Color display tube system with reduced spot growth

A color display tube system having an electron gun producing three co-planar electron beams which are non-convergent, i.e. parallel or divergent. A convergence element located at a distance from the electron gun and coaxial with the tube axis then produces convergence of the electron beams at the display screen by generating a static 45.degree. 4-pole field. This combination reduces growth of the scanning beam spot in the horizontal direction at the ends of the horizontal axis, which is a problem encountered with electron guns which are convergent.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a colour display tube system comprising: a) an 
evacuated envelope having a neck, a cone and a display window, b) an 
electron gun in the neck, which gun has a beam-forming part for generating 
a central electron beam and two outer electron beams whose axes are 
co-planar, and an electrode system which in operation constitutes a main 
lens, c) a deflection unit for generating deflection fields for deflecting 
the electron beams in the horizontal and vertical directions and for 
scanning the display window by means of convergent beams. 
2. Description of the Related Art 
Colour display tube systems of the type described in the opening paragraph 
are commonly referred to as 3-in-line systems. Generally, the guns of 
these systems are constructed so that the outer beams converge upon 
leaving the gun, and comprise self-convergent deflection units which in 
operation generate non-uniform magnetic fields for horizontal and vertical 
deflection (particularly a barrel-shaped field for the vertical deflection 
and a pincushion-shaped field for the horizontal deflection) so that the 
three electron beams generated by the electron gun and focused on the 
display screen by the main lens converge throughout the display window. 
However, these deflection fields cause the horizontal spot growth to 
increase by a given factor in the case of deflection, which factor may be 
more than two in 110.degree. colour display tube systems. This notably 
means that in a normal self-convergent system, in which the three guns are 
located in a horizontal plane, a circular central spot becomes narrower in 
the vertical direction and very elongate in the horizontal direction when 
scanning the screen. As a result a loss of resolution occurs in the 
horizontal direction and there is a risk of Moire problems owing to the 
spot becoming narrower vertically and the existence of horizontal dams in 
the shadow mask. The increasingly strict requirements imposed on the 
definition of the image, notably in high-resolution colour monitor tubes 
or when using colour display tubes for high-definition television with an 
aspect ratio of approximately 9:16 of the display window, require that at 
the ends of the horizontal axis the spot should be small in the horizontal 
direction. 
SUMMARY OF THE INVENTION 
It is one of the objects of the invention to provide a colour display tube 
of the type described in the opening paragraph in which the spot 
dimensions at the ends of the horizontal display screen axis are reduced 
in the horizontal direction (and in which the vertical spot dimension is 
preferably enlarged). 
To achieve this object, a colour display tube according to the invention is 
characterized in that the electron gun produces non-convergent beams and 
in that an element producing convergence is arranged between the electron 
gun and the side of the deflection unit facing the display window, which 
element generates a static magnetic field exerting a force on each outer 
electron beam having a component in the plane of the electron beams 
directed towards the central electron beam. 
The invention is based on recognition of the following. In operation, the 
electron gun produces outer electron beams which are parallel to the 
central beam or which may diverge for obtaining an even greater effect. 
These outer beams are then bent towards each other at a given distance 
from the gun. The two effects, introduced by the invention, on the 
convergence of the electron beams are dimensioned in such a way that there 
is convergence at the screen. The object of the invention is achieved in 
that the apex angle of each outer electron beam is separately enlarged in 
the horizontal direction (i.e. in a direction parallel to the plane of the 
non-deflected beams), which results in a reduction of the spot in the 
horizontal direction. The apex angle is understood to mean the angle 
between the outer electron paths of a given beam. 
The magnetic field to be generated for producing the desired effect on 
convergence may comprise local dipole fields at the location of each of 
the two outer beams. For an improved focusing of the electron beams, 
however, a preferred embodiment of the invention is characterized in that 
the element producing convergence is constructed so that in operation it 
generates a 45.degree. magnetic 4-pole field. The extent of 
non-convergence and convergence can be adjusted in such a way that a 
desired reduced spot dimension is realised in the horizontal direction at 
the ends of the horizontal display screen axis. The spot in the centre is 
then also reduced. Since the effect of spot growth in the horizontal 
direction, inherent in the use of self-convergent fields, is now 
substantially reduced, the spot in the centre will be smaller than the 
spot at the ends of the horizontal display screen axis. The invention is 
based, inter alia, on the recognition that this is no drawback because 
before the spot can never become too small in the horizontal direction the 
bandwidth of the video amplifier will become the limiting factor. The 
magnetic field to be generated for realising convergence may be generated, 
for example, by means of permanent magnets or by means of a configuration 
of electric coils which are energized with a (substantially constant) 
direct current. 
If the magnetic field used for producing convergence is generated by means 
of a configuration of electric coils, these coils may be wound on an 
annular core coaxially surrounding the neck of the tube between the 
electron gun and the deflection unit. If the configuration of electric 
coils of the convergence unit is arranged on the annular core of the 
deflection unit itself, the deflection to produce convergence is effected 
at a larger distance from the gun, which is favourable for the envisaged 
effect. The same holds for the use of a permanently magnetized ring for 
producing convergence. The larger the distance from the gun, the greater 
the effect. A location within the deflection unit is thus very favourable, 
for example, a location within and coaxial with the system of line 
deflection coils or between the annular core and the system of line 
deflection coils. The permanently magnetized ring may alternatively be 
arranged within the tube.

Where applicable, identical reference numerals are used for identical 
components. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a cross-section of a colour display tube system according to the 
invention. A glass envelope 1, which is composed of a display window 2, a 
cone 3 and a neck 4, accommodates an electron gun 5 in this neck, which 
gun generates three non-converging electron beams whose axes are located 
in the plane of the drawing. In the non-deflected state, the axis of the 
central electron beam coincides with the tube axis 9. The display window 2 
has a large number of triplets of phosphor elements on its inner side. The 
elements may consist of, for example, rows or dots. Each triplet comprises 
a green-luminescing phosphor, a blue-luminescing phosphor and a 
red-luminescing phosphor. A shadow mask 11 is arranged in front of the 
display screen, which mask has a large number of apertures 12 through 
which the electron beams pass and each impinge upon phosphor elements of 
one colour only. The three non-converging electron beams are deflected by 
a deflection unit 20 comprising a system 13 of line deflection coils and a 
system 13' of two diametrical field deflection coils, as well as an 
annular core 21 coaxially surrounding at least the system 13 of line 
deflection coils. 
Characteristic of the invention is a coaxial element 14 arranged at a 
relatively large distance from the gun 5 for generating a magnetic field 
configuration which drives the (non-convergent) outer electron beams in 
the plane of the electron beams towards each other for realising 
convergence, all this in such a manner that the spot is small enough in 
the horizontal direction at the ends of the horizontal display screen axis 
X' (see FIG. 1B). 
The magnetic field configurations to be used may comprise local dipole 
fields generated by means of permanent magnets or by configurations of 
coils at the location of the outer beams 6 and 8 (see FIG. 2A). Magnetic 
pole shoes (not shown) may be arranged on the tube neck 4 so as to provide 
the dipole fields at the correct locations. However, magnetic field 
configurations comprising a 45.degree. 4-pole field are preferably used. 
Such 4-pole fields may be generated, for example, by means of systems of 
permanent magnets. It is alternatively possible (see FIG. 2B) to generate 
these fields by means of a ring 14' of permanent magnetic material in 
which a suitable configuration of magnetic poles is induced. 
In the embodiment shown in FIG. 2B element 14' comprises an annular core 
15' of a magnetizable material which coaxially surrounds the tube neck (4) 
and on which four coils 16', 17', 18' and 19' are wound in such a way that 
a 45.degree. 4-pole field having the orientation shown with respect to the 
three co-planar beams 6, 7 and 8 is generated upon energization. (A 
45.degree. 4-pole field may be generated in an alternative way by means of 
two wound C cores, as shown in FIG. 6, or by means of a stator 
construction, as shown in FIG. 7). 
The use of the colour display tube system according to the invention is 
particularly suitable in high-resolution monitors and in future HDTV 
apparatuses, particularly in those cases where the aspect ratio of the 
display screen is larger than 4:3, notably 16:9. The recognition on which 
the invention is based will be further described with reference to FIGS. 3 
and 4 showing diagrammatic cross-sections of colour display tubes. FIG. 3 
shows a state-of-the-art colour display tube with an electron gun 52 which 
produces statically converged electron beams and a dynamically converging 
system 53 of deflection coils. The electron beams converge throughout the 
display window. 
FIG. 4 shows the principle of a colour display tube system according to the 
invention with a system 13 of line deflection coils. Electron gun 5 is 
constructed in such a way that the outer beams diverge. 
Convergence-producing element 14 compensates this divergence. As a result, 
the spot dimension in the horizontal direction at the ends of the 
horizontal display screen axis is reduced with respect to that occurring 
in the system of FIG. 3. A further advantage is that the spot shape may be 
more homogeneous (more circular). In the known state of the art, the 
horizontal dimension of the spot at the edges of the display screen is 
considerably larger than the vertical dimension. A more homogeneous spot 
shape is desired, particularly for data displays. 
FIG. 5 shows a modification in which electron gun 5 produces parallel beams 
and element 14 ensures the convergence. The advantage of this modification 
is that the electron gun may have a simpler construction. Such a gun may 
comprise, for example, three parallel electrically insulated tubes whose 
inner surfaces carry high ohmic resistance structures which constitute a 
focusing lens. 
It can be ensured with the aid of the afore-described means that the spot 
is very small in a colour display tube using self-convergent deflection 
fields. For high-resolution applications the spot should not only be small 
but it should also remain in focus as much as possible when it is 
deflected across the screen. An additional advantage is obtained if the 
convergence means according to the invention are used in combination with 
an electron gun having a dynamic astigmatic focusing facility, known as 
the DAF gun. The required dynamic focusing voltage of such a gun appears 
to be considerably decreased due to the presence of the 45.degree. 4-pole. 
The required dynamic focusing voltage decreases as the 45.degree. 4-pole 
is arranged further remote from the gun. This is particularly important 
when using elongate guns or when using guns having a focusing lens 
extending over a considerable axial area, such as focusing lenses 
constituted by a (helical) high-ohmic resistance structure. Elongate guns 
with a dynamic astigmatic focus require an extra large amount of dynamic 
voltage. FIG. 8 shows an embodiment which provides the possibility of 
arranging the 45.degree. 4-pole element 14 within the coil system 13 at a 
large distance from gun 5. In this case element 14 is of the permanent 
magnet type shown in FIG. 2A. It is to be noted that the (funnel-shaped) 
annular element 14 of FIG. 2A may have a corrugated outer edge with which 
a sinusoidal variation of the field strength along the inner surface is 
achieved, which improves the purity of the 4-pole field. 
FIG. 9 shows an alternative embodiment of a colour display tube system 
according to the invention. In this embodiment the tube has a 
convergence-producing element 54' for driving the puter electron beams 
toward each other, which element comprises a coil configuration arranged 
on the annular core 51 of the deflection unit. FIG. 10 shows the annular 
core 51 of the deflection unit with coil configuration 56, 57, 58 and 59, 
which is connectable to a voltage source in such a way that a 4-pole field 
having an orientation for driving the outer beams towards each other is 
generated. In this case the neck 4' of the colour display tube system 1' 
may be shorter than the neck 4 of the system 1 in FIG. 1A.