Patent GB-1230162-A (GB)
IPC: H01J29/70
Filed: 1968-03-19
Abstract: 1,230,162. Cathode-ray tube operating circuits. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 19 March, 1968 [22 March, 1967], No. 13260/68. Heading H4T. In a colour television display employing a single or plural beam cathode-ray tube "anisotropic" astigmatism produced by the beam deflection coils which causes an electron beam having an elliptical or a circular cross-section to be given the form of a tilted (i.e. rotated) ellipse in the case of single beam tubes and produces a "rotation" of the "triangular" landing points of the beams, in the case of three-beam tubes (see Figs. 2a and 2b respectively), such effects arising at the corners of the (rectangular) display, is corrected by forming the horizontal (line) deflecting coils in two halves symmetrically positioned about the neck of the tube and supplying to the coils in addition to the normal horizontal deflection saw-tooth wave, a correction current which varies at frame frequency, such correction current being arranged to flow in opposite directions in the two halves of the coil. The beam deflection is therefore not affected by the correction current and is produced by the field resulting from the unequal currents in the two halves of the coil. The difference between the currents in the two coil halves however results in the production of a "quasi-quadripole" field the strength of which varies at frame frequency and this field produces the compensating "rotation" of the beam(s) which is required to correct the astigmatism. The manner in which the astigmatism is produced by the action of a "quasiquadripolar" field is described with reference to Figs. 3 and 4a, 4b (not shown) and the present invention corrects this by the provision of our opposite "quasi-quadripolar" field superimposed on the field for the horizontal deflection. Fig. 1 shows the general arrangement in which the horizontal scan generator 9 feeds the two halves 7<SP>1</SP>, 7<SP>11</SP> of the horizontal deflection coils through a correction means 11 the operation of which is controlled as a function of the vertical scan generator 10 via a means 12. Additionally, a single depramic, electrostatic radial convergence unit 6 together with the (anastigmatic) deflection coil system 7, 8, ensures colour coincidence along the deflection axes 21, 22 (Figs. 2a, 2b). In one embodiment (Fig. 5) the correction means 11 comprises a modulator 51 supplied with vertical scan information from the means 12 and horizontal deflection information from means 52, the modulator operating to produce the correction curent ic which, by means of transformer 54 causes correction currents ic<SP>1</SP> to flow in opposite directions through the coils 7<SP>1</SP>, 7<SP>11</SP> The modulator 51 may employ valves or transistors or may utilize the Hall effect (Fig. 6, not shown). In a second embodiment (Fig. 7), the correction means 11 comprises two identical transductors 71<SP>1</SP>, 71<SP>11</SP> the cases of which are made from a material having a non-linear BH characteristic which is premagnetized by windings 73<SP>1</SP>, 73<SP>11</SP> supplied from D.C. sources 74<SP>1</SP>, 74<SP>11</SP>, or by means of permanent magnets. This arrangement, in addition producing the required correction current also provides a reduction in EAST-WEST pin-cushion distortion. A further embodiment (Fig. 8, not shown), utilizing a single transductor also effects pincushion distortion correction in addition to producing the required correction current. The effect of "anisotropic astigmatism" may also be eliminated by utilizing the vertical deflection coil system 8<SP>1</SP>, 8<SP>11</SP> to produce the "quasiquadripole" field and an arrangement operating in this manner and utilizing field-sensitive resistors is described with reference to Fig. 9 (not shown).