PICTURE DISPLAY SYSTEM INCLUDING A DEFLECTION UNIT WITH A DOUBLE SADDLE COIL SYSTEM

Abstract: Self-convergent picture display system with a colour display tube (1) and an electromagnetic deflection unit (5). The deflection unit (5) comprises a field deflection coil (8) and a line deflection coil (7) which are both of the saddle type and are particularly wound directly on a support. The deflection unit also comprises a pair of magnetically permeable portions (12, 13) which are arranged symmetrically with respect to the plane of symmetry of the field deflection coil (8) on either side of the tube axis. The magnetically permeable portion withdraw magnetic flux from the end of the yoke ring in order to extend the vertical deflection field. A self-convergent system can be realised with different screen formats by choosing different lengths of the magnetically permeable portions.

Description: The invention relates to a picture display system including a colour display tube having a neck accommodating an electron gun assembly for generating three electron beams lying in one plane, and an electromagnetic deflection unit surrounding the paths of the electron beams which have left the electron assembly, said deflection unit comprising a field deflection coil of the saddle type having a front and a rear end for deflecting electron beams generated in the display tube in a vertical direction; a line deflection coil of the saddle type likewise having a front and a rear end for deflecting electron beams generated in the display tube in a horizontal direction, and a yoke ring of ferromagnetic material surrounding the two deflection coils and having front and rear end faces extending transversely to the tube axis, the electron beam traversing the coils in the direction from the rear to the front ends when the deflection unit is arranged on a display tube. For some time a colour display tube has become the vogue in which three electron beams are used in one plane; the type of such a cathode ray tube is sometimes referred to as "in-line". In this case, for decreasing convergence errors of the electron beams, a deflection unit is used having a line deflection coil generating a horizontal deflection field of the pincushion type and a field deflection coil generating a vertical deflection field of the barrel-shaped type. Deflection units for in-line colour display tube systems can in principle be made to be entirely self-convergent, that is to say, in a design of the deflection unit which ensures convergence of the three electron beams on the axes, anisotropic y-astigmatism errors, if any, can simultaneously be made zero in the corners without this requiring extra correction means. Whilst it would be interesting from a point of view of manufacture to have a deflection unit which is self-convergent for a family of display tubes of the same deflection angle and neck diameter, but different screen formats, the problem exists, however, that a deflection unit of given main dimensions can only be used for display tubes of one screen format. This means that only one screen format can be found for a fixed maximum deflection angle in which a given deflection unit is self-convergent without a compromise (for example, the use of extra correction means). The Netherlands Patent Specification 174 198 provides a solution to this problem which is based on the fact that, starting from field and line deflection coils having given main dimensions, self-convergent deflection units for a family of display tubes having different screen formats can be assembled by modifying the effective lengths of the field and line deflection coils with respect to each other. This solution is based on the recognition that, if self-convergence on the axes has been reached, the possibly remaining anisotropic y-astigmatism error (particularly the y-convergence error halfway the diagonals) mainly depends on the distance between the line deflection point and the field deflection point and to a much smaller extent on the main dimensions of the deflection coils used. If deflection units for different screen formats are to be produced while using deflection coils having the same main dimensions, the distance between the line and field deflection points may be used as a parameter to achieve self-convergence for a family of display tubes having different screen formats but the same maximum deflection angle. The variation in the distance between the line and field deflection points necessary for adaptation to different screen formats is achieved in the prior art by either decreasing or increasing the effective coil length of the line deflection coil or of the field deflection coil, or of both - but then in the opposite sense - with the main dimensions of the deflection coils remaining the same and with the dimensions of the yoke ring remaining the same, for example, by mechanically making the coil or coils on the rear side smaller and longer, respectively, by a few millimetres, or by positioning, with the coil length remaining the same, the coil window further or less far to the rear (so that the turns on the rear side are more or less compressed). To be able to realise this, saddle-shaped line and field deflection coils of the shell type were used. These are coils having ends following the contour of the neck of the tube at least on the gun side. This is in contrast to the conventional saddle coils in which the gun-sided ends, likewise as the screen-sided ends, are flanged and extend transversely to the tube surface. When using saddle coils of the shell type it is possible for the field deflection coil (and hence the vertical deflection field) to extend further to the electron gun assembly than the line deflection coil, if the field design so requires. However, there are also deflection units with deflection coils of the conventional saddle type, which means that - as stated - they have front and rear ends located in planes extending at an angle (generally of 90°) to the tube axis. (A special type of such a deflection unit with conventional saddle coils is, for example, the deflection unit described in EP 102 658 with field and line deflection coils directly wound on a support). In this case it has until now been impossible to extend the vertical deflection field further to the electron gun assembly than the horizontal deflection field, because the field deflection coil is enclosed between the flanges of the line deflection coil. It is an object of the invention to provide a picture display system with a deflection unit having conventional saddle coils in which this problem is solved. To this end a picture display system of the type described in the opening paragraph is characterized in that first and second axially extending magnetically permeable members are arranged in the said plane symmetrically with respect to the tube axis, each magnetically permeable member having a first end directly facing the rear end face of the yoke ring and a second end located at the neck of the display tube in the vicinity of the exit end of the electron gun assembly, the length of the first and second magnetically permeable members and their distance to the yoke ring being dimensioned for providing a self-convergent picture display system. The invention is based on the recognition that the first ends of the magnetically permeable portions draw a field deflection flux from the yoke ring and channel it to their second ends. The quantity of flux which is taken up is adjusted by means of the distance between the first ends and the yoke ring, and the length of the magnetically permeable portions determines how far the vertical deflection field is extended to the rear. A practical embodiment of the picture display system according to the invention is characterized in that regions of the rear end face of the yoke ring located in the plane of the electron beams are left free by the rear end of the field deflection coil and in that the first ends of the magnetically permeable portions are located opposite said regions. The invention can particularly be used to advantage if the field deflection coil and the line deflection coil are directly wound on a support. The invention also relates to an electromagnetic deflection unit suitable for use in a picture display system as described hereinbefore. For use in a display tube having a larger screen format than the display tube for which it is designed, the invention provides the possibility of moving apart the deflection points of the horizontal deflection field and the vertical deflection field generated by a given deflection unit having saddle coils and of moving them towards each other for use in a display tube having a smaller screen format. The great advantage of the invention is that only a modification of the length of the magnetically permeable portions (providing or omitting them, respectively) is required to adapt a deflection unit to different screen formats of a display tube family. The invention will now be described in greater detail by way of example with reference to the drawings in which Figure 1 is a diagrammatic elevational view of a vertical cross-section through a colour display tube comprising a deflection unit according to the invention, Figure 2 is a diagrammatic elevational view of a horizontal cross-section through the colour display tube of Figure 1, Figure 3 is a rear view of the yoke ring with field deflection coils of the deflection unit of Figures 1 and 2, Figure 4 is a rear view of the colour display tube of Figure 1 having a first set of magnetically permeable portions according to the invention, Figure 5 is a rear view of the colour display tube of Figure 1 having a second set of magnetically permeable portions according to the invention, Figure 6 shows the axial variation of the respective deflection fields in a first combination of a display tube and a deflection unit, and Figure 7 shows the axial variation of the respective deflection fields in a second combination of a display tube and a deflection unit. Figure 1 is a diagrammatic elevational view of a vertical cross-section through a colour display tube 1 of the "in-line" type having a display screen 2, a tube neck 3 and three co-planar electron guns 4. A deflection unit 5 mounted on the display tube comprises a yoke ring 6, a saddle coil 7 for the horizontal deflection (the so-called line deflection coil) and a saddle coil 8 for the vertical deflection (the so-called field deflection coil). The yoke ring 6 has a gun-sided end 9, the field deflection coil 8 has a (flange-shaped) gun-sided end 10 and the line deflection coil has a (flange-shaped) gun-sided end 11. The deflection unit 5 is provided with magnetically permeable portions 12 and 13 (see also Figures 2 and 4) which are arranged symmetrically with respect to the plane of symmetry of the field deflection coil 8 (the y-z plane), and which extend up to a small distance from the gun-sided end 9 of the yoke ring 6. The portions 12 and 13 take up magnetic flux from the yoke ring 6 and due to their special positioning and construction they produce an extra (vertical deflection) field. The location of this extra field, as well as its quantity can be adjusted by the shape of the portions 12 and 13. The problem of screen-format adaptation (trilemma problem) when using deflection units with a set of double saddle coils can be solved thereby. A type of deflection unit with a double set of saddle coils which is currently in the limelight is particularly the type in which the two saddle coils are directly wound on a hollow support, either on the same support, or each on a separate support. It will be evident that it is necessary to form the (flange-shaped) gun-sided end 10 of the field deflection coil 8 in such a manner that regions A and B of the gun-sided end 9 of the yoke ring 6 located on either side of the plane of symmetry of the line deflection coil 7 (the x-z plane) are accessible. The regions from which the magnetically permeable portions 12 and 13 must withdraw magnetic flux, the regions A and B, are denoted by arrows in Figure 3. In other words, the insides of the saddle heads of the gun-sided field deflection coil end 10 must be sufficiently high to leave the gun-sided end 9 of the yoke ring 6 locally free. For the sake of clarity the line deflection coil 7 is not shown in Figure 3. The line deflection coil 7 must of course also enable the regions A and B of the gun-sided end 9 of the yoke ring 6 to be accessed, but in general no special measures need to be taken for this purpose. However, it must be ensured that the saddle heads of the gun-sided line deflection coil end leave open a sufficiently large region on either side of the y-z plane in order to reach the yoke ring 6 with magnetically permeable portions 12 and 13 of some height h (see Figure 1). Finally it will be evident that also the support (not shown in the Figures) of the field deflection coil 8 must have apertures to provide access to the regions A and B of the yoke ring 6 with the magnetically permeable portions 12 and 13. The above-described measures combined comprise the preparation of a deflection unit for providing magnetically permeable portions extending the vertical deflection field. As already mentioned hereinbefore, the length of the portions 12 and 13 determines the location of the extra field and the distance between parts 12 and 13 and the yoke ring 6 determines the quantity of the field. The ends of the portions 12 and 13 remote from the yoke ring 6 preferably have inwardly extending arms. Such inwardly extending arms are denoted by the reference numerals 14 and 15 in Figures 2 and 4. The value of the angle  which they embrace (see Figure 4) determines the sixpole/dipole ratio of the extra field. Possibly remaining convergence errors can be corrected by controlling the sixpole/dipole ratio. In Figure 4 the embraced angle  is large, which means that the extra field has a positive sixpole component. (Compare the field distribution indicated by the broken lines). In Figure 5 the angle embraced by the inwardly extending arms 14' and 15' is very small, which means that the extra field not only has a dipole component but also a negative sixpole component (compare the field distribution indicated by the broken lines). A configuration generating an extra field with substantially only a dipole field lies in between the configurations shown in Figures 4 and 5. By changing the length of the portions 12 and 13, the distance between the line and field deflection points is changed and hence a deflection unit is obtained which is self-convergent for another screen format This is explained with reference to Figures 6 and 7. A vertical deflection field H B and a horizontal deflection field H L are generated by means of a deflection unit of the type shown in Figure 1. The field distribution measured in the direction of the axis of the display tube is as shown in Figure 6. The Gauss deflection points of the two fields are a distance D apart. A vertical deflection field and a horizontal deflection field having a field distribution as shown in Figure 7 are generated by means of a deflection unit having longer magnetically permeable portions 12 and 13. In this case the distance between the Gauss deflection points is D' with D' - D = ▵D .

Claims: A picture display system including a colour display tube having a neck accommodating an electron gun assembly for generating three electron beams lying in one plane, and an electromagnetic deflection unit surrounding the paths of the electron beams which have left the electron assembly, said deflection unit comprising a field deflection coil of the saddle type having a front and a rear end for deflecting electron beams generated in the display tube in a vertical direction; a line deflection coil of the saddle type likewise having a front and a rear end for deflecting electron beams generated in the display tube in a horizontal direction, and a yoke ring of ferromagnetic material surrounding the two deflection coils and having front and rear end faces extending transversely to the tube axis, the electron beam traversing the coils in the direction from the rear to the front ends when the deflection unit is arranged on a display tube, characterized in that first and second axially extending magnetically permeable members are arranged in the said plane symmetrically with respect to the tube axis, each magnetically permeable member having a first end directly facing the rear end face of the yoke ring and a second end located at the neck of the display tube in the vicinity of the exit end of the electron gun assembly, the length of the first and second magnetically permeable members and their distance to the yoke ring being dimensioned for providing a self-convergent picture display system. A picture display system as claimed in Claim 1, characterized in that regions of the rear end face of the yoke ring located in the said plane are left free by the rear end of the field deflection coil and in that the first ends of the magnetically permeable members are located opposite said regions. A picture display system as claimed in Claim 1 or 2, characterized in that the field deflection coil and the line deflection coil are directly wound on a support. An electromagnetic deflection unit suitable for use in a picture display system as claimed in Claim 1, 2 or 3.

Inventors: SLUYTERMAN, ALBERTUS AEMILIUS SEYNO
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1988
Main classification: H01J29
IPC codes: G09G1, H04N9, H01J29