Patent GB-1452489-A (GB)
Title: Colour display apparatus
IPC: G09G5/02
Filed: 1973-12-20
Abstract: 1452489 Cathode-ray tube displays INTERNATIONAL COMPUTERS Ltd 20 Nov 1974 [20 Dec 1973] 59177/73 Heading H4T In a colour cathode-ray tube data display apparatus a computer generates a video signal which produces a display of successive horizontal rows of data (Alpha-Numeric characters) separated by spaces the video signals corresponding to such spaces carrying digital information specifying the colours in which the data are to be produced. In the described embodiment each row contains sixty-four characters, each row being formed from seven raster lines and being separated from the adjacent row by a space consisting of at least one blank raster line. Fig. 1 shows the overall arrangement in which the video signal from computer 10 is supplied via a decoder 12 to a three-gun shadow-mask colour display cathode-ray tube 14 the raster scanning of which is controlled by scanning circuits 16 and details of the decoder are shown in Fig. 2. Referring to Fig. 2, a line counter 23 supplies a decoder 35 which produces outputs on lines 36 and 37 when the blank lines between character rows are detected. The occurrence of a line synch pulse from synch separator 20 on line 21 sets a bi-stable 24 the set output of which conditions AND gate 25 to pass clock signals from oscillator 26. The latter signals are at sixteen times the character rate of the system and a four stage counter 27 (¸16) receiving the output of AND gate 25 thus produces character rate signals outputs. The latter signals are supplied to a sampling circuit 38, an AND gate 34 and a further counter 28. The latter is connected to a decoder 29 which supplies outputs on lines 30, 31 and 32 corresponding respectively, to the first character position and to the last character position in a raster line and to the end of such raster line. The first character position signal on line 30 sets a bi-stable 33 the set output of which conditions AND gate 34 to pass the character rate signals from counter 27 as shift pulses for a "three-bit wide" shift register comprising three "one-bit wide" shift registers 41. When each inter-row blank raster line occurs AND gates 40 are conditioned to pass three-bit colour specifying signals from sampling circuit 38, in response to sampling character-rate signals from counter 27, to respective ones of the shift registers 41. The shift pulses from AND gate 34 then write these three bit signals successively, into the registers so that at the end of each blank line the registers contain sixty-four three-bit colour codes specifying the colours in which the following row of characters are to be produced. The occurrence of the last character position signal on line 31 then reset bi-stable 33 and terminates the shift pulses. During this writing-in the blank-line signal on line 37 inhibits AND gates 42 so that the guns of the display tube which are connected to lines 13 are not driven, and when the end of raster line signal occurs on line (conductor) 32 bi-stable 24 is reset and remains in this state until the occurrence of the next line synch. pulse which again sets bi-stable 24. The arrangement now operates generally as before such that as the character data for each raster line of a character row passes to AND gates 42 the shift registers supply outputs at the characters rate (due to the shift pulses from AND gate 34) to the AND gates 42 so that each item of character data is gated via one or more of the latter gates to the Red, Green and Blue guns in accordance with the three-bit colour specifying output of the shift registers 42. The video signals on line 11 may be fed to a monochrome display tube to produce a monochrome display and by arranging that the colour information transmitted is of smaller amplitude than the data information the colour information may be rendered invisible by increasing the contrast and reducing the brightness.