Patent Application
20050146276
The present invention relates to a magnetic device for correcting geometrical defects in the image created on the screen of a cathode-ray tube and is more particularly adapted to tubes whose front face has a high radius of curvature.
A cathode-ray tube intended to generate colour images generally comprises an electron gun emitting three electron beams, each beam being intended to excite a phosphor of a defined primary colour (red, green or blue) on the screen of the tube.
The electron beams scan the screen of the tube under the influence of the deflection fields created by a deflection device, also called a deflector, fixed to the neck of the tube, comprising coils for horizontally and vertically deflecting the said beams. Conventionally, a ring made of a ferromagnetic material surrounds the deflection coils so as to concentrate the deflection fields in the appropriate region.
The three beams generated by the electron gun must always converge on the screen of the tube otherwise an error called a convergence error is introduced, which distorts in particular the rendition of the colours. In order to make the three coplanar beams converge, it is known to use astigmatic deflection fields called self-converging fields; in a self-converging deflection coil, the intensity of the field or the lines of flux caused by the horizontal deflection winding are generally in the form of a pincushion in the region of a portion of the coil which is located somewhat to the front of the latter on the side of the screen of the tube. This amounts to introducing, into the distribution of the turns forming the line coil, a large positive third harmonic of the ampere-turn density to the front of the coil.
Moreover, due to the action of uniform horizontal and vertical deflection magnetic fields, the volume scanned by the electron beams is a pyramid whose apex is coincident with the centre of deflection of the deflector and whose intersection with a non-spherical screen surface presents a geometrical defect called a pincushion. This geometrical distortion of the image is greater the greater the radius of curvature of the screen of the tube. The self-convergent deflectors generate astigmatic deflection fields making it possible to modify the north/south and east/west geometry of the image and, in particular, partially compensate for the north/south pincushion distortion. The east/west geometrical defects are generally corrected by an electronic circuit associated with the deflector.
However, the current trend towards tubes having an increasingly flat screen surface, or even a completely flat surface, especially amplifies the problems of image geometry; the result of this is that the self-convergent deflectors can no longer completely correct the geometry of the north/south pincushion, while moreover, the east/west geometrical defects require increasingly strong corrections.
To correct these problems of image geometry, linked to the flatness of the screen and to the self-convergent deflection device equipping the tube, there are various solutions such as, for example, electronic correction devices illustrated by Patents U.S. Pat. No. 3,916,254 and U.S. Pat. No. 3,748,531, or transducers to correct the east/west pincushion defects as in application EP 776125.
However, the solutions described only make it possible to correct either the north/south pincushion, or the east/west pincushion which especially complicates the design of the device for correcting the geometrical defects of the tube equipped with its deflector. Moreover, the electronic correction circuits are generally developed by designers of the electronic frame generating all the functions of the television set in which the tube is inserted, the said designers preferring to buy, for reasons of cost, a tube where these geometrical faults are corrected beforehand.
The object of the invention is to provide an overall solution, correcting both the north/south and east/west geometrical defects, a solution taking the form of a magnetic device which can be loaded with the deflector so as to produce a tube which does not require an electronic geometrical correction device.
For this, the subject of the invention is a magnetic device for correcting the geometry of the image created on the screen of a cathode-ray tube, in the form of a saturable magnetic device comprising:
The invention and its various advantages will be better understood using the description below and the drawings of which:
a and 8b show the variations of the deflection current as a function of time when they are modulated by the correction device according to the invention.
The invention aims to correct these geometrical defects by modification of horizontal and vertical deflection currents.
The presence of an east/west pincushion distortion means that the amplitude of the horizontal deflection current Ih at the centre of the screen is insufficient and that it must be compensated for by reducing the amplitude of Ih on the edges of the screen A, B, C, D. This effect must be obtained by modulating the amplitude of the horizontal deflection current at the frequency (1/Tv) of the vertical deflection current. The shape of the horizontal current is then similar to that illustrated by
The presence of a north/south pincushion distortion means that the amplitude of the deflection current Iv must be increased from the centre towards the upper and lower parts of the screen so as to “pull” the middles of the horizontal edges, AB and CD, of the image outwards, the amplitude of the increase having to be proportional to the value of the uncorrected vertical deflection current and the modulation frequency having to correspond to the frequency of the horizontal deflection current (1/Th).
The core is magnetically saturated in the absence of a current in the coils 1, 2, 3, 4 by the flux Φs of the magnets 5 and 6: the inductance of the coil series is therefore a minimum.
When the amplitude of the current in the coils 1 to 4 increases, a progressive desaturation then occurs in the core, proportional to the current amplitude, under the pair of coils where the flux created by the deflection current opposes the flux Φs; it follows that the inductance of this pair increases with the current while the inductance of the other pair remains substantially constant. When the current Ih goes from its minimum value (−Ih,max) to its maximum value (Ih,max), the inductance Ls of the coils 1 to 4 connected in series will then change according to the curve illustrated by
However this prior art does not solve the problems of correcting the north/south geometry of the image.
The correction device comprises a first set of four coils 33, 34, 35, 36 successively connected in series and coiled around a saturable magnetic core separated into two parts 40, 41 by means of a permanent magnet 39 saturating the magnetic core by means of a longitudinal flux Φs. The coils 33 to 36 are coiled around the magnetic core such that the magnetic fluxes created by the pair of coils 33,34 (ΦH1,ΦH2) are oriented longitudinally in one direction and the fluxes created by 35, 36 (ΦH3,ΦH4) are oriented longitudinally in the opposite direction. The coils are supplied by all or some of the horizontal deflection current intended to supply the horizontal deflection coils of the cathode-ray tube.
The correction device furthermore comprises a second set of coils consisting of an external coil pair 31, 32, in series one with the other and placed around the magnetic core so as to contain the magnetic fluxes created by the set of coils 32 to 36, the two coils being traversed by at least part of the vertical deflection current intended to supply the vertical deflection coils of the cathode-ray tube; the coils 31 and 32 are arranged so as to create, in the magnetic core, a magnetic flux Φv1 oriented in one direction for one of the pairs of coils (34,35) of the first set and a magnetic flux ΦV2 oriented in the opposite direction in the second pair ( 36, 33) of the first set.
The various flux orientations indicated in
The force of the permanent magnet is preferably chosen so as to saturate the core (40,41) with a magnetic flux Φs in the absence of a horizontal and vertical deflection current.
As illustrated in
The correction device operates as follows:
For the first set of coils 33 to 36:
For the second-set of coils 31, 32:
Let us consider the situation for which the current l is at its maximum positive value.
In the configuration illustrated in
In contrast, the flux ΦV2 created by the coil 31 will desaturate the magnetic circuits around which the coils 33, 36 are coiled. If the coils 33 and 36 are considered as identical, the flux (ΦV2+ΦH1) in the circuit around which the coil 33 is coiled is greater than the flux (ΦV2−ΦH4) in the circuit around which the coil 36 is coiled. The variation in flux between the two coils 33 and 36 will induce a current IV0 in the coil 31 which will be superimposed with the current already flowing in the said coil.
An identical phenomenon will occur when the current Iv is at its maximum negative value, in this case, the coils 32, 34 and 35 acting respectively like coils 31, 33 and 36.
Moreover, since the desaturation of the magnetic circuits is proportional to the amplitude of the said current Iv, the increase IV0 in the amplitude of the current Iv will also be proportional to the said current. The result of this is a modulation of the current Iv according to
In this way, the current in the vertical deflection coils will be according to
The principle of the invention is not limited to the embodiment described above.
In another embodiment of the invention (not shown) the permanent magnet 39 of the geometrical correction device is replaced by a coil coiled on a core placed in contact with the core (40, 41), the coil being supplied by a D.C. source in order to generate a longitudinal magnetic flux Φs in the said core. Although this configuration is more expensive, it has the advantage of being able to provide an element for additional adjustment of the flux Φs, making it possible to adapt the same geometrical correction device to several types of deflectors equipping various families of cathode-ray tubes.
In order to decrease the costs of fabrication and assembly of the device according to the invention, it is possible, as illustrated in
| Number | Date | Country | Kind |
|---|---|---|---|
| 0116465 | Dec 2001 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP02/13698 | 12/4/2002 | WO |
