Electronic character generating

Claim 1

An electronic character generating apparatus of the kind in which a display cathode ray tube which is subjected to deflection in accordance with a television type raster is caused to display an approximation to a selectable character by applying to said tube signals occurring during selected dot positions each occurring during a predetermined fraction of a television line period and a predetermined fraction of a television frame period, said positions together constituting a rectangular matrix of such positions, and comprising means for so varying the lengths of the television line fractions in each dot position during which said signals are applied that the shape of the display produced in each such dot position is not rectangular but substantially smaller in dimension, in the line direction at the beginning and end of the aforesaid fraction of the television frame period than in the middle of said fraction the signals applied to the display tube being obtained by a combination of a character signal generator adapted to produce signals, which, if directly applied to the tube, would produce rectangular dot position displays and, in cascade therewith a signal converting circuit arrangement time controlled by the television line and frame freqUencies and adapted to convert the constant length line fraction signals from said known generator into the required varying length line fraction signals, the display being by means of an interlaced raster and said signal converting circuit arrangement comprising a first pair of positive NAND gates each having two inputs of which one is fed from said signal generator with substantially rectangular pulses at the frame frequency and the other is fed from said signal generator with substantially rectangular pulses at half the line frequency; and OR gate having three inputs of which one is fed with output from one of the first pair of gates, another is fed with output from the other of said pair of gates and the third is fed rectangular waves of dot frequency derived from a master oscillator and delayed by one quarter of the dot period; and a further gate having two inputs of which one is fed with output from said third gate and the other is fed with character signals (which if directly applied to the display tube, would produce rectangular dot position displays) from said character signal generator, output from said further gate, which is adapted, when positive output signals from said OR gate are fed thereto, to pass character signals, being applied via a polarity inverter as brighten-up signals to the display tube.