Color cathode ray tube having electron gun with reduced eddy current loss at shield cup

A color cathode ray tube has a three beam, in-line electric gun in one end of the tube focused on a phosphor screen on the other end of the tube. A common magnetic deflection field controls all three beams, thereby causing a coma distortion, if not corrected. The three beams pass through three aligned apertures in the bottom of a shield cup which corrects the distortion. As a result, eddy currents might form around the apertures to defeat its corrective functions. These eddy currents are eliminated by slender cuts formed in the shield cup and radiating from the two outside apertures.

BACKGROUND OF THE INVENTION 
This invention relates to a color cathode ray tube, and particularly, to a 
color cathode ray tube having an in-line electron gun for radiating three 
co-planar beams and to a self-convergence system in which rasters formed 
on a phosphor screen by the three beams have an equal size, under a common 
deflecting magnetic field. 
The three co-planar beams of an in-line electron gun are deflected 
horizontally and vertically by a deflection yoke disposed on a funneled 
part of a glass envelope, to form rasters on a phosphor screen. To work 
the color cathode ray tube on a self-convergence system whereby a dynamic 
convergence correction is not required, a coma distortion is minimized by 
adjusting a horizontal deflecting magnetic field of the deflection yoke to 
give a strong pincushion distortion and a vertical deflecting magnetic 
field to give a strong barrel distortion, thus forming an accordant raster 
on the phosphor screen. In this case, however, the raster scanned by the 
central beam of the three beams is generally smaller in both horizontal 
and vertical dimensions, than the rasters formed by each of the outside 
beams. A mismatching of the rasters is due to a coma distortion of the 
deflection yoke. In order to attain a coincidence of the rasters by 
removing the coma distortion, a field control element consisting of a high 
permeability magnetic member is disposed on the bottom of a shield cup 
formed in a bottomed cylinder with a non-magnetic material which is 
mounted on a tip of the electron gun to which a rear leakage magnetic 
field of the deflection yoke is exerted. 
Recently, a color display tube with a high resolution characteristic has 
been employed for display of various data, thereby giving alphanumeric 
character, symbol, Chinese characters, diagram, etc. in high density. 
For a high density display, it is necessary for a resolution of the color 
cathode ray tube to be high; a focusing characteristic to be uniform; and 
a frequency band of a video signal circuit to be wide to improve a 
horizontal resolution of the displayed picture. Many scanning lines are 
required to improve the a vertical resolution. 
To increase the number of scanning lines as an available means for high 
density display, a horizontal deflecting frequency f.sub.h is enhanced to 
a value higher than the 15.734 KHz which is used in the current standard 
TV system. In this case, however, a coma distortion arises on the rasters 
formed by the central beam and by the beams on both sides according to a 
horizontal deflecting field which was not observed at the horizontal 
deflecting frequency f.sub.h =15.734 KHz. Thus, a problem is quite 
unavoidable because a grade of the picture displayed on a phosphor screen 
is severely deteriorated thereby. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide a color cathode ray tube using an 
in-line electron gun of a self-convergence system, wherein a 
misconvergence does not occur on the rasters formed by beams on both 
outsides and a central beam, as a result of coma distortion even at an 
increased horizontal deflecting frequency. 
This invention is characterized in that a plurality of slender cuts are 
formed around transmission apertures for beams. The cuts are on both 
outsides, cut in an in-line array in the bottom of a cylindrical shield 
cup consisting of a non-magnetic metallic material which is mounted on a 
tip on an electron beam emitted side of an in-line electron gun used in a 
color cathode ray tube. With this structure, field control elements 
disposed on apertures for both outside electron beams can be prevented 
from experiencing an occurrence of eddy currents due to a high-frequency 
horizontal deflecting frequency component. Due to coma distortion, an 
asymmetric misconvergence may arise on the rasters formed by the central 
beam and beams on both outsides can be removed despite an increase in 
horizontal deflecting frequency, thus working the in-line electron gun as 
a superior gun capable of displaying high density data. 
It is further desirable to form a plurality of slender cuts in the field 
control elements provided around the outer beam apertures so as to match 
the slender cuts around the outer beam in the bottom of a bottomed 
cylindrical shield cup.

DESCRIPTION OF THE PRIOR ART 
FIG. 1 is an axial sectional view of a cathode ray tube using an in-line 
electron gun of a self-convergence system which requires no dynamic 
convergence correction means, generally used heretofore. A central beam 
B.sub.1 and a pair of both outside beams B.sub.2, B.sub.3 are radiated 
from an in-line electron gun 1 within the same plane, and are deflected 
horizontally and vertically by a deflection yoke 5 disposed on a funneled 
part of a glass envelope 2 to form a raster on a phosphor screen 4. Screen 
4 is on the end of the glass envelope 2 and is fitted inside with a 
plurality of phosphor picture elements which are luminous in three colors. 
The beams are radiated through a shadow mask 3 which is opposite screen 4. 
To work the color cathode ray tube on the self-convergence system requiring 
no dynamic convergence correction, a horizontal deflecting field of the 
deflection yoke 5 is adjusted to cause a strong pincushion distortion and 
a vertical deflecting field is adjusted to cause a strong barrel 
distortion. As shown in FIG. 2, a coma distortion of a pair of beams 
B.sub.2, B.sub.3 is removed by these deflecting fields, thereby forming an 
almost accordant raster 6 on the phosphor screen 4. However, a raster 7 
resulting from the central beam B.sub.1 is still smaller than the raster 
caused by both the outside beams B.sub.2, B.sub.3, smaller both 
horizontally and vertically. 
A mismatching of the rasters is due to a coma distortion of the deflection 
yoke 5. For removing the coma distortion to make the rasters coincide with 
each other, U.S. Pat. No. 3,772,554 disclosed a method wherein field 
control elements consisting of a high permeability magnetic member are 
disposed on a bottom 11 of a shield cup 10 formed in a bottomed cylinder. 
A non-magnetic material is mounted on a tip of the electron gun 1 to which 
a rear leakage field of the deflection yoke 5 is exerted. 
FIG. 3 represents one example of the field control element, which is 
constituted of a pair of disc magnetic enhancers 15, 16 that are opposite 
each other. These enhancers are put in line with a central beam aperture 
12 formed in the bottom 11 of the shield cup 10 on a vertical axis Y--Y 
coming in a short axis of the phosphor screen 4. Magnetic shield rings 17, 
18 are disposed to surround both outside beam apertures 13, 14 formed on a 
horizontal axis X--X, in a long axis of the phosphor screen 4. 
The magnetic enhancers 15, 16 operate for the central beam B.sub.1 to 
increase the deflection sensitivity of a horizontal deflecting field 
F.sub.H of the deflection yoke 5 so that the sensitivity is greater in the 
center than the sensitivity of each outside beams B.sub.2, B.sub.3. The 
magnetic shield rings 17, 18 operate for both outside beams B.sub.2, 
B.sub.3 to decrease a deflection sensitivity of both horizontal and 
vertical deflecting fields F.sub.H, F.sub.V of the deflection yoke 5 to a 
level which is lower than the level of the central beam B.sub.1. The 
central beam B.sub.1 has an increased deflection sensitivity of the 
vertical deflecting field F.sub.V which is greater than it is for both 
outside beams. 
Accordingly, the raster 7 resulting from the central beam B.sub.1 is 
expanded both horizontally and vertically by the field control elements 
15, 16 and 17, 18. The raster 6 resulting from both outside beams B.sub.2, 
B.sub.3 is reduced thereby. Thus, the coma distortion according to the 
deflecting fields is removed to make the rasters 6, 7 coincide completely 
with each other. 
Recently a color display tube with a high resolution characteristic has 
been employed for display of various data, thereby giving alphanumeric 
character, symbol, Chinese characters, diagram, etc. in high density. 
High density display requires a high resolution of the color cathode ray 
tube, a uniform focusing characteristic, a wide frequency band of a video 
signal circuit which improves a horizontal resolution of the displayed 
picture, and many scanning lines which improve a vertical resolution 
thereof. 
To increase the number of scanning lines for high density display, a 
horizontal deflecting frequency f.sub.h is enhanced to be higher than the 
15.734 KHz of the currently standard TV system. In this case, however, a 
coma distortion arises on rasters 6', 7' caused by both outside beams and 
the central beam, according to a horizontal deflecting field which was not 
observed at the horizontal deflecting frequency f.sub.h =15.734 KHz. 
As shown in FIG. 4, the raster 6' resulting from both outside beams is 
expanded somewhat horizontally against the raster 7', resulting from the 
central beam. The ratio of the expansion is then discrepant, both left and 
right, on the phosphor screen 4. An asymmetry arises wherein an expanded 
dimension d.sub.1 of the left side is larger than an expanded dimension 
d.sub.2 of the right side. The displacement of the rasters indicates a 
convergence error, which is capable of severely deteriorating the grade of 
pictures displayed on the phosphor screen. For example, in a 20-inch 
90-degree deflection color cathode ray tube, the above displacements are 
d.sub.1 =0.7 mm and d.sub.2 =0.3 mm near the effective phosphor screen 
when the horizontal deflecting frequency f.sub.h =15.734 KHz is doubled as 
f.sub.h =31.5 KHz. 
The displacement due to a coma distortion arising horizontally on the 
rasters 6', 7' results from both outside beams and the central beam, 
according to an increase in the horizontal deflecting frequency f.sub.h. A 
description of the cause of this distortion is as follows. First of all, 
an eddy current is generated around both outside beam transmission 
apertures 13, 14 and in the magnetic shield rings 17, 18 which are 
disposed around the outer beam apertures 13, 14. The eddy current is 
caused by a horizontal deflecting field component induced in the bottom 11 
of the shield cup 10 and penetrating the plan of the bottom. As a result, 
a magnetic flux is generated to prevent a magnetic flux change in the 
magnetic shield rings 17, 18, thus decreasing the effectiveness of the 
magnetic shield. A loss of the magnetic flux due to the eddy current can 
be thoroughly neglected at the conventional horizontal deflecting 
frequency f.sub.h =15.73, or so. However, the loss of the magnetic flux 
due to the eddy current cannot be neglected, as the frequency increases. 
As shown in FIG. 4, the raster 6' produced by each of the outside beams is 
expanded horizontally against the raster 7', by the central beam. 
On the other hand, a saw tooth current is used in a horizontal deflecting 
coil of the deflection yoke 5, for horizontal scanning as shown in FIG. 5. 
In this figure, a time period t.sub.1 from a point a to a point b is a 
horizontal scanning time, and a time period t.sub.2 from the point b to a 
point c is a horizontal blanking time. Normally, time t.sub.2 is about 1/5 
of t.sub.1. The positions of points a or c come to correspond with each 
other on the right-hand end. The left end position of a raster corresponds 
to the termination of the horizontal blanking time t.sub.2 and the right 
end corresponds to the termination of the horizontal scanning time 
t.sub.1. A magnetic field is generated according to a current which is 
changing at a velocity of about five times of the horizontal scanning time 
t.sub.1. This field is generated in the bottom 11 of the shield cup and 
the magnetic shield rings 17, 18 during the horizontal blanking time 
t.sub.2. Accordingly, a loss of the magnetic shielding effect of the 
magnetic shield rings 17, 18 occurs according to an eddy current loss 
responsive to the higher-order harmonic component field. The loss of 
shielding is larger on the left side of the phosphor screen than on the 
right side. Therefore, FIG. 4 shows a larger horizontal expanded width 
d.sub.1 on the left side of the raster 6', than on the right side d.sub.2, 
giving rise to an asymmetry of the coma distortion horizontally. These 
expansions are caused by both outside beams. The time period t.sub.1 is 51 
to 53 .mu.sec. at f.sub.h =15.734 KHz which is employed in a conventional 
standard color TV system (NTSC system), and the eddy current loss caused 
thereby can be totally neglected. Thus, the above-mentioned coma 
distortion and the asymmetry could not be found out, essentially. However, 
a difference arising between time periods t.sub.1 and t.sub.2 in 
accordance with an increase in f.sub.h and further the blanking time 
t.sub.2 for increasing the effective scanning time t.sub.1 are set to be 
as small as possible. Thus, an asymmetry of the eddy current loss becomes 
too large to neglect, giving rise to the above-mentioned phenomenon. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 6 is a perspective view of a shield cup 20 given in one embodiment of 
this invention. A central and a pair of outside beam transmission 
apertures 22, 23, 24 are formed in line in a bottom 21 of the shield cup 
20 formed in a bottomed cylinder, with a non-magnetic material of 
stainless steel which is mounted on a tip of the electron gun, at regular 
intervals on the X--X axis, corresponding to a long axis of the phosphor 
screen. Slender cuts 25 are formed around both outside beam transmission 
apertures 23, 24 in the direction of X--X axis and also perpendicularly 
thereto. A field control element comprising a high permeability magnetic 
member similar to that of a conventional one is disposed on the bottom of 
the shield cup 20 as shown in FIG. 7. Namely, a pair of magnetic enhancers 
15, 16 are disposed opposite each other to put in the central beam 
transmission aperture 22, on the vertical axis Y--Y, which is a short axis 
of the phosphor screen 4. The magnetic shield rings 17, 18 are disposed to 
surround both outside beam transmission apertures 23, 24 formed on the 
horizontal axis X--X. A function of these field control elements 15, 16, 
17, 18 is exactly the same as the function in the above-described 
conventional example. 
However, if the horizontal deflecting field is induced to the bottom of the 
shield cup 20 and if there is present a component penetrating the plane, 
an eddy current is prevented from arising at the apertures, by a plurality 
of slender cuts 25 formed around both outside beam transmission apertures 
23, 24. 
Accordingly, a generation of such magnetic flux is minimized to prevent a 
change of magnetic flux in the magnetic shield rings 17, 18 responsive to 
the eddy current if the horizontal deflecting frequency becomes higher 
than f.sub.h =15.73 KHz, the magnetic shield effect is never decreased. 
Consequently, if the horizontal deflecting frequency f.sub.h increases, 
the rasters by both outside beams will not be expanded against the raster 
created by the central beam. Or, the rate of expansion will not be 
asymmetric due to a difference between the horizontal scanning time and 
the horizontal blanking time. 
FIG. 8 is a perspective view of a magnetic shield ring 27 (28) used for 
another embodiment of this invention. As illustrated therein, there are 
formed, on the magnetic shield ring 27, two slender cuts 29A on one 
diameter of the two concentric circles. These cuts extend from an edge of 
the outside circle in the direction, toward the inside circle. Further, 
two slender cuts 29B are formed on a diameter which is orthogonal to the 
above diameter, extending from an edge of the inside circle in the 
direction of the outside circle. Each cut has a width, at least in the 
thickness dimension of the shield ring 27, which does not penetrate from 
the inside to the outside circle. 
As shown in FIG. 9, the magnetic enhancers 15, 16 and the magnetic shield 
rings 27, 28 are disposed on a bottom of the shield cup 20 shown in FIG. 
6. Namely, a pair of magnetic enhancers 15, 16 are opposite each other, to 
put in the central beam transmission aperture 22 on the vertical axis 
Y--Y. The magnetic shield rings 27, 28 are disposed to surround both 
outside beam transmission apertures 23, 24 provided on the horizontal axis 
X--X. In this case, the slender cuts 25 are formed around both outside 
beam transmission apertures 23, 24 of the shield cup bottom 21. The 
slender cuts 29A, 29B for the magnetic shield rings 27, 28 are positioned 
to coincide with each other and then are welded in place. The deflecting 
function of these field control elements 15, 16, 27, 28 is exactly the 
same as that described in the foregoing conventional example. 
Even if the horizontal deflecting field F.sub.H is induced in the bottom 21 
of the shield cup 20, and if there is present a component penetrating the 
plane, an eddy current is prevented from arising on the magnetic shield 
rings 27, 28 because a plurality of slender cuts 25, 29A, 29B are formed 
around both outside beam transmission apertures 23, 24 and magnetic shield 
rings 27, 28. 
Accordingly, a generation of magnetic flux is minimized to prevent a change 
of magnetic flux in the magnetic shield rings 27, 28, in response to the 
eddy current. Even in case the horizontal deflecting frequency becomes 
higher than f.sub.h =15.73 KHz, which is employed in the current standard 
color TV system, the magnetic shield effect is never decreased regardless 
of the higher frequency. 
The above description has referred to the case wherein field control 
elements comprises a combination of a pair of magnetic enhancers and 
magnetic shield rings. Each is used for the correction of a coma 
distortion of the rasters by the central and both outside beams, which ar 
related as shown in FIG. 2. However, the invention is not necessarily 
limited only thereto. It can be applied to the correction of a coma 
distortion having various patterns and also on field control elements 
having other shapes. 
For example, cuts 39A, 39B are formed in field control elements 37, 38 as 
shown in FIG. 10, after the slender cut 25 is formed on a bottom aperture 
of the shield cup 20. These elements are effective to correct the coma 
distortion shown in FIG. 2. Their function is then such that a horizontal 
raster produced by each of the outside beams is reduced until it comes to 
coincide with the horizontal raster produced by the central beam, by 
adjusting the size of an annular part 39C of the field control elements 
37, 38. A vertical raster is expanded until it comes to coincide with the 
vertical raster produced by both of the outside beams, by increasing a 
sensitivity of the central beam to the vertical deflecting field by means 
of a projection 39D facing the central beam transmission aperture 22 side 
on the axis X--X. In this case, an eddy current is also prevented from 
arising by the cuts provided on the shield cup bottom and the field 
control elements. Thus, a dependence on the operation of the field control 
elements is removed against the horizontal deflecting frequency. 
Furthermore, if this invention is applied to a random scanning system with 
the scanning speed undefined instead of a line-sequential raster scanning 
system with the scanning speed constant during an available period of 
scanning, the coma distortion will not arise in this case. The 
effectiveness becomes remarkable. 
According to this invention, the field control element disposed on the 
shield cup bottom will not necessarily be optimized to exclusive use at 
every working horizontal deflecting frequencies. However, one and the same 
field control element can be used in common at all the frequencies. 
As described above, according to this invention, dependence on action of 
the field control elements against horizontal deflecting frequency and 
also the difference in action due to a difference between horizontal 
scanning time and horizontal blanking time can be removed by forming a 
plurality of slender cuts around both of the outside beam transmission 
apertures formed on the shield cup bottom mounted on a tip of the in-line 
electron gun of the self-convergence system, or around both of the outside 
beam transmission apertures and the magnetic shield rings disposed on the 
shield cup. Consequently, an asymmetric misconvergence can be thoroughly 
removed. This misconvergence is due to a coma distortion on the rasters 
formed by central and both outside beams, despite an increase in the 
horizontal deflecting frequency. Thus, such an in-line electron gun is 
capable of displaying data in high density and superior in characteristics 
accordingly to an exceedingly high practicability. 
Those who are skilled in the art will readily perceive how to modify the 
invention. Therefore, the appended claims are to be construed to cover all 
equivalent structures which fall within the true scope and spirit of the 
invention.