Digital image correction circuit for cathode ray tube displays

Abstract

A digital electron optics correction circuit for correcting the focus and astigmatism in a displayed image on a CRT is disclosed. The circuit includes a clock generator for generating a selected number of pulses between each horizontal blanking synchronization pulse. These clock pulses are then counted by an X-address counter and a first address signal is generated corresponding to a particular horizontal position on the CRT screen for each count of the X-address counter. A pulse generator circuit is also provided which is coupled to receive the vertical blanking synchronization pulses and which provides a selected number of output pulses for each vertical blanking synchronization pulse. A Y-address counter counts the output pulses from the pulse generator circuit and provides a second address signal corresponding to a particular vertical position on the CRT screen for each count. These X- and Y-addresses are then input to focus and astigmatism ROMS which are preprogrammed with an appropriate control correction signal for each X-Y-address. In a first embodiment of the invention, the pulse generator circuit inclues a dividing circuit which is coupled to receive the horizontal blanking synchronization pulses. The dividing circuit then provides an output pulse for every N2 pulses received, and each such output pulse is then utilized to increment the Y-address counter. In a second embodiment, the pulse generator circuit includes a second clock generator for generating N3 pulses between successive vertical blanking synchronization pulses, with each clock pulse being used to increment the Y-address counter. Also disclosed in a specialized clock generator cirucit for generating a preselected number of pulses between successive gating pulses.