Color CRT with cross-misconvergence correction device

Details Color CRT with cross-misconvergence correction device Color CRT with cross-misconvergence correction device

1999-10-20

2001-12-04

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Cathode ray tube circuits

Cathode-ray deflections circuits

C313S440000

Reexamination Certificate

active

06326742

ABSTRACT:

BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a color cathode ray tube (CRT) used as a monitor, a television receiver, or the like, and particularly relates to a means that corrects cross-misconvergence occurring in a horizontal strip in the central part of both the upper and lower halves of a fluorescent screen of the color CRT.
(2) Related Art
When blue and red rasters are projected onto a fluorescent screen of a conventional color CRT provided with an in-line type electron gun, so-called “cross-misconvergence” occurs as shown in FIG.
1
. This cross-misconvergence takes place due to a delicate interrelationship between a distorted distribution of a magnetic field generated by a deflection device (or, a deflection yoke) and a shape of an inner surface of a front panel of the color CRT.
The cross-misconvergence refers to a phenomenon in which blue and red rasters vertically deviate from each other in a horizontal strip in the central part of both the upper and lower halves of the effective display region of a fluorescent screen
50
. As shown in
FIG. 1
, the upper half includes a first quadrant indicated as (I) and a second quadrant indicated as (II), while the lower half includes a third quadrant as (III) and a fourth quadrant as (IV). Hereinafter, each horizontal strip in the central parts of the quadrants is referred to as the “central strip.”
As shown in
FIG. 1
, blue rasters
1
B to
4
B and red rasters
1
R to
4
R are projected in different slanting directions. In the quadrants (I) and (III), blue rasters
1
B and
3
B drawn in dashed lines are located respectively above red rasters
1
R and
3
R drawn in solid lines. In the quadrants (II) and (IV), blue rasters
2
B and
4
B are located respectively below red rasters
2
R and
4
R.
Here, Japanese Laid-Open Patent No. 64-84549 discloses a method to reduce the occurrence of such cross-misconvergence. This method is specifically explained as follows. Vertical deflection coils of a deflection device include a pair of coils for generating magnetic fields distorted in a pincushion and a pair of coils for generating magnetic fields distorted in a barrel. Two diodes in parallel, with their polarities being opposite, are connected in series to the pair of coils generating the pincushion magnetic fields. With this construction, the magnetic field is switched between the pincushion and barrel magnetic fields at a timing at which electron beams are deflected to the central strip, in order that the stated cross-misconvergence can be prevented.
Meanwhile, Japanese Laid-Open Utility Model No. 63-80756 discloses another method of correcting cross-misconvergence. In the disclosure, at least four permanent magnets are set around a front rim of a bobbin of a deflection device, each permanent magnet having the magnetic poles parallel to the axial direction of the bobbin and being set on an extended diagonal line of the bobbin. By means of these permanent magnets, raster distortion and cross-misconvergence in the central strips can be simultaneously corrected.
Although the cross-misconvergence is reduced using the method disclosed in the former reference, the linearity of vertical rasters is deteriorated due to sudden switches from the barrel magnetic field to the pincushion magnetic field. This leads to deterioration in images displayed on the fluorescent screen of the CRT.
Using the technique disclosed in the latter reference, meanwhile, because of the permanent magnets, cross-misconvergence next occurs in areas around the horizontal axis of the fluorescent screen, where it has never occurred. Thus, although the cross-misconvergence occurring in the central strips is corrected, convergence quality of the entire screen cannot be improved.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a color cathode ray tube which reduces cross-misconvergence using a construction that can be easily realized at low costs without deteriorating the linearity of vertical rasters and causing misconvergence to other areas of the fluorescent screen.
The object of the present invention can be achieved by a color cathode ray tube made up of: a glass bulb which has a front panel and a fluorescent screen that is set on an inner surface of the front panel; an in-line electron gun which is provided in the glass bulb and projects a plurality of electron beams onto the fluorescent screen; a deflection unit including a horizontal deflection coil that deflects the electron beams in a horizontal direction and a vertical deflection coil that deflects the electron beams in a vertical direction, the horizontal and vertical deflection coils being arranged outside the glass bulb; and a correction unit for generating a corrective magnetic field that is used for correcting cross-misconvergence, a strength of the corrective magnetic field being changed in accordance with an amount of deflection of the electron beams in the vertical direction.
With this construction, the corrective magnetic fields having a strength appropriate to the amount of cross-misconvergence can be generated, thereby reliably correcting the cross-misconvergence that changes with the amount of deflection of the electron beams.
The cross-misconvergence can be corrected more effectively by the color cathode ray tube having the correction unit that changes the strength of the corrective magnetic field so that the strength affecting the electron beams becomes largest when the electron beams are deflected to an area where a greatest amount of correction for the cross-misconvergence is needed and that the strength affecting the electron beams becomes smallest when the amount of deflection of the electron beams in the vertical direction is zero.
The correction unit includes: a plurality of correction coils which respectively generate corrective magnetic fields; and a control unit which controls a current to be supplied to the correction coils. The control unit increases the current to be supplied to the correction coils in accordance with the amount of deflection of the electron beams in the vertical direction, wherein each of the correction coils is formed by winding a solenoid around a saturable core, a strength of the corrective magnetic fields being largest when the current supplied to the correction coils reaches a predetermined value and being decreased owing to saturation of the saturable cores after the current exceeds the predetermined value.
With this simple construction, when the electron beams are deflected to the horizontal axis of the fluorescent screen, the strength of the corrective magnetic fields generated by the correction coils is small. Meanwhile, when the electron beams are deflected to a horizontal strip in the central part of the upper or lower half of the fluorescent screen, the strength of the corrective magnetic fields is largest. Thereafter, as the electron beams are deflected upward or downward to reach the top or bottom edge of the screen, the current supplied from the vertical deflection coils is increased and then the saturable cores are saturated. After the saturation of the saturable cores, the strength of the magnetic fields decreases. Since the correction coils operate in the saturation region of the saturable cores, the strength of the magnetic fields generated by the correction coils can be set largest when the electron beams are deflected to the areas where the correction for cross-misconvergence is needed.
Moreover the control unit supplies a current to the correction coils, the current changing proportional to a vertical deflection current supplied to the vertical deflection coil. As a result, the current is supplied to the correction coils in sync with the vertical deflection of the electron beams, and is increased in accordance with the amount of deflection of the electron beams.
The current changing proportional to the vertical deflection current refers to a current that changes in the same cycle as the vertical deflection current and whose current value changes proportional to the curren

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