Astigmatic focus correction circuit

In a display system having a source of horizontal and vertical deflection signals, apparatus for correction of the image distortion produced by the astigmatism error generated within a cathode ray tube (CRT), comprising:

• a CRT, having an electron gun for generating an electron beam;
• means including a magnetic deflection coil assembly disposed around the outside perimeter of said CRT for deflecting the electron beam in directions radial to the beam axis in response to deflection signals provided thereto by the horizontal and vertical deflection signal source;
• an annular magnetic core disposed around the outside perimeter of said CRT between said electron gun and said magnetic deflection coil assembly;
• a set of four coils evenly disposed upon said annular core with their geometric centers along the circumference of said core displaced by an arc angle of 90.degree. from the centers of adjacent coils in the same set, and wound upon said core in an electrical series such that adjacent coils in the same set induce mutually opposing magnetic flux lines in said core;
• a second set of four coils evenly disposed upon said annular core in the same manner as said first set of coils and interposed with the coils of said first set, with the axes formed by the four coils of said second set being in alignment with the horizontal and vertical axes formed by said deflection coils, and being displaced by an arc angle of 45.degree. from the axes formed by the four coils of said first set;
• means for providing current excitation for said first set of coils in proportion to the difference between the square of the horizontal deflection signal and the square of the vertical deflection signal;
• and
• means for providing current excitation to said second set of coils in proportion to the product of said horizontal and vertical deflection signals.