Patent GB-1510536-A (GB)
Title: Display system
IPC: H04N9/285
Filed: 1975-05-02
Abstract: 1510536 Cathode-ray tube circuits RCA CORPORATION 2 May 1975 [20 May 1974] 18555/75 Heading H4T [Also in Division H1] In a colour display system including a colour picture tube having three co-planar beams, the beams are converged at the screen centre by static convergence means, the winding distribution of the horizontal deflection coils is so arranged as to substantially converge vertical lines at the central screen region, the winding distribution of the vertical deflection coils is so arranged as to substantially converge horizontal lines at all screen positions and the residual mis-convergence of the vertical lines (at the top and bottom of the screen) is substantially corrected by means of a quadripolar field varied at the vertical scanning rate. The quadripolar field may be produced by four suitably positioned coils adjacent the scanning yoke or it may be produced by unbalancing the scanning current in the vertical deflection coils. Fig. 1 shows one embodiment in which the deflection yoke comprises a ferrite core 26 carrying windings 27 comprising the vertical and horizontal deflection coils, a static convergence assembly 28 and a colour purity adjusting device 29 mounted about the neck of the tube "behind" the yoke. The latter may also include the quadripolar field producing windings in which case the field is produced by supplying to the latter windings a substantially parabolic current waveform 35 produced by a diode bridge converter 32 from the vertical current sawtooth waveform 31. In the absence of the quadripolar field a "cross-hatch" pattern type of input signal would be displayed as shown in Fig. 6 and to correct for the residual misconvergence in the vertical direction the quadripolar field comprising four poles 43a, 43b, 43c and 43d (Fig. 4) modulated by the parabolic vertical frequency function is employed. The fully corrected display is as shown in Fig. 7. The quadripolar field may be produced by arrangements such as shown in Figs. 8a, 8b or 8c comprising, respectively, four windings on a toroidal core, two windings on a pair of U-shaped cores or four radially mounted cores, a waveform generator 32, such as that shown in Fig. I producing the required drive. Current waveforms other than purely parabolic waveforms may be employed, e.g. a sawtooth waveform may be combined with the parabolic waveform. Instead of employing the latter arrangements the quadripolar field may be produced by unbalancing the scanning current in the vertical deflection coils 61a, 61b (Fig. 9) by introducing diodes 63, 64 and adjustable resistor 65 into the circuit, the flux field shown being for the top righthand portion of the raster, i.e. with diode 63 conducting and diode 64 reverse biased. When the quadripolar field is to be produced by the addition of windings to the deflection yoke (which has the winding distribution referred to above) such additional windings are distributed as shown in Fig. 10 where # indicates the quadripolar field producing windings. O, indicates the horizontal deflection windings and X indicates the vertical deflection windings for one quadrant W of a complete deflection yoke. Thus four such sections are toroidally wound about the ferrite core 26 (Fig. 11) with the reference lines X and Y (Fig. 10) coinciding with the X and Y axes of each quadrant.