Patent GB-1280877-A (GB)
Title: Line timebase circuit
IPC: H04N3/16
Filed: 1969-09-04
Abstract: 1280877 Cathode-ray tube operating circuits; line time bar generators PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 4 Sept 1969 [7 Sept 1968 6 June 1969 8 July 1969] 43840/69 Headings H3T and H4T In a colour television display employing a single or plural beam cathode-ray tube "anisotropic" astigmatism produced by the beam deflecting 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 ease of single beam tubes and produces a "rotation" of the "triangular" landing points of the beams, in the case of three-beam tubes, 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 sawtooth 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. In one arrangement (Fig. 1), the two halves 13, 14 of theline deflection coil are supplied with the deflection sawtooth current via respective transformers 9, 10 from respective generators comprising transistors 5, 6 driven by switching pulses from the secondary windings 20, 21 of the output transformer 22 of the line drive stage and to produce the required correction current the operating potential is modulated at frame frequency by including in the connection between the junction of the two transistor generators (terminal 30) and the (centre) tap on potentiometers 32 connected across the operating voltage source +Vb, the secondary 33 of a transformer 34 the primary 37 of which is supplied from the frame sawtooth generator 35. The winding 33 is by-passed for line frequency by a capacitor 31 and the linearity of scan is controlled by known corrector means 15, 16. In a modification of this arrangement the two transformers 9, 10 are each replaced by a separate tapped winding as a common core and the frame frequency modulation is introduced via a transformer connected between the two transistors (Fig. 2 not shown). Fig. 3 shows a second arrangement in which the frame sawtooth from generator 35 is introduced via an emitter follows circuit 62 (Darling- ton configuration) which in effect reduces the impedance of the voltage source supplied to the transistors 5, 6 from potentiometer 32 (which must have a high resistance since it is connected across the supply voltage at terminal 2) so that the effect of asymmetry, due e.g., to the transistors 5, 6 not being identical, is reduced. To improve the symmetrical operation of the circuit a pair of complementary transistors 64 operates to control the magnitude of the switching voltage supplied to transistor 5 (via resistor 66) and to maintain the potential at terminal 30 (the point of symmetry) at the same value as that at the emitter output of the circuit 62. A resistor 66<SP>1</SP> of similar value to resistor 66 is included in the circuit of transistor 6. For the connection of EAST-WEST pincushion distortion the D.C. operating potential supplied at terminal 2 may be modulated by a parabolic function of frame frequency so that the line deflection sawtooth is also modulated but because of the symmetrical arrangement the correction current produced in the deflection coil halves 13, 14 is not modulated by this function. Centring of the beam is effected by a known circuit 67 and EHT a transformer 70 the primary of which is driven by the transistors 5, 6 supplies the D.C. operating potentials V 1 , V 2  for the complementary pair transistors 64 from a secondary winding 71 and provides the EHT voltage for the tube from a further secondary winding.