WO2004025686A1

WO2004025686A1 - Color picture display device with a magnetic quadrupole coil

Info

Publication number: WO2004025686A1
Application number: PCT/EP2003/009744
Authority: WO
Inventors: Albertus Aemilius Seyno Sluyterman
Original assignee: Lg. Philips Displays
Priority date: 2002-09-09
Filing date: 2003-09-01
Publication date: 2004-03-25
Also published as: AU2003270141A1

Abstract

A cathode ray tube (1) has a quadrupole coil (14), which is positioned between the display screen (2) and the deflection unit (20). The deflection unit (20) comprises a frame coil set (13') for generating a vertical deflection of electron beams (6,7,8) over the display screen (2) and a line coil set (13) for generating a horizontal deflection. In operation, the quadrupole coil (14) is driven by a current that is proportional to the square of a frame-current that drives the frame coil (13'). In this way the sensitivity of the line deflection coil (13) is increased, East/West pincushion distortion is reduced and the landing variation is reduced.

Description

Color picture display device with a magnetic quadrupole coil

The invention relates to a color picture display device comprising a cathode ray tube having a display screen, means for generating an electron beam, deflecting means for deflecting the electron beam over the display screen, quadrupole means for generating an magnetic quadrupole field, which is positioned on an axial axis between the display screen and the deflecting means. The invention relates further to a display apparatus.

Conventional color picture display devices comprising cathode ray tubes are of the 3-in-line type, i.e. having three electron beams that are positioned in one plane. In general these tubes comprise a self-convergent deflection unit which, in operation, generates magnetic fields for horizontal and vertical deflection of the beams such that the three electron beams generated by an electron gun and focussed on a display screen converge throughout the display screen.

US-6,072,547 discloses a cathode ray tube in which a coil that generates a magnetic quadrupole field is used for correcting landing properties of the electron beams. The quadrupole coil is positioned between the deflection unit and the display screen and is driven with a signal derived from the video information (for example a signal which is proportional to the strength of the instantaneous beam current) in order to remedy landing errors caused by space-charge repellence between electron beams. It is desired to further improve the quality of the pictures that are displayed on such picture display devices.

It is an aim of the invention to provide a color picture display device on which the displayed pictures have an improved quality. To this end the color picture display device according to the invention is characterized in that it further comprises means for driving the quadrupole means with a current that is proportional to the square of a current through the deflecting means. The inventor has realized that if the quadrupole means is positioned between the screen and the deflection means, and is driven with the indicated current this will have several advantages that lead to an improved quality of the displayed images.

The line coil sensitivity will be increased. Furthermore, East/West pincushion distortion is reduced and the variation of landing, being different in the comers of the screen compared at the line axis is reduced. The quadrupole means are, in preferred embodiments, positioned sufficiently far away from the front of the deflection yoke so that it hardly influences the convergence of the electron beams. The quadrupole means are activated such that the line scan is increased. By applying a parabolic frame current, the scan in the comers is reduced, thereby increasing the sensitivity of the deflection and reducing pincushion distortion.

The dependent claims describe advantageous embodiments of the invention. These and other objects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

In the drawings: Fig. 1 shows a longitudinal cross-section of a cathode ray tube according to the invention,

Fig. 2 shows the quadrupole current as a function of the frame-current, Fig. 3 shows an embodiment of the quadrupole coil according to the invention, Fig. 4 shows another embodiment of the quadrupole coil according to the invention, and

Fig. 5 shows an advantageous embodiment of the quadrupole coil according to the invention.

The figures are not drawn to scale. In the figures, like reference numerals generally refer to like parts.

Fig. 1 is a cross-section of a color picture display device according to the invention. A glass envelope 1 is composed of a display window 2, a cone 3 and a neck 4. The neck accommodates an electron gun 5, which generates three electron beams 6, 7 and 8 whose axes are in the plane of the drawing. In the undeflected state, the axis of the central electron beam 7 coincides with the longitudinal axis 9 of the tube. An inner surface of the display screen 2 is provided with a large number of triads of phosphor elements. Each triad comprises an element consisting of a phosphor luminescing in green, an element consisting of a phosphor luminescing in blue and an element consisting of a phosphor luminescing in red. Opposite the display screen, there is provided a shadow mask 11 having a large number of apertures 12 for allowing passage of the electron beams 6, 7 and 8, which each impinge on phosphor elements of only one color. The three electron beams are deflected by means of a deflection unit 20, which comprises a line-deflection coil system 13 and a frame deflection coil system 13' as well as a ring core 21 that coaxially surrounds the line-deflection coil system 13. In operation, the line-deflection system and the frame-deflection system are driven by line- and frame-currents, respectively.

The picture display device according to the invention is provided with a quadrupole coil 14 for generating a magnetic quadrupole field, said quadrupole coil being positioned on the axial axis between the screen 2 and the deflecting unit 20. In operation, the quadrupole coil is driven by a quadrupole current that is proportional to the square of the current through the frame-deflection coil system 13'. The magnetic field generated by the quadrupole coil additionally deflects the outer electron beams that are not positioned in the center of the tube. The current through the quadrupole coil is such that the horizontal deflection is increased and the deflection in the vertical direction is decreased.

The quadrupole current Iq as a function of the frame-current I_v is shown in Fig.2. The quadrupole current is proportional to the square of the frame deflection current with a negative offset Ioffset of approximately half of the nominal dynamic amplitude. The offset current is the current on the line axis, and the difference of the current in the comers with respect to the line axis is called the dynamic (or the deflection dependent) current. In principle both currents can be freely adjusted. The offset value is adjusted to determine the sensitivity of the horizontal deflection at the longitudinal axis of the tube. The dynamic amplitude is then adjusted to determine how much the sensitivity is decreased as a function of vertical deflection. The quadrupole coil 14 is, in preferred embodiments, positioned sufficiently far away from the front of the deflection yoke 20 so that convergence of the electron beams is hardly influenced, e.g. at least 5 mm (e.g. between 1 cm and 2,5 cm) beyond the windings of the line coil system. The little influence that might be present can be corrected in the yoke, e.g. via adaptation of the wire distribution of the deflection coils. The effect of the position of the quadrupole coil and the current through it is such that the line scan, i.e. the horizontal deflection is increased during operation. By applying a parabolic frame current, the scan in the comers can be reduced. This results in an increased sensitivity and reduced pincushion distortion. A picture of a raster of parallel horizontal and vertical lines may be distorted by the deflection unit such that a pin-cushion shape on the screen is created. The deflection unit is by its nature inclined to produce a pincushion shaped picture. This is caused by the fact that the deflection angle is to a great extent linear with the current through the deflection unit, but the position of the beams on the screen to their point of deflection is not. The application of the quadrupole 14 reduces this effect.

The deflection unit 20 is mounted such that it touches the glass of the cathode ray tube. This implies that pullback is gained, which means that this gain in pullback can be used to further increase the deflection sensitivity. Pullback is the degree to which a deflection coil can be pulled back (in the direction of the gun) without the risk that the deflected electron beams touch the inner surface of the glass envelope. The further the deflection field of the coils can be located in the direction of the gun the more sensitive the coil is, but at the same time the risk is increased that the electron beams will touch the surface of the envelope. Hence if the deflection unit requires less pullback, this degree of freedom can be used to design a unit with increased sensitivity. Also the landing characteristics of the tube are improved. 'Landing' is to be understood to mean a correct landing angle of the electron beams on the screen. The three electron beams which must excite the phosphor dots of a specific pixel must pass through the same hole of the color selection electrode shadow mask) at slightly different predetermined angles. The landing at the line axis is adjusted by the offset value of the current through the quadrupole coil 14, whereas the landing in the comers of the screen 2 is corrected by means of the parabola current through the quadrupole. More precisely, the landing in the comers of the screen is determined by the actual current through the quadrupole. Hence, the offset value and the dynamic amplitude together determine the landing in the comers. The procedure then is to adjust first the landing on the line axis by means of the offset value, and after that to adjust the landing in the comers by means of the dynamic amplitude. In this way the landing spread can be reduced.

A further aspect of the invention provides a display apparatus comprising the color picture display device 1 according to the invention, and circuitry 730 for providing control signals 732 and display signals 734 to the display device. Good results were obtained when a so-called 45° quadrupole coil for generating the magnetic quadrupole field was used.

In Figs. 3, 4, and 5 advantageous embodiments of the 45° quadrupole coil are shown. In the embodiment shown in Fig.3, the quadrupole coil comprises a ring core

15 of a magnetizable material, which coaxially surrounds the cone 3 of the tube and on which coil elements 16, 17, 18 and 19 are wound in such a manner that, upon excitation, a 45° magnetic 4-pole field is generated, whose orientation relative to the three beams 6, 7 and 8 corresponds to that shown in the drawing. Alternatively, a 45° magnetic 4-pole field can be generated by means of two

C-cores that are provided with a winding, as shown in Fig. 4, or with a stator construction as shown in Fig.5.

In summary the invention relates to a cathode ray tube 1 having a quadrupole coil 14, which is positioned between the display screen 2 and the deflection unit 20. The deflection unit 20 comprises a frame coil set 13' for generating a vertical deflection of electron beams 6,7,8 over the display screen 2 and a line coil set 13 for generating a horizontal deflection. In operation, the quadrupole coil 14 is driven by a current that is proportional to the square of a frame-current that drives the frame-coil 13'.

In this way the sensitivity of the line deflection coil 13 is increased, East/West pincushion distortion is reduced and the landing variation is reduced.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of other elements or steps than those listed in a claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

Claims

CLAIMS:

1. A color picture display device comprising a cathode ray tube (1) having a display screen (2), means (5) for generating an electron beam (6,7,8), deflecting means (20) for deflecting the electron beam (6,7,8) over the display screen (2), quadrupole means (14) for generating a magnetic quadrupole field, the quadrupole means being positioned on an axial axis (9) between the display screen (2) and the deflecting means (20), and means for driving the quadrupole means with a quadrupole current (Iq) that is proportional to the square of a current (I_v) through the deflecting means.

2. A color picture display device according to claim 1, wherein the quadrupole current (Iq) comprises a DC component (Ioffset)-

3. A color picture display device according to claim 1, wherein the quadrupole means (14) comprise a quadrupole coil (14) for generating a 45° magnetic field.

4. A color picture display device according to claim 1, wherein the deflection means (20) comprises a frame coil set (13') and the current (I_v) through the deflection means is the current through the frame coil set (13') when in operation.

5. A display apparatus, comprising: the color picture display device according to claim 1, and means (730) for providing control signals (732) and display signals (734) to the display device.