Blue droop correction circuit with a single supplemental correction coil

Claim 1

In a color television system comprising (1) a cathode-ray tube having an image screen bearing a multiplicity of phosphor dot triads individually including a dot of green, a dot of blue and a dot of red phosphor and further having a neck section enclosing three electron guns for producing a delta array of three electron beams for exciting an assigned color phosphor of said dot triads, (2) a magnetic deflection yoke for producing across the paths of said beams a scanning field for deflecting said beams over said screen in a repeating series of parallel lines, (3) a dynamic convergence system, including a plurality of electromagnetic field developing means individually associated with assigned ones of said beams, respectively, and energized by signals of horizontal and vertical frequencies, for developing dynamic convergence fields to maintain said three beams converged to define an equilateral beam spot triad in registration with said dot triads as said beams scan said screen but which, in the presence of astigmatism in said deflection field, distorts said spot triad by displacing two of said beams on the same sense relative to the center of said equilateral triad and displacing the third beam in an opposite sense relative to said center of said equilateral triad, the improvement which comprises a single supplemental electromagnetic field developing structure positioned externally of said neck section and adjacent that one of said guns producing said third beam and energized by the horizontal or vertical frequency component of the dynamic signal applied only to that one of said electromagnetic means which is associated with said third beam to develop a supplementing dynamic field component at said horizontal or vertical frequency, which is opposite in polarity to, and has a timeintensity characteristic related to that of, the corresponding dynamic field component developed by said one electromagnetic means, for prebending said third beam before that beam enters the convergence field of said one electromagnetic means to compensate for the displacement of said third beam attributable to said astigmatism.