CRT astigmatism correction apparatus with stored correction values

Claim 1

A raster scan cathode-ray tube display apparatus, comprising a cathode-ray tube having a display screen and an electron gun for forming an electron beam directed towards said screen, an electromagnetic deflection yoke associated with the cathode-ray tube, deflection circuitry for driving the deflection yoke to generate a magnetic field for deflecting said beam across said screen in first and second mutually perpendicular directions at first and second frequencies respectively so as to define a raster pattern on said screen, the deflection yoke producing cross-sectional distortion of the electron beam dependent on the deflection of the beam, and said deflection circuitry also generating a first synchronization signal at said first frequency and a second synchronization signal at said second frequency, a plurality of electrodes disposed along the path of said beam and through which the beam passes before reaching the deflection yoke, memory means storing values proportional to voltages appropriate for application to said electrodes to compensate for the cross-sectional distortion produced by the deflection yoke, said memory means having m×n separately addressable memory locations in which respective values are stored, and memory accessing means responsive to said first synchronization signal to generate less significant address bits at a frequency that is m times and first frequency and responsive to said second synchronization signal to generate more significant address bits at a frequency that is n times said second frequency, said memory accessing means also being operative to apply the more significant and less significant adress bits to the memory means for accessing the stored values and to apply said voltages to said electrodes to produce an electrostatic field that dynamically compensates for the distortion produced by the deflection yoke.