To provide measurement of horizontal and vertical spatial errors, as well as shading errors, the active video picture is divided horizontally into 13 equal blocks, and vertically into 14 bands of checkers. During the setup mode, the signal from a selected camera is fed to an A/D converter in the video...http://www.google.ca/patents/US4326219?utm_source=gb-gplus-sharePatent US4326219 - Digital error measuring circuit for shading and registration errors in television cameras
