Color display tube system with reduced spot growth

A color display tube system comprising an electron gun for producing three co-planar electron beams, and including a main deflection system which in operation generates deflection fields of the self-convergent type, and an auxiliary deflection system which in operation generates a 45.degree. magnetic 4-pole field facing the electron gun and moving the outer electron beams away from each other, and a 45.degree. magnetic 4-pole field facing the display screen and moving the outer beams towards each other.

BACKGROUND 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 a first and a second electrode system which in 
operation jointly constitute a main lens and are connectable to means for 
supplying an energizing voltage, and 
(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. 
Colour display tube systems of the type described in the opening paragraph 
are of the conventional 3-in-line type. They generally 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 self-convergent 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 flat in the 
vertical direction and very elongate in the horizontal direction, 
particularly when using a DAF gun and 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 flatter and the 
existence of horizontal dams in the shadow mask. The increasingly stricter 
requirements imposed on the definition of the image, notably in 
high-resolution colour display tubes or when using colour display tubes 
for high-definition television, imply that the spot at the ends of the 
horizontal axis should be smaller 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 at the 
ends of the horizontal display screen axis is 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 a first element influencing convergence is arranged 
between the beam-forming part of the electron gun and the side of the 
deflection unit facing the display window, which element generates a 
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, and in that a second element influencing convergence is 
arranged between the first element influencing convergence and the 
beam-forming part of the electron gun, which element generates a magnetic 
field exerting a force on each outer electron beam having a component in 
the plane of the electron beams directed away from the central electron 
beam. 
The invention is based on the following recognition. Due to the two 
elements influencing convergence the outer electron beams are, in 
operation, subjected to a force which initially drives these electron 
beams apart (underconvergence) and then bends them towards each other 
(overconvergence). The two effects introduced by the invention, in the 
case of deflection, on the convergence of the electron beams substantially 
compensate each other. 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 one beam. 
The magnetic fields to be generated for the desired effects on convergence 
may comprise local dipole fields at the location of each of the two outer 
beams. 
For an improved focusing possibility of the electron beams a preferred 
embodiment of the invention is, however, characterized in that each 
element influencing convergence is adapted to generate a 45.degree. 
magnetic 4-pole field. The extent of underconvergence and overconvergence 
caused by the two elements influencing 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 
not 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: the spot can 
never become too small in the horizontal direction because the bandwidth 
of the video amplifier will then become the restrictive factor. 
The magnetic fields in question may be substantially constant in time. In 
this case they may be generated, for example, by means of an arrangement 
of permanent magnets or by means of a configuration of electric coils 
which are energized with a (substantially constant) direct current. It is 
alternatively possible to energize the configuration of electric coils 
with a DC signal whose value only depends on the amplitude of the line 
deflection signal. Only a simple circuit is required in the two 
last-mentioned cases and no circuit at all is required in the 
first-mentioned case. 
In the special case of using 45.degree. 4-pole fields the apex angles of 
the outer beams are enlarged in the horizontal direction, with the 
above-described effect of reducing the horizontal spot enlargement factor, 
but simultaneously the apex angles of the outer beams are reduced in the 
vertical direction so that the vertical spot dimension increases. Using 
4-pole fields which are constant in time may result in a too large 
vertical dimension of the spot in the centre. 
One way to prevent this risk is dynamic control of the configurations of 
coils generating the 4-pole fields such that the vertical dimension of the 
spot in the centre is sufficiently small. To achieve this, the means for 
producing the 45.degree. 4-pole fields may be fed, in operation, for 
example with currents which are approximately proportional to the square 
value of the line deflection current (i.e. the means for generating the 
45.degree. 4-pole fields can be energized by means of a line-parabolic 
voltage). This can be realised by means of a simple circuit, as will be 
further described. The currents should be applied in such a way that the 
generated 4-pole fields have an opposed orientation. 
It can be ensured with the aid of the afore-described means that the spot 
in the horizontal direction at the ends of the horizontal display screen 
axis is very small in a colour display tube using self-convergent 
deflection fields. 
However, a second drawback of using self-convergent fields is that there is 
vertical overfocusing when deflecting the electron beams across the 
display screen. This cannot always be tolerated for applications in which 
increasingly stricter requirements are imposed on the definition, for 
example, in high-resolution colour monitors. In these cases it is 
advantageous to combine the elements according to the invention causing 
underconvergence and overconvergence with an electron gun which is 
provided with a (magnetic or electric) quadrupole field lens controlled by 
means of a static or dynamic voltage for compensating the astigmatic 
defocusing. 
If the magnetic fields used for influencing convergence are generated by 
means of two configurations of electric coils, each coil may be wound on 
an annular core coaxially surrounding the neck of the tube. This requires 
a relatively long tube neck. The tube neck may be shorter if the 
screen-sided configuration of electric coils is arranged on the annular 
core of the deflection unit itself.

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, accomodates an electron gun 5 in this neck, which gun 
generates three electron beams 6, 7 and 8 whose axes are located in the 
plane of the drawing. In the non-deflected state, the axis of the central 
electron beam 7 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. The relevant 
embodiment shows row-shaped elements. Each triplet comprises a row of a 
green luminescing phosphor, a row of a blue luminescing phosphor and a row 
of a red luminescing phosphor. The phosphor rows are perpendicular to the 
plane of the drawing. A shadow mask 11 is arranged in front of the display 
screen, which mask has a large number of elongate apertures 12 through 
which the electron beams 6, 7 and 8 pass and each impinge upon phosphor 
rows of one colour only. The three co-planar 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 the generation of a first, gun-sided 
magnetic field configuration which drives the electron beams 6 and 8 apart 
in the plane of the electron beams, and a second, screen-sided magnetic 
field configuration which drives the electron beams 6 and 8 towards each 
other in the plane of the electron beams, 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), while maintaining 
convergence. 
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. Magnetic pole shoes (not 
shown) may be arranged in the tube neck 4 so as to guide the dipole fields 
to the correct locations. A drawback of using (metallic) pole shoes is, 
however, that eddy currents may occur in them when using high-frequency 
line deflection fields. 
The use of pole shoes may be dispensed with if each magnetic field 
configuration to be used comprises a 45.degree. 4-pole field. These 4-pole 
fields may be generated, for example, by means of systems of permanent 
magnets. It is alternatively possible to generate these fields by means of 
elements 14 and 14' (see also FIGS. 2A and 2B) which comprise suitable 
configurations of electric coils. 
In the embodiment shown element 14 (FIG. 2A) 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 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). Element 14' (FIG. 2B) has a construction with an annular core 
15' and coils 16', 17', 18' and 19', comparable with the construction of 
element 14. The coils are, however, wound in such a way and the direction 
in which, in operation, a current flows through the coils is such that a 
45.degree. 4-pole field is generated with an orientation which is opposed 
to that of the 45.degree. 4-pole field in FIG. 2A. 
The embodiment shown in FIG. 1 and FIGS. 2A and 2B comprises a 
(self-convergent) main deflection unit and an auxiliary deflection unit 60 
having two coil configurations each generating a 4-pole field, which unit 
is arranged in front of the main deflection unit. A circuit for driving 
the coil configurations generating 4-pole fields may be arranged on the 
deflection unit 20. 
For energizing the coil configurations of the elements 14 and 14' it is 
possible to use direct currents which are not coupled in any way to the 
line deflection signal, or direct currents whose amplitude is coupled to 
the amplitude of the line deflection signal. A circuit for realising the 
last-mentioned possibility is shown in FIG. 10 in which the line 
deflection coils 13, the coils of element 14, the coils of element 14', 
four diodes by D.sub.1, D.sub.2, D.sub.3 and D.sub.4, respectively, and a 
capacitor C are shown schematically. 
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 and a self-convergent 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. The underconvergence 
induced by an element 14 influencing convergence and moving the outer 
beams away from each other, and the overconvergence induced by a 
subsequent element 14' influencing convergence compensate each other so 
that the self-convergence is maintained. 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. 
One way to achieve this is dynamic control of the configurations of the 
coils generating 4-pole fields such that the vertical dimension of the 
spot in the centre is sufficiently small. To achieve this, the means for 
producing the 45.degree. 4-pole fields may be fed, in operation, for 
example with currents which are a substantially quadratic function of the 
line deflection current (i.e. the means for generating the 45.degree. 
4-pole fields can be energized means of a line-parabolic voltage). This 
can be realised by means of the circuit shown in FIG. 5, as will be 
further described. The currents should be applied in such a way that the 
generated 4-pole fields have an opposed orientation. The function which 
the above-mentioned line parabola represents may have its minimum value on 
the zero line. In those cases where the spot dimension in the x-direction 
at the ends of the horizontal axis is sufficiently small, but not in the 
y-direction, the dimension in the y-direction can be realised 
satisfactorily by putting the minimum value of the above-mentioned 
function below the zero line. 
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. To realise this, the means according to the 
invention can be combined with an electron gun having a static, or 
particularly dynamic astigmatic focusing facility. An example of such a 
gun is a so-called DAF gun. 
The principle of an electron gun using D(ynamic) A(stigmatic) F(ocus) will 
be described in greater detail with reference to FIG. 6. 
For the purpose of illustration, FIG. 8 is a longitudinal section of an 
electron gun suitable for use in a colour display tube system according to 
the invention. This electron gun comprises a common cup-shaped electrode 
20 in which three cathodes 21, 22 and 23 are secured, and a common 
plate-shaped screen grid 24. The three electron beams whose axes are 
co-planar are focused by means of the electrode systems (G3) and (G4) 
which are common for the three electron beams. Electrode system G3 
comprises two cup-shaped parts 27 and 28 whose ends face each other. A 
main lens is constituted by applying suitable voltages to the first 
electrode system G3 and the second electrode system, or anode G4. 
Electrode system G4 has one cup-shaped part 29 adjoining G3 and a centring 
bush 30 whose bottom has apertures 31 through which the electron beams 
pass. Electrode part 28 has an outer edge 32 extending towards electrode 
part 29 and electrode part 29 has an outer edge 33 extending towards 
electrode part 28. A recessed portion 34, which extends transversely to 
the plane through the axes 35, 36 and 37 of the electron beams 6, 7 and 8, 
has apertures 38, 39 and 40. A recessed portion 41, which extends parallel 
to recessed portion 34, has apertures 42, 43 and 44. The recessed portions 
34 and 41 form one assembly with the electrode parts 28 and 29, 
respectively. For obtaining desired focusing fields, the apertures in the 
recessed portions may be, for example, circular or provided with collars, 
or they may be polygonal and without collars. In the latter case a 
polygonal gun is concerned. 
In this embodiment an astigmatic element is formed in electrode system G3 
by providing the open ends of the parts 27 and 28 with auxiliary 
electrodes 25, 26 in the form of flat plates having elongate (vertical) 
apertures 45, 46 and 47 and elongate (horizontal) apertures 48, 49 and 50, 
respectively. The apertures may have any shape leading to the formation of 
a 4-pole field for the electron beams passing through the apertures, for 
example, a rectangular, an oval or a diamond shape. 
In operation, electrode 27 can be coupled to means, which are not shown in 
this Figure, for applying a constant focusing voltage V.sub.foc. In this 
embodiment electrode 28 can be coupled to means for applying a control 
voltage V.sub.foc +V.sub.C. 
FIG. 9 shows the auxiliary electrodes 25 and 26 of the electrode system of 
FIG. 8 in a front elevation. The axes of the electron beams 6, 7 and 8 are 
shown in this Figure by means of crosses and substantially coincide with 
the centres of gravity of the (vertical) apertures 45, 46 and 47. The 
centres of the 4-poles formed in the apertures substantially coincide with 
the beam axes. The auxiliary electrodes may alternatively comprise two 
parallel electrode plates, one of which has three substantially vertical 
apertures and the other has one substantially horizontal, elongate 
aperture. 
The embodiment shown should not be considered as limitative. For example, 
only one auxiliary electrode, controlled by V.sub.foc, may be arranged 
between the electrode parts 27 and 28, with a control voltage V.sub.foc 
+V.sub.C being applied to the two electrodes 27 and 28. More generally, 
any type of electron gun having a static or dynamic astigmatic focus can 
be used within the scope of the invention. 
FIG. 11 shows an alternative embodiment of a colour display tube system 
according to the invention. In this embodiment the tube has a gun-sided 
element 54 influencing convergence for driving apart the outer electron 
beams of the type having its own annular core which is shown in FIG. 12A. 
In this embodiment the screen-sided element 54' influencing convergence 
for driving the outer beams towards each other comprises a coil 
configuration which is arranged on the annular core 51 of the deflection 
unit. FIG. 12B 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.