Note: Descriptions are shown in the official language in which they were submitted.
1~936ZS
PHN 8492
The invention relates to a colour television
display device comprising a display tube of the shadow mask
type having a neck portion accomodating an electron gun
assembly for generating a plurality of electron beams and
a funnel-shaped portion whose widest end is terminated in a
display screen; and a system of deflection coils coaxially
arranged about the tube at the transition between the neck .
and funnel-shaped portions w'nich deflection coil system com-
prises first and second deflection coil units each coil unit
having two substantially symmetrical coil halves situated
opposite each other, the first deflection coil unit being
positioned 90 with respect to the second unit such that the
deflection coil units, when energised, deflect the electron
beams in first and second orthoginal deflection directions.
It is an object of the invention to provide
such a colour television display device where cushion or
barrel shaped raster distortion can be corrected by magnet
fields.
The invention provides a colour television
display device comprising a display tube of the shadow mask
type having a neck portion accomodating an electron gun
assembly for generating a plurality of electron beams and a
funnel-shaped portion whose widest end is terminated in a
display screen, a system of deflection coils coaxially
arranged about the tube at the transition between the neck
and funnel-shaped portions which deflection coil system com-
prises first and second deflection coil units and each coil
unit having two substantially symmetrical coil halves situat-
ed opposite each other, the first deflection coil unit being
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~0936Z5
PHN 8492
positioned 90 with respect to the second unit such that
the deflection coil units, when energised, deflect the
electron beams in first and second orthoginal deflection
directions; and means for generating first and second static
eight pole magnetic fields the poles of which are situated
between the axes of a system of axes which alternatively
colncoide with and lie diagonally between the said first
and second deflection directions, said first eight pole
field being generated at or beyond the front end of said
deflection coil system which front end faces the display
screen whilst the second eight pole field is generated at
or beyond the rear end of said deflection coil system which
rear end faces the electron gun assembly, correspondingly
positioned poles of said first and second eight pole fields
being of opposite magnetic polarity.
The means for generating the first and
second eight pole fields may comprise first and second
arrangements of permanent magnets, each arrangement compris-
ing four or eight permanent magnetics. The arrangements
could alternatively be made up of electromagnets.
One of the eight pole fields may be generated
by a pair of rings each being magnetised to have eight
magnetic poles, the rings being coaxially mounted and
adapted to rotate with respect to each other in order that
the strength of the eight pole field so produced may be
varied.
The invention also provides the combination
of a system of deflection coils and means for generating
first and second static eight pole magnetic fields for use
~0936Z5
P~N. 8492.
with a colour television display device as described above.
The invention additionally provides a colour television
display tube of the shadow mask type and the said combina-
tion.
In our U.S. Patent 4,088,930 - Barten - May
9, 1978 a system is described with which a convergence cor-
rection for the isotropic astigmatism can be obtained with-
out the landing being noteworthily influenced by it by a
dynamic energization of four toroidal windings provided on
the core of the deflection coil. For this dynamic energiza-
tion parabolic or sawtooth voltages are necessary which have
to be generated in separate circuits.
As already shown in the above mentioned
Patent Application, an n-pole field can be described with
_ _ (1/2n-1)
B = Av (1)
in which B iS the vector denoting the magnetic field
strength B in the complex plane
A is an (integration) constant, and
v is a complex auxiliary quantity which
can be expressed in the coordinates x and _ so that:
B = By + i BX (2)
and
v = y - i x (3)
During the deflection the centre of the three electron
beams in the n-pole field moves towards the position
xO, yO. Description of the field with respect to said
new centre yields the relationship:
B = A (vO - v ~)(l/2n - l)
wherein
O vO = yO - i xO
v' = y' - i x' (6)
B
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10936Z5
PHN 8492
and x' and y' are the coordinates in the shifted system
of coordinates. For the eight--pole used in this invention
it follows from (4):
B = A(vo + 3vov + 3vov' + v' ) (7)
Now is, generally, with larger deflection ¦vO¦ 7 ¦v ¦ '
in other words, the displacement in the eight-pole fleld
caused by the deflection then is large with respect to the
eccentricity of the beams. As a result of this the terms
with v 2 and v 3 are negligible, so that it holds to an
approximation that:
B = A (vO + 3Vov ) (8)
The first term Avo represents a homogeneous field the
strength of which varies with the third power of the
deflection and the direction of which depends on the
direction of the deflection. This field exerts an
identical influence on the three electron beams on the
raster shape but not on the convergence. The second term
3Avo v represents a static four-pole field the strength
of which varies with vO, so with the square of the
deflection and thus influences the convergence. In order
to be able to use the eight-pole for correction of the
raster shape in a colour display tube the influence on
the convergence should preferably be minimum. Therefore,
the second term of formula (8) should be small with respect
to the first term. This can be achieved by causing the
eight-pole field to extend preferably not over the whole
deflection field but by providing it only on the display
screen side of the deflection field.
The static eight-pole magnetic field may be
constructed in a number of manners. For example, four
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~V936ZS
PHN 8492
or eight permanent magnets or electromagnets may be
used which may be situated, for example, to form a ring,
or radiate from the display tube. The magnets may be
incorporated in the housing of the system of deflection
coils, be placed substantially against it or by secured
at some distance therefrom, for example, against the
envelope of the display tube.
As the influence on the convergence may still
be too large when a single eight pole field is used there
is generated at or beyond the other end of the system of
deflection coils situated a second static eight-pole field
of opposite polarity. The strength of the second eight-
pole may then be chosen to be so that the second term of
formula (8) is compensated for and hence substantially no
influence exists of the convergence any longer. The first
term of formula (8) which determines the raster distortion
is only little counteracted by the first term of the
second eight-pole field. This is because vO in the
second eight-pole is much smaller. Just as the first
eight-pole, the second eight-pole may be manufactured
in a number of manners by means of coils, electromagnets
and permanent magnets.
The invention can be used with colour display
tubes of inter alia the "delta"type in which the electron
beams pass through the corners of an equilateral triangle
which is situated in a plane substantially at right angles
to the tube's longitudinal axis. However, the invention
also proves suitable for colour display tubes of the
"-in-line" type in which the electron beams are situated
substantially in one plane.
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~936ZS PHN 8492
As static eight-poles are used, no voltages
or currents varying with the deflection are necessary.
Thus no extra circuits are necessary to generate these
currents or voltages. In the invention, th~ dynamic
character of the required correction is obtained by the
deflection of the electron beam itself.
The invention will now be described in
greater detail with reference to a drawing, in which
Figure 1 shows a colour display tube of the
"in-line" type according to the invention,
Figure 2 shows an eight--pole field and the
forces acting on the electron beam as a result thereof,
Figure 3 shows the effect of the eight-pole
on the frame shape, and
Figures 4 to 12 show a number of possibilities
for generating an eight-pole field,
The colour display tube shown in Figure 1 -
comprises a glass envelope 1 which has a neck 2 and a
funnel or cone-shaped part 3. Three electron guns 7, 8
and 9 are provided in the neck 2. The colour display tube
is of the "in-line" type so that the axes of the guns 7,
8 and 9 are situated substantially in one plane. The
electron beams generated by these electron guns are
deflected by the system of deflection coils 11 and impinge
upon the display screen 4 through the apertures 6 in the
colour selection electrode 5 (The shadow mask). The three
electron beams pass through the apertures 6 at a small angle
with each other (the so-called colour selection angle) and
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~93~Z5 PHM 8492
consequently each impinge only upon phosphor strips of
one colour. The system of deflection coils 11 may comprise
a number of saddle-shaped coils and/or toroidal coils. An
important property of the tube is the convergence. Con-
vergence is the substantial coincidence of the threeelectron beams 27, 28 and 29 in one point in the apertures
6 of the colour selection electrode 5. The beams must not
only converge in one point in the centre of the colour
selection electrode but this must remain also during the
deflection with the system of deflection coils 11. This is
necessary because otherwise the red, green and blue rasters
no longer coincide entirely and coloured edges at the
transitions are formed in the picture. Good convergence
in the centre of the colour selection electrode of a col-
our display tube can be obtained with simple means.However,it is particularly difficult to maintain a good
convergence also for the parts of the colour selection
electrode in which a large deflection of the electron
beams has taken place, the cause of this being the un-
equal influence of the deflection field on the threeelectron beams. Another important property is the extent
of raster distortion. The raster distortion which is
expressed in a cushion~like and/or barrel-like distort-
ion of the picture occurs for the greater part as a re-
sult of a non-linear deflection by the system of de-
flection coils. Such distortion also occurs in deflect-
ion coils for black-and-white tubes where the design of
deflection coils may be mainly directed to the minimiz-
ation of the extent of the distortion. However, this
necessity is omitted when an eight pole is used-In colour
B -7-
lV936Z5
PHN 8492
display tubes the convergence is most important. The con-
siderable distortion which often occurs in deflection coils
providing a good convergence has so far been corrected for
by passing currents through the individual winding groups
of the system of deflection coils. The correction currents
must be generated by extra circuits, so that this solution
is cumbersome and expensive. The correction in a colour
display tube is carried out by means of static magnetic
eight-pole fields one of which is situated in or near the
front end of the system of deflection coils 11 which faces
the display screen 4 and is generated by a number of magnets
12. This eight-pole field can be generated by means of
permanent magnets or by means of electromagnets or coils.
The coils or magnets may be incorporated in the system of
deflection coils 11. It is alternatively possible for them
to be assembled at some distance therefrom closer to the -~
display screen, and supported, for example by means of ;
braces or strips. As the influence of this eight-pole
field on the convergence can prove to be too large,
a second . eight-pole field is generated at the other (rear)
end of the deflection coil system which focus the electron
gun assembly the polarity of which is opposite to that of
the first eight-pole field by means of a number of magnets
- 13 so that said influence is compensated for without con-
siderably influencing the positive influence on the raster
distortion.
Fig. 2 shows diagrammatically an eight-pole
field by means of a number of field lines 14. The axes X
and Y coincide with the axes of the deflection directions.
B
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~0936Z~i PHN 8492
The forces F acting on each electron beam are shown in
the Figure. The north and south magnetic oles (N and Z)
are situated between the axes. The eight~pole field need
not be completely symmetrical. ~y making the eight pole
not quite symmetrical, higher-order multiple fields are
obtained in addition (for example a twelve-pole, sixteen-
pole) with which higher order errors in the raster can
be corrected for.
Fig. 3 shows a cushion-like distortion of the
raster (solid line). By using the invention, the forces F
act on the electron beam(s) in such manner that the raster
is corrected (broken line). The raster distortion may also
be barrel-shaped. In order to correct for raster distortion,
the polarity of the eight-pole must be such that the forces
F are directed just in the opposite direction.
Figures 4 to 12 show a number of possibilities
for generating an eight-pole.
Figure 4 shows how an eight-pole field can be
generated by means of four permanent magnets 15, 16, 17
and 18.
Figure 5 shows how this is possible with a
permanent magnetic ring 19 having pole pieces.
Figures 6 and 7 show the electromagnets 20
and 21 which can replace the permanent magnets of Figures 3
and 4.
Figure 8 shows an eight-pole field generated
by eight permanent magnets 22.These magnets may be in-
corporated in the system of deflection coils 11 or be
assembled at a small distance therefrom.
Figure 9 shows that it is also possible to
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_g
~V936ZS
PHN 8492
generate an eight-pole by means of a number of coils 23.
Figure 10 shows the toroidal construction
for generating an eight-pole field. A number of coils 25
are provided around a magnetic yoke 24.
Figure 11 shows two magnetic rings 26 and 27
constructed to produce an eight-pole with magnetic regions
42. By rotating these rings relative to each other, the
strength of the eight-pole field can be adjusted. Rotating
the two rings at the same time may be carried out by means
of a rack and pinion transmission which is shown in Fig. 12.
Internal teeth 28 are provided on ring 26 and external teeth
29 are provided on ring 27 which are coupled by means of
- a pinion 30. Such an adjustable eight-pole is perferably
used for producing the second eight-pole field.
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