Raster distortion correction for a video display apparatus, such as one including a 'square planar' picture tube.

摘要

A video display apparatus incorporates a picture tube, such as a square-planar picture tube, that includes a phosphor screen located on a faceplate having an aspherically curved geometry that produces a relatively flat surface contour. Deflection circuitry generates line and field scanning currents in respective line and field deflection windings to enable the electron beams of the picture tube to scan a raster on the phosphor screen. The electron beams are subject to a scanning error that results in raster distortion, such as side pincushion distortion. A parabola generator produces a parabolic modulation of the scanning of the electron beams to generally correct the raster distortion while retaining a residual error due to the aspherically curved geometry of the faceplate. A nonlinear waveshaping circuit modifies the parabolic modulation of the scanning current in accordance with the aspherically curved geometry to provide additional modulation of the electron beam scanning for correcting the residual error. The nonlinear waveshaping circuit includes a current source and a switched current divider that is responsive to a threshold level of the parabolic voltage developed by the parabola generator.