An improved system for continuously detecting and correcting the display in a cathode ray tube for errors in size and linearity. The system employs a crystal oscillator as a reference frequency to determine absolute error which is represented as "size" on the display screen, and relative error which is represented as linearity. The error is determined from the index pulse feedback, which is an output of a phase lock loop voltage controlled oscillator which is compared to the reference frequency to provide an error voltage level. This output level is then digitized and processed in terms of general control, interpolation, extrapolation, filtering, etc., testing, and stored in digital memory. The digital error data is converted back to analog levels for insertion into a horizontal auxiliary amplifier circuit for linearity correction. A main horizontal amplifier circuit receives size correction. Size correction is taken from the linearity data as a single data point per raster line which forms a lower frequency correction for each raster line. Linearity error includes approximately 256 points per raster line which provides for generation of complex correction wave forms.