Electric lamp and discharge devices: systems – Cathode ray tube circuits – Cathode-ray deflections circuits
Reexamination Certificate
2000-09-25
2003-06-03
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Cathode ray tube circuits
Cathode-ray deflections circuits
C315S364000, C315S400000
Reexamination Certificate
active
06573668
ABSTRACT:
This application is based on an application No. 11-281322 filed in Japan, the content of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a color cathode ray tube used in television sets, computer displays and the like, and in particular to an apparatus for correcting convergence in a color cathode ray tube (hereafter CRT) that corrects raster distortion using magnets.
2. Related Art
One method used to correct convergence in a color CRT that uses an inline electron gun is a self-convergence method. This method corrects convergence involving pincushion distortion of the horizontal deflection field and barrel distortion of the vertical deflection field. The self-convergence method enables apparatuses with a simple construction and an excellent cost-performance ratio to be manufactured, and is consequently in widespread use.
In a conventional color CRT using the self-convergence method, for example a color CRT with a deflection angle of 90°, and a large screen curvature, the vertical deflection field experiences barrel distortion, thereby causing the horizontal component (hereafter referred to as ‘Bh’) of the vertical deflection field to become larger nearer to the right and left edges of the CRT.
FIG. 1A
is a graph plotting Bh against a horizontal axis H of the CRT. If a central point along the horizontal direction of the CRT is taken as an origin O, line
1
showing Bh is symmetrical about the origin O, and slopes upward more steeply the further it is from the origin O.
According to Fleming's Law, the vertical deflection force applied to the electron beams will increase as Bh becomes larger. Therefore, in a color CRT using the self-convergence method, electron beams passing closer to a vertical axis V will receive a weaker vertical deflection, and electron beams passing further away from the vertical axis V will receive a stronger vertical deflection. When an inline electron gun is used, three electron beams corresponding to the three colors RGB (red, green and blue) are horizontally aligned, so that, if we ignore a case in which the central beam of the three electron beams coincides with the vertical axis V, there will be some variations in the vertical deflection force applied to electron beams on either side of the vertical axis V.
FIG. 1B
shows vertical deflection forces Fr, Fg and Fb, received respectively by the red, green and blue electron beams R, G and B. Electron beams emitted by an inline electron gun are usually arranged in the order B, G and R from left to right as seen from in front of the screen. In this specification, it is assumed that all electron beams are arranged in this order. When the electron beam G coincides with the vertical axis V, in other words when it is positioned so as to correspond to the origin O of the horizontal axis H, vertical deflection forces Fr and Fb are equal, and vertical deflection force Fg is smaller than both vertical deflection forces Fr and Fb. When the electron beam R is further away from the origin O than the electron beam B, however, the vertical deflection forces received by the electron beams are such that Fb<Fg<Fr. Conversely, when the electron beam B is further away from the origin O than the electron beam R, the vertical deflection forces received are such that Fb>Fg>Fr.
As a result, when horizontal magenta lines are displayed at the top and bottom edges of the screen, the misconvergence shown in
FIG. 2
is caused. Here, a red component R (the solid line in the drawing) and a blue component B (the broken line in the drawing) in each magenta line on a display screen
2
, diverge vertically towards the corners of the screen. Since Bh is largest when the amount of vertical deflection reaches its maximum, this misconvergence is particularly marked at the corner areas of the screen. This type of misconvergence is hereafter referred to as PQV pincushion pattern misconvergence.
Japanese Laid Open Patent 8-98193 discloses a color CRT that corrects PQV pincushion pattern misconvergence by weakening the barrel distortion of the vertical deflection field.
FIG. 3A
is a graph plotting the values of Bh, both before and after barrel distortion of the vertical deflection field has been weakened, against the horizontal axis H. As a result of weakening barrel distortion, the variation in Bh changes from line
1
to line
3
in the drawing. Thus, as shown in
FIG. 3B
, the variations in Bh along the horizontal are reduced, and PQV pincushion pattern misconvergence is corrected.
If the barrel distortion of the vertical deflection field is weakened, this in turn weakens the ability of the CRT to correct misconvergence using a self-convergence method. Here, if a magenta line is displayed vertically down the center of the display screen
2
, the misconvergence shown in
FIG. 4
will be generated. This misconvergence is hereafter referred to as YH pincushion pattern misconvergence. The color CRT disclosed in the related art corrects this type of misconvergence using a four-pole coil.
FIG. 5
is a view of such a four-pole coil, seen from the front of the screen. Here, a four-pole coil
4
includes coils
5
and
8
, and U-shaped cores
6
and
7
. The U-shaped cores
6
and
7
are arranged in opposition on the side of the deflection yoke nearer to the electron gun, so that the electron beams pass between the two cores
6
and
7
. When a vertical deflection current is passed through the coils
5
and
8
after being rectified by a diode, force is exerted on the electron beams B and R emitted from the left and right of the electron gun, pushing them away from the vertical axis V, and thereby correcting YH pincushion pattern misconvergence.
In recent years, color CRTs with a virtually flat screen and a wide deflection angle have become increasing commonplace. In such CRTs, the distance the electron beams travel to reach the screen after being emitted from the electron gun varies markedly for each point on the screen surface. This results in increased raster distortion. Of this raster distortion, that which occurs when the top and bottom edges of the raster area scanned by the electron beams bow inward is referred to as top/bottom pincushion distortion, and is conventionally corrected by attaching magnets to the deflection yoke.
FIG. 6
is a view of a deflection yoke to which magnets have been attached, seen from in front of the display screen. Magnets
10
and
13
are attached to the front surface of an insulating frame
11
of a deflection yoke
9
at the top and bottom, and a horizontal deflection coil
12
is mounted on the inner surface of the insulating frame
11
. When viewed from in front of the display screen, the magnets
10
and
13
are arranged so that the north pole of the magnet
10
is on the right side and the south pole on the left side, while the south pole of the magnet
13
is on the right side and the north pole on the left side.
FIG. 7
illustrates magnetic flux generated by the magnets
10
and
13
. If the magnets
10
and
13
are arranged in this fashion, forces F are applied to the electron beams according to Fleming's Law, as shown in
FIG. 7
, thereby correcting the top/bottom pincushion distortion.
However, a horizontal component Mh of the magnetic fields generated by the magnets
10
and
13
grows weaker at points further away from the magnets.
FIG. 8A
is a graph plotting Mh against the horizontal axis H. If a point at the center of the horizontal axis H is taken as an origin O, line
14
showing component Mh is symmetrical about the origin O, growing smaller and sloping down more steeply as it moves further away from the origin O.
FIG. 8B
shows forces Fr, Fg and Fb received by electron beams R, G and B. When the electron beam G coincides with the vertical axis V, in other words when it is positioned so as to correspond to the origin O of the horizontal axis H, vertical deflection forces Fr and Fb are equal, and vertical deflection force Fg is larger than both vertical deflection forces Fr and Fb. W
Matsushita Electric Ind. Co., Ltd.
Tran Chuc D
Wong Don
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