direction of the display tube along the surface of the display tube, and a foremost and a rearmost coil head where the conductors extend substantially perpendicular to the conductors in the active parts. The said coil heads having approximately the shape of a part of a flat ring which is perpendicular to the axis of the display tube, the ends of said ring being arranged at a given angle distance from each other.

A commonly used type of color television display tube comprises three electron guns which are arranged at the vertices of an equilateral triangle and which can emit electrons which are accelerated in the direction of a display screen which comprises a large number of phosphor dots which, when struck by electrons, luminesce in one of the colors green, red or blue. The phosphor dots are grouped such that each time three dots of a different color are situated at the vertices of an equilateral triangle. In order to ensure that the electrons originating from each of the electron guns can be incident on phosphor dots of one color only, a shadow mask is arranged between the electron guns and the display screen, the said shadow mask being a plate with openings for intercepting electrons which travel to phosphor dots of the wrong color. For proper operation of this system the three electron beams must pass simultaneously through the same opening of the shadow mask (convergence) and the landing spot of each of the three beams on the display screen must lie completely within the circumference of the relevant phosphor dot. These requirements must be satisfied while the electron beams are deflected, so that they scan the entire display screen. A convergence unit is generally arranged behind the deflection coil system on the cylindrical part of the display tube. This unit comprises means for generating a constant magnetic field which corrects the direction of each of the three electron beams such that the electron beams converge in the center of the screen (i.e., without deflection taking place). This convergence unit furthermore comprises means for generating magnetic fields which vary with the strength of the magnetic fields generated by the deflection coil system in order to correct also convergence errors at other locations on the display screen which appear upon deflection of the electron beam.

It was found that, particularly at large deflection angles (for example 110°), a satisfactory convergence can thus be achieved, but also that inadmissibly large landing errors occur at some areas of the display screen. The landing of the blue electron beam imposes problems, particularly in the vicinity of the end of the horizontal axis of the display screen. The invention is based on recognition of the fact that this situation can be improved by introducing a slight asymmetry in the part of the magnetic field generated by the horizontal deflection coil which is traversed by the electron beams incident on the relevant parts of the display screen. By a suitable choice of this asymmetry, the landing errors can be reduced to values within acceptable limits while proper convergence is maintained. A deflection coil system having a suitable asymmetry according to the invention is characterized in that the value of the said angle distance is different for the two horizontal deflection coils.

It was found that the best result can be obtained if the angle distance at the upper horizontal deflection coil is smaller than that at the lower horizontal deflection coil.

The invention will be described in detail hereinafter with reference to the drawing.

FIG. 1 is a substantially simplified longitudinal sectional view of a display tube for color television about which a deflection coil system is arranged.

FIG. 2 is a plan view of a saddle-shaped deflection coil,

FIG. 3 is a front view of a set of horizontal deflection coils forming part of a deflection coil system according to the invention,

FIGS. 4a and b are landing images obtained by deflection coil systems commonly used thus far.

FIG. 4c shows a landing image obtained by the deflection coil system according to the invention, and

FIG. 5 is a front view of a display screen as used in the display tube shown in FIG. 1.

FIG. 1 is a diagrammatic longitudinal sectional view of a color television display tube 1, comprising a rearmost cylindrical part 3 which changes over into a foremost flared part 5. The cylindrical part 3 comprises three electron guns 7 which are arranged at the vertices of an equilateral triangle, only two electron guns being visible in FIG. 1. The flared part 5 is bounded on the front side by a display screen 9, which is shown in front view in FIG. 5 and which is covered with a mosaic of phosphor dots, each of which luminesces, when struck by an electron beam, in one of the colors blue, green or red as is shown at an increased scale in the FIGS. 4a to c to be described hereinafter. Arranged between the display screen 9 and the electron guns 7 is a shadow mask 11 which consists of a metal plate provided with openings through which only the electrons can pass which travel from one of the electron guns to a phosphor dot of the color associated with the relevant gun.

At the area of the transition from the cylindrical part 3 to the flared part 5, a deflection coil system 13, only shown diagrammatically in FIG. 1, is arranged about the display tube 1. Arranged on the rear of the deflection coil system 13 is a convergence unit 15, again only diagrammatically shown, which comprises coils for correcting the direction of the electron beams emitted by the electron guns 7 such that the three electron beams intersect each time in one point at the area of the shadow mask 11. The construction of the deflection coil system 13 is generally known and is not separately shown herein. The deflection coil system comprises an annular ferromagnetic core, saddle-shaped or toroidal deflection coils for the vertical deflection of the electron beams, and saddle-shaped deflection coils for the horizontal deflection of the electron beams.

FIG. 2 is a plan view of one of the saddle-shaped coils for horizontal deflection. This figure shows a window 17 about which turns of conductive wire extend (not separately shown). Arranged on both sides of the window 17 are two active parts 19 and 21 where the conductors which constitute the turns extend, after arrangement of the coil on the display tube 1, approximately according to the longitudinal direction of the display tube and parallel to the surface thereof, i.e., approximately from the bottom upwards in FIG. 2. Arranged on the front of the coil is a foremost coil head 23, while a rearmost coil head 25 is arranged on the rear. In these coil heads the conductors extend substantially perpendicular to the conductors in the active parts 19, 21, i.e., from left to right in FIG. 2. The coil heads 23, 25 are bent such that the conductors in these coil heads remain at some distance from the surface of the display tube 1. As a result, each coil head 23, 25 has the shape of a part of a flat ring which is perpendicular to the axis of the display tube 1, the said part extending over a given angle α as can be seen in the front view shown in FIG. 3. This Figure shows two horizontal deflection coils, one of which (27) envelops the lower half of the display tube 1 (denoted in FIG. 3 by broken lines), while the other (29) envelops the upper half. If current is passed through their constituent conductors, the two horizontal deflection coils 27 and 29 together generate a magnetic field, the lines of force of which extend substantially vertically and which horizontally deflects the electron beams generated by the electron guns 7. For the deflection in the vertical direction, the deflection coil system 13 comprises a second coil pair which is not shown. After the deflection, each electron beam must always strike a phosphor dot of the correct color on the display screen 9. Errors may then occur, as will be described hereinafter with reference to the FIGS. 4a to c.

Each of the FIGS. 4a to c shows a group of three phosphor dots 31, 33 and 35 which, when struck by an electron beam, luminesce in the colors green, red and blue, respectively. As already stated, the display screen 9 is completely covered with such phosphor dots. FIGS. 4a to c also show the cross-sections 37, 39 and 41 of the electron beams which originate from the electron guns 7 and which strike the phosphor dots 31, 33 and 35, respectively. It will be obvious that the cross-section of each bundle must lie completely within the circumference of the associated phosphor dot. If this is not so, color defects occur in the displayed colors. Therefore, the ideal situation is that where each electron beam strike the relevant phosphor dot exactly in its center. As it was found that this cannot be realized in practice, the cross-section of the electron beams is chosen to be smaller than the surface of the phosphor dots, with the result, however, that each phosphor dot is struck by a smaller number of electrons which has an adverse effect on the brightness of the image. Therefore, attempts are made to approximate the ideal situation as closely as possible. It was found that the deviations between the center of the electron beam and the centre of the phosphor dot, referred to as landing errors, are not equally large over the entire surface of the display screen 9. The display screen 9, shown in front view in FIG. 5, can be divided into four quadrants by a horizontal axis 43 and a vertical axis 45. It appears that the largest landing errors occur near the ends of the horizontal axis 43, i.e., in the areas denoted by 47. The landing images shown in FIGS. 4a to c relate to these areas. It was found that the landing errors in the areas 47 can be influenced by variation of the angle distance α between the ends of the foremost coil heads 23 of the horizontal deflection coils 27 and 29. FIG. 4a shows the landing image which appears if the two angles are substantially equal to 180°. It is obvious that in that case the cross-section of each of the three electron beams 37, 39 and 41 lies barely within the circumference of the associated phosphor dot 31, 33 and 35, respectively. A small variation in one of the many parameters determining the path of the electrons can then already exert an adverse effect on the color purity of the image.

In order to improve this situation, it was previously proposed to choose the said angle distances α of the two horizontal deflection coils 27 and 29 to be smaller than 180°, for example, 155°. The landing image then arising is shown in FIG. 4b. It appears that the green and the red phosphor dots 31 and 33 are struck almost exactly in their center by the associated electron beams 37 and 39, while the blue electron beam 41 is incident just within the blue phosphor dot 35. This situation is an improvement with respect to the landing image shown in FIG. 4a, but the landing of the blue beam 41 is still so marginal that disturbances can still readily occur.

The latter problem is solved by means of the deflection coil system according to the invention, the horizontal deflection coils 27 and 29 of which are shown in FIG. 3. As appears from FIG. 3, the angle distances α are now different for the two coils, i.e., the value α for the upper coil 29 is smaller than that for the lower coil 27. The result is shown in the landing image of FIG. 4c. All three beams 37, 39, 41 lie within the circumference of the associated phosphor dots 31, 33 and 35, respectively, with a given reserve. This reserve is slightly smaller for the green and the red beams 37 and 39 with respect to the situation shown in FIG. 4b, but this drawback is more than compensated for in that now an acceptable landing reserve is for the first time obtained for the blue beam 41.

It was found that the arrangement shown in FIG. 3, where the angle distance α of the upper coil 29 is smallest, offers the best results in display tubes where, as is common practice, the blue electron gun is arranged at the top vertex of the triangle.