Color CRT and driving method of the same

Details Color CRT and driving method of the same Color CRT and driving method of the same

2001-06-21

2002-07-23

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Cathode ray tube circuits

Cathode-ray deflections circuits

C313S414000, C313S449000

Reexamination Certificate

active

06424104

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a color cathode ray tube (CRT), and more particularly, to a color CRT for correcting distortion of a profile of an electron beam according to an increase of a deflection angle of the electron beam emitted from an electron gun, and to a driving method of the same.
2. Description of the Related Art
A typical color CRT is shown in FIG.
1
. As shown in the drawing, a color CRT includes a panel
12
having a fluorescent film
11
formed an inner surface thereof, a shadow mask frame assembly
13
installed inside the panel
12
and including a shadow mask
13
a
having a color selection function of an electron beam with respect to fluorescent substances of three colors and a frame
13
b
supporting the shadow mask
13
a
, a funnel
14
sealed to the panel
12
, an electron gun
20
for installed inside a neck portion
14
a
of the funnel
14
forming a seal, and a deflection yoke
15
installed at a cone portion of the funnel
14
for deflecting an electron beam emitted from the electron gun
20
.
In the color CRT having the above structure, as a predetermined electric potential is applied to the electron gun
20
, an electron beam emitted from the electron gun
20
is selectively deflected according to the position of scanning and excites fluorescent substances so that an image is formed.
In the above color CRT, an electron beam does not accurately land on a fluorescent point of a fluorescent film at the peripheral portion of a screen surface due to lowering of a focus property caused by making a screen surface flat and a wide deflection angle. That is, as shown in
FIG. 2
, when a deflection angle of an electron beam increases (from 102° to 120°) and a screen has a predetermined curvature, distortion of a spot S
1
of an electron beam B
1
is not severe at the peripheral portion of the screen. However, in the case of a flat screen, an incident angle of an electron beam B
2
scanned onto the peripheral portion of the screen decreases so that the electron beam is distorted and a spot S
2
increases. Also, when the deflection angle increases as described above, since densities of a pincushion magnetic field (MP) and a barrel magnetic field (MB) increase at the peripheral portion of an area where an irregular magnetic field is formed by a deflection yoke, as shown
FIG. 3
, an electron beam is severely distorted. As shown in
FIG. 4
, since the overall length of a CRT
10
b
having a relatively small deflection angle is shorter than that of a CRT
10
a
having a relatively large deflection angle, a difference in length of focusing at the central portion of a screen and the peripheral portion thereof increases. The difference in the length of focus makes the profile of an electron beam landing at the central portion and peripheral portion of the screen large.
According to a conventional technology to solve the above problem, at least one quadrupole lens is adopted in an electron gun in the CRT and a dynamic focus voltage synchronized with a deflection signal is applied to one of electrodes forming the quadrupole lens. Thus, the magnification of the quadrupole lens and the shape of the profile of an electron beam are changed, and simultaneously, a difference in voltage between an electrode forming the quadrupole lens and another electrode forming an electron lens installed adjacent to the electrode is reduced, so that the length of focus is changed.
However, the above methods of correcting the profile of an electron beam by using the quadrupole lens and adjusting the length of focus by changing the magnification of the electron lenses are not able to sufficiently correct distortion due to the distortion of the profile according to an increase of the deflection angle and the irregular magnetic field of the deflection yoke.
In particular, in the case of an electron gun forming at least one quadrupole lens, the shape of a waveform of a dynamic voltage fitting into a quadratic equation is substantially not useful because application of the dynamic voltage applied to the electrode forming the quadrupole lens of the electron gun in an area other than a raster area where a video signal of an image is applied does not affect at all a surface of the image. Thus, since a dynamic parabolic voltage is effective only in the raster area to which the video signal of an image is applied, the shape of a dynamic waveform of a screen should be considered by assuming that the raster area makes 100%.
When the shape P
1
of the dynamic horizontal voltage is fitted into a quadratic equation in the raster area as shown in
FIG. 5
, since a voltage lower than a necessary voltage is applied at the central portion of a screen, a halo phenomenon in which the profile of an electron beam landing at the central portion of the screen is vertically crushed is generated. If the voltage is raised by moving the center of the horizontal voltage waveform upward as shown in
FIG. 13
, to remove the halo phenomenon, a parabolic voltage in a horizontal direction which is much higher than a necessary voltage is applied at the central portion of the screen. Thus, the profile of the electron beam is vertically elongated as much as the difference between the necessary voltage and the actually applied voltage. When the elongated electron beam is deflected by an irregular magnetic field of the deflection yoke toward the peripheral portion of the screen, the electron beam receives a divergent force in a horizontal direction, considerably lowering resolution of a screen. As shown in
FIGS. 5 and 13
, a rapid increase in the applied voltage in the outer area of a screen results in a rapid increase in the voltage in an area other than the screen, so that reliability of a high voltage circuit is lowered.
When the waveform is formed according to a quadratic equation, the horizontal dynamic parabolic voltage has a ratio of 1.8 between a slope in a unilateral area of 90% of the raster signal and a slope in a unilateral area of 50% thereof. Thus, since the difference from a fitting trace of an electron beam having a sharp slope at the peripheral portion of a screen surface according to an increase in a deflection angle increases, the electron beam does not accurately land on a fluorescent point of the fluorescent film.
SUMMARY OF THE INVENTION
To solve the above problems, it is an object of the present invention to provide a color CRT which can prevent lowering of a focusing property of an electron beam due to distortion in the profile of the electron beam and a change in the length of focus according to an increase of a deflection angle of the electron beam by the deflection yoke, and a driving method of the same.
Accordingly, to achieve the above object, there is provided a color CRT comprising a panel having a screen surface on which a fluorescent film is formed in a predetermined pattern, a funnel sealed to the panel, an electron gun installed at a neck portion of the funnel and having electrodes for forming at least one quadrupole lens, and a deflection yoke installed throughout the neck portion and a cone portion of the CRT, and a dynamic voltage waveform having a ratio of slopes of 6.85 or more between a unilateral area of 90% and a unilateral area of 50% of a raster area to which a video signal of an image is applied, is applied to at least one electrode forming the quadrupole lens.
It is preferred in the present invention that the horizontal dynamic parabola voltage waveform is applied to at least one of electrodes forming the quadrupole lens of the electron gun.
It is preferred in the present invention that the inclination of a voltage relatively decreases in a unilateral area of 90% or more of the raster area to which a video signal of an image is applied.
Alternatively, to achieve the above object, there is provided a driving method of a color CRT comprising the steps of focusing and accelerating an electron beam emitted from a cathode by forming a plurality of electron lens including a quadrupole lens by applying a predetermined voltage to t

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