Patent GB-1446956-A (GB)
Title: Colour television display apparatus incorporating a television display tube
IPC: H04N3/26
Filed: 1973-12-12
Abstract: 1446956 Cathode-ray tube circuits PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 12 Dec 1973 [15 Dec 1972] 57482/73 Heading H4T Convergence errors in a three-beam colour television tube of the kind in which the beams are generated in the same plane, the error being due to tolerance errors in the tube construction (the orientation of the plane of the beams) and/or in the X, Y deflection coils, are corrected by splitting at least one of the latter coils in two equal portions arranged to produce a magnetic quadripolar field the polar axes of which coincide with the X, Y directions and the field strength of which is proportional to the integral of the deflection current flowing in either deflection coil. In the arrangement disclosed it is assumed that deflection errors due to coma and anisotropic astigmatism are substantially zero and the invention is distinguished from known systems for correcting such errors in which the quadripolar field is a function of the product of the X and Y deflection currents. The invention is also distinguished from known systems for the correction of isotropic astigmatism in which the quadripolar field varies with the square of either deflection current but in which the axes of such field lie along the diagonal(s) between the axes of deflection and further is generated by separate windings and not by the deflection coil or coils. Fig. 1, shows a construction tolerance error in which the plane D of the three beams L (left), C (Centre) and R (right) makes an angle α with the X direction of deflection and Fig. 2, shows the (vertical) convergence errors on the tube screen 2 which occur as a result of this tolerance error. Correction is effected by producing a quadripolar magnetic field the polar axes of which as shown in Fig. 1, coincide with the X and Y deflection axes and produce forces FL, ER in the directions shown to cause the beams L and R to effectively lie in the plane containing the X deflection axis. Fig. 3, shows one arrangement for producing the required field in which the X deflection generator supplies a line frequency sawtooth current to the line deflection coil halves 4<SP>1</SP>, 4<SP>11</SP> connected in parallel for the line current iH which also flows through differentially adjustable low inductance coils 5<SP>1</SP>, 511 to compensate any asymmetry of the deflection field. In operation, a sawtooth voltage, the polarity and amplitude of which depend on the position of the slider is developed across potentiometer 6 the resistance of which, e.g. 4À7 ohms, is low compared with the impedance of the line deflection coils 4<SP>1</SP>, 411 at line frequency. The coils, therefore, pass an additional current which is proportional to the integral of the voltage across potentiometer 6 and therefore constitutes the parabolic correction difference current i KH  which is required for convergence correction. This current flows in the same direction as the deflection current i H , in coil 411 and in the opposite direction in the other coil 4<SP>1</SP> and produces the quadripolar field depicted in Fig. 1. The arrangement of Fig. 3 in which the coils 4<SP>1</SP>, 4<SP>11</SP> are connected in parallel for the deflection current may be modified to allow series connection of these coils (Fig. 7, not shown). Fig. 4 shows an embodiment in which the potentiometer 6 is coupled to the remainder of the circuit via an auto-transformer to allow an increase in the amplitude of the current i KH  without increasing the resistance of the potentiometer which step would result in increased dissipation, give rise to a linearity error of the deflection current and introduce higher-order components into the current i KH . The correction may also be effected by means of the field frequency deflection coils which because such coils are predominantly resistive for the field frequency the circuits of Figs. 3 and 4 cannot be used. Figs. 5 and 5a show two suitable arrangements connected to the field deflection current generator 10. If the deflection coils are wound toroidally (i.e. not of the saddle type) the parabolic difference currents must be generated by separate generators and introduced into the deflection coil halves via tappings.