Patent GB-1474289-A (GB)
Title: Display apparatus
IPC: G09G1/00
Filed: 1974-08-07
Abstract: 1474289 Cathode-ray tube displays TELETYPE CORP 7 Aug 1974 [14 Aug 1973] 34720/74 Heading H4T In a cathode-ray tube character display system a memory stores signals representative of characters to be displayed together with a plurality of "enhancement" signals each of which causes a selected character to be displayed in a distinctive manner. In the described embodiment the "enhancement" signals provide; a Protect signal which, in the case of a character which is not to be altered, e.g. a character forming part of a fixed format portion of a display, causes such character to be displayed with a reduced intensity; a Highlight signal which causes a selected character to be "flashed"; a Cursor signal which produces a selected character as black against a white background instead of vice-versa; an Underline signal which underscores a selected character and; a Tabulate signal which produces a tabulator dot aligned with a marginal position. Fig. 2, shows the various "enhancement" signal outputs of the memory 10 which also supplies on lines 13 a seven digit ASCII character code. The latter is supplied to a decoder 23 and thence over nine lines to a signal inverter 62 the output of which, after conversion to serial form in a shift register 66, controls the intensity of the beam of the cathode-ray display tube 26. The decoder 23 is controlled by the outputs 46 of a scan line counter 44 (Fig. 1) to produce the characters in a 7 x 9 dot matrix form within a character space of 9 x 14 dot positions as shown in respect of the character E. In operation, when a displayed character is to be protected, the Protect signal output at terminal 22 acts through an OR gate 71 and an intensity control circuit 72 to produce a reduction in the overall intensity of the beam during the production of such character. When a Highlight signal occurs at output 20 such signal together with a one per second signal from a timer 76 act via AND gate 74 and OR gate 71 to pulse the intensity control circuit 72 so that the character being displayed is rendered visually noticeable. During the occurrence of a Cursor signal at output 78 and in the absence of character data on lines 13 an exclusive OR gate 78 supplies a signal to the invert input 64 of inverter 62 so that a full character space of 9 x 14 dots is illuminated to form the cursor. When, however a character is also to be displayed during the display of the cursor "wash-out" of the character is prevented since the character data input signals to inverter 62 are also inverted so that the character is produced as an unilluminated shape within the remaining illuminated dots which forms the cursor as 34 (Fig. 1). When Underline is required a signal at output 16 operates via AND gate 82 and exclusive OR gate 78 on the occurrence of the 12th line count signal at input 48 (from output 48 of scan line counter 44, Fig. 1) as an input signal to invert input 64 of the inverter 62 so that all of the dot postions in line 12 are illuminated as shown at 30 (Fig. 1). When the Cursor and the Underline outputs are simultaneously present the coincidence of these signals at the input of the exclusive OR gate 78 removes the input to invert input 64 during the writing of the 12th line so that the latter is blanked to produce the underline as a unilluminated line 80 (Fig. 1) in the cursor. When Tabulate is required the coincidence of the signal at output 14, the 12th line circuit signal at input 48 and the output of an inverter 87, which is high in the absence of an Underline signal at output 16 causes AND gate 84 to supply a signal to the second column line 88 so that a tabulate dot 32 (Fig. 1) is produced. If the Tabulate and Underline signals are simultaneously present the absence of an output from inverter 87 removes the output of gate 84 from column line 88. However, the Underline signal is effective via exclusive OR 78 at the invert input 64 and via AND gate 89 and diodes 90, 91 at the first and third column lines 92, 94 so that the first and third dot positions are blanked as shown at 96 (Fig. 1).