1. Field of the Invention
The present invention relates to a deflection device for use in an in-line color cathode-ray tube, which is designed to deflect three electron beams passing in the same plane, and more particularly to a deflection device which has a convergence-correcting coil for eliminating mis-convergence in an in-line color cathode-ray tube of a self-convergence type.
2. Description of the Related Art
Most color cathode-ray tubes have the structure shown in FIG. 1. As shown in FIG. 1, each tube comprises an envelope 2 formed of a panel 1 and a funnel 9 integral with the panel 1. It further comprises a phosphor screen 3 formed on the inner surface of the panel 1, a shadow mask 4 located in the envelope 2, an electron gun unit 8 located in the neck 6 of the envelope 2, and a deflection device 10 surrounding the adjoining portions of the neck 6 and large-diameter portion 9 of the envelope 2. The screen 3 consists of blue-light emitting phosphor stripes, green-light emitting phosphor stripes, and red-light emitting phosphor stripes. The shadow mask 4 opposes the phosphor screen 3 and has a number of apertures. The electron gun unit 8 has three electron guns for emitting three electron beams 7B, 7G, and 7R, respectively, toward the phosphor screen 3. The deflection device 10 is designed to generate horizontal and vertical deflection magnetic fields. The electron beams 7B, 7G, and 7R emitted from the gun unit 8 are deflected by the deflection magnetic fields generated by the unit 10, then pass through the apertures of the shadow mask 4, and finally applied to the phosphor screen 3. Excited by the beams 7B, 7G, and 7R, the phosphor stripes of the screen 3 emit blue light rays, green light rays, and red light rays. As a result, the cathode-ray tube displays a color image.
The electron gun unit 8 is a so-called "in-line type" designed to emit three electron beams, i.e., a center beam 7G and two side beams 7B and 7R which pass in the same plane. The horizontal deflection magnetic field generated by the unit 10 is shaped like a pin-cushion as is shown in FIG. 2A. By contrast, the vertical deflection magnetic field generated from the device 10 is shaped like a barrel as is illustrated in FIG. 2B. The magnetic fluxes 12H of the pincushion-shaped magnetic field deflects the electron beams 7B, 7G, and 7R in a horizontal plane, while the magnetic fluxes 12v of the barrel-shaped magnetic field deflects the electron beams 7B, 7G, and 7R in a vertical plane.
Self-convergence type in-line color cathode-ray tubes, described above, are commonly used in practice.
As FIG. 1 shows, the deflection device 10 comprises a horizontal deflection coil 13H for generating the pincushion-shaped horizontal deflection magnetic field, and a vertical deflection coil 13v for generating the barrel-shaped vertical deflection magnetic field. Generally, the coils 13H and 13V are a saddle type and a toroidal type, respectively. The pincushion-shaped horizontal deflection magnetic field 12H converges the electron beams 7B, 7B, and 7R toward one another in the horizontal plane extending in an x axis, whereas the barrel-shaped vertical deflection magnetic field 12V converges the electron beams 7B, 7B, and 7R toward one another in the vertical plane extending in a y axis.
Even if the beams 7B, 7B, and 7R are so converged by the pincushion-shaped magnetic field 12H and the barrel-shaped magnetic field 12V, however, misconvergence of the beams occurs at the corners of a display screen 14 as shown in FIG. 3. In other words, a blue-beam spot 15B, a green-beam spot 15G, and a red-beam spot 15R are vertically displaced from one another at the corners of the display screen 14. In most cases, the mis-convergence can be eliminated by adjusting the distance between the deflection center of the horizontal deflection coil 13H and that of vertical deflection coil 13V.
As has been indicated, the deflection device 10 generates a horizontal deflection magnetic field 12H shaped like a pincushion, and a vertical deflection magnetic field 12V shaped like a barrel. Hence, the three electron beams 7B, 7G, and 7R can be converged at any position in the horizontal and vertical axes of the display screen 14. As shown in FIG. 4, however, mis-convergence of the beams takes place in intermediate regions between the corners and the horizontal and vertical axes of the screen 14. The red-beam spot 15R, for example, is formed closer to the center of the screen 14 than the blue-beam spot 15B in the right half of the screen 14, and is located farther from the center of the screen 14 than the blue-beam spot 15B in the left half of the screen 14. The mis-convergence of the electron beams inevitably deteriorates the quality of the image the cathode-ray tube displays.
The mis-convergence occurring at a position between the vertical axis y of the screen 14 and the any corner thereof may be minimized by altering the distribution of the magnetic fluxes generated by the deflection device 10 distribution. In this case, the mis-convergence is increased in the corners of the screen. Consequently it is no longer possible to improve the quality of the image displayed.
Recently, not only the distance between the deflection centers of the deflection coils 13H and 13V is adjusted, but also a saturable reactor is used, varying, differentially at the vertical deflection frequency, the horizontal-deflection currents supplied to the upper and lower coils constituting the horizontal deflection coil 13H. Mis-convergence in any corner of the screen 14 is eliminated almost completely. Mis-convergence can, therefore, be sufficiently reduced at any position in the horizontal and vertical axes of the screen 14 and at any corner thereof, but not at a position between the vertical axis y of the screen 14 and the any corner thereof. That is, as shown in FIG. 4, mis-convergence remains between the axis y and each corner, such that the red-beam spot 15R is located farther to the center of the screen 14 than the blue-beam spot 15B in the right half of the screen 14, and is located nearer the center of the screen 14 than the blue-beam spot 15B in the left half of the screen 14. The display screen 14, as a whole, has but poor convergence characteristic.
The mis-convergence occurring between the axis y of the screen 14 and each corner thereof can be reduced by two alternative methods. The first is to alter the distribution of deflection magnetic fluxes. The second is said same method used to minimize the mis-convergence at the corners of the screen 14. If either alternative method is performed, however, a prominent mis-convergence will occur at each corner of the display screen 14, inevitably degrading the convergence all over the display screen 14.
With the conventional method of eliminating or reducing mis-convergence in an in-line cathode-ray tube of self-convergence type, it is impossible to minimize the mis-convergence between the axis y of the screen 14 and each corner thereof, without degrading the convergence all over the display screen 14.