It is fundamental to all commercially-available colour television tubes that the picture displayed on the screen is made up of three primary colours, red, green, and blue, generated when electrons originating from a three-gun assembly in the neck of the tube strike the phosphor-coated viewing screen.
In practice, it is impossible to achieve the required accuracy without some form of additional correction to obtain the necessary convergence.
Traditionally, correction is achieved by means of an electrical correction system which is designed to influence the three beams after they leave the guns by means of additional components mounted on the neck of the tube. Special electrical circuits are needed to achieve this, together with a complex adjustment process of typically 14 controls which must be adjusted by the manufacturer of the receiver; it is also important that the accuracy of adjustment is maintained during the life of the receiver. It has long been recognised that the process of achieving correct convergence without recourse to these cumbersome methods is a goal that must be achieved, and a number of attempts to realise an alternative tube system have been made from time to time in various parts of the world.
The realisation of such a system requires a number of important changes to the previously-accepted picture tube and its associated deflection coil, each of which is associated with more closely controlled manufacturing processes than has been possible in the past. This change is matched by a conversion of the colour screen from a pattern of small phosphor dots to one where each coloured phosphor is applied as a strip running from top to bottom of the tube face, each strip being precisely located with respect to its neighbouring colour. Of equal importance are changes to a major component associated with the colour tubes, the deflection coil. As with some of the earlier 110′ tube designs the magnetic shield is fitted inside the glass cone. The electron guns are mounted side by side, the two outer guns (red and blue) being slightly inclined towards the centre gun (green).
One pair of rings magnetised as a vertical two-pole magnet has only the strength and polarity adjustable, not the direction, and thus provides the purity adjustment which requires a horizon­tal displacement only. The remaining pair of rings (N-S Symmetry), magnetised as a horizontal two-pole magnet, again adjustable in strength and polarity only, corrects any vertical misalignment between the beams and the axis of the tube-yoke system.
The system includes magnetic field coils optimized for uniform field generation accurately printed on flexible circuit board.
The image seen by the viewer is the result of light emitted when the electrons strike coloured phosphors deposited on the back of the screen.
To achieve this, it is obvious that all three beams must scan together with great accuracy to avoid imperfections in the final registration of the beams with their respective phosphors.

For example, the delta gun format is now replaced by a three-in-line array to remove one source of misalignment of the beams. The curvature of field is compensated for by increasing the astigmatism of the deflection field in such a way as to eliminate misconver­gence in the horizontal direction at the expense of increasing it in the vertical direction.
These adjustment compensations are few (7), and relatively straight­forward and expend very little energy compared with a com­parable delta-gun system. The N-S raster distortion is inherently barrel shaped but is decreased by shaping the deflection coil ferrite ring.
Advantage has been taken of the unlimited vertical landing reserve inherent in a vertical stripe picture tube system by rotating the degaussing coils through 90: compared to previous designs.
The green beam is positioned between the other two in order to minimise the visual effect of small residual convergence errors (the human eye is more sensitive to con­vergence errors between red and green, or blue and green, than between red and blue). The object of static correction (convergence) is to cause the landing positions of the two outside electron beams (red and blue) to coincide with the central (green) beam in the centre of the picture tube screen. Convergence errors in the outer areas of the display are corrected by means of four additional electro­magnetic four-pole fields.
Traditionally, the phosphor pattern consists of around a million accurately-located red, blue, and green dots each of which can be illuminated as required by the modulated beam from the appropriate gun.
However, the latter is of no consequence, since the horizontal-in-line gun array eliminates mis-convergence in a vertical direction, resulting in good convergence at all positions on the screen. Shaped cut-outs at the gun-end of the ring affect only the vertical deflection field, giving a N-S raster shape which is acceptable without further correction. By this means, the vertical component of residual fields that may cause horizontal landing errors is completely eliminated.
By turning each pair of rings in opposite directions the strength of the field is influenced, while turning the two rings together, results in a change of the direction of the field.
The vertically slotted shadowmask makes colour purity independent of beam landing in a vertical direction. Two of these are produced by means of four small toroidal windings on the deflection coils ferroxcube ring and located on the horizontal and vertical axes of deflection.

The curvature of the image field causes misconvergence of the three beams which now form an equi­lateral triangle.
Because the mask material is not interrupted by holes in the direction of the de­gaussing field, the required magnetomotive force can be smaller thus allowing an appreciable reduction in the ampere-turns of the degaussing coils. The static correction unit is of the permanent magnet type and is combined with the purity adjuster and a N-S raster symmetry adjuster. One pair of magnets forms a four-pole field, which moves the outside beams (red and blue) equally in opposite directions.
The series-connected four-pole windings are driven by field and line frequency sawtooth or parabola currents derived from the deflection currents.
The additional effect of astigmatism causes these image spots to change to isosceles triangles.
Both this and the E-W pincushion distortion is corrected by means of the E-W diode modulator circuitry. The com­bination consists of four pairs of magnetised plasto-ferrite rings, each pair coupled by pinion gears, and is fitted on the tube neck behind the deflection coils.
The other pair forms a six-pole field, which moves the outer beams equally in the same direction. The other two four-pole fields are produced by means of balancing sawtooth or parabola currents through each half of the field and line deflection coils. The principal effect of coma in a delta-gun tube is to displace the blue beam relative to the red and green beams towards the direction of deflection. When adjusting the four-pole or six-pole rings, the centre beam is unaffected, and it is thus possible to bring all three beam spots into coincidence. In a system with horizontal-in-line guns, coma caused by the horizontal and vertical deflection displaces the centrally positioned green beam relative to the red and blue beams against the direction of deflection.

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