Beam repositioning circuitry for a cathode ray tube calligraphic display

Abstract

A beam repositioning system for a calligraphic display system is described in which beam repositioning time is minimized. When the beam repositioning signal is applied to the deflection circuits of the CRT, the deflection signal, either yoke current or deflection voltage, is sensed and a 'beam slewing' logic signal is generated as long as the deflection signal is changing indicating that the beam is being repositioned. This logic signal activates a control circuit which generates a 'hold' signal for the symbol generating or 'stroke' circuits which persists as long as the logic signal remains at the logic 'one' level. When the beam is repositioned as indicated by the fact the deflection signal no longer changes, a logic 'zero' signal is generated which actuates a first timing circuit which generates a first control signal if the logic signal remains at the zero level for a first predetermined period. This initial period is to allow beam settling and to avoid preturbations due to beam overshoot or undershoot. The first control signal blocks the beam slewing logic signal so that the changes in deflection signals due to generation of the actual symbols have no effect on the beam repositioning circuits. After a further period of time, a second control signal is generated which enables the symbol writing or stroking circuits of the cathode ray tube device to permit the symbols to be generated and displayed. In this fashion, the blanking of the signal beam and the repositioning of the beam is closely and accurately controlled to reposition the beam in the minimum time possible.