Patent GB-1251891-A (GB)
IPC: G09G1/16
Filed: 1969-01-14
Abstract: 1,251,891. Cathode-ray tube displays. HARRIS-INTERTYPE CORP. 14 Jan., 1969 [4 March, 1968], No. 2253/69. Heading H4T. In a cathode-ray tube display system (employed in photo typesetting apparatus) in which characters produced on the tube screen, as selected by instructions from a control record, are located in the required horizontal position by means of a horizontal beam deflection signal, compensation for the non-linear relationship between the magnitude of the deflection signal and the magnitude of the resulting deflection of the beam spot across the screen is effected by forming the horizontal deflection signal in two parts the first of which comprises a signal having step values each linearly related to the magnitude of the required beam deflection and a second portion comprising a signal having step values each of magnitude non-linearly related to the required beam deflection and determined by the actual horizontal position of the beam specified by the first part of the deflection signal. The beam focus of the tube is also controlled as a function of beam position. Fig. 1 shows the general arrangement in which a control record 40 selects characters stored, in digital form, in a memory 10 and causes a character generator 20 to produce the actual signals which form the selected character on the display cathode-ray tube 30. The characters may be produced by "bright-up", of selected points in a line scanned raster or, of selected points between which the beam is deflected by appropriate X, Y co-ordinate deflection signals. The control record 40 also supplies a control circuit 35 (see also, Fig. 2) with signals indicative of character size, interword spacing together with leading instructions and vertical rule commands so that a justified line of characters is produced. In operation digital signals from the character generator 20 are supplied to D/A converters 62, 63 which provide the horizontal and vertical beam conditions required for character production the magnitudes of the analogue outputs being determined by analogue inputs from D/A converters 65, 66 which convert the horizontal and vertical scale values from registers 56 and 57. The actual horizontal position at which the production of a character commences is determined by the width of the previous character and the space between it and the character to be produced. A digital value corresponding to this magnitude is inserted in accumulator 60 (at the completion of each previous character) and the state of the accumulator is then employed to control the production of the two parts of the horizontal deflection signal. These signals are produced by utilizing outputs from the registers representing the six most significant digits to produce, via D/A converter 70, horizontal deflection voltages linearly related to the state of these registers and via D/A converter 75, horizontal deflection voltages selected from one of 48 different possible magnitudes dependent on the state of the six registers (i.e. dependent on the current horizontal position) and such that when added to the first deflection signal part result in equal changes in the horizontal position of the beam for equal changes in the state of the registers. This provides compensation for non-linearity of deflection over 48 horizontal segments (of a line of characters) and by means of an interpolation D/A converter 76 similar to D/A converter 70 each of the 48 segments is divided into 255 segments using as a reference an output from D/A converter which is employed for horizontal character size control. The three outputs viz from D/A converters 70, 75 and 76 are then combined in the horizontal deflection amplifier 25 and supplied to the horizontal deflection coil. The D/A converter 75 also supplies. outputs controlling the vertical character size and the leading instruction and vertical rule commands and in addition supplies output signals for controlling the focus of the tube beam as a function of horizontal position.