GB1515309A - Character display - Google Patents

Abstract

1515309 Alpha-numeric character generation MULLARD Ltd 25 Sept 1975 [16 Sept 1974] 40244/74 Heading H4T In a system in which coded digital pulse groups represent alpha-numeric characters which on receipt are generated and displayed, special groups serve both as inter-word space markers and as control groups to initiate or cancel some form of enhancement of the display of succeeding characters. In a teletext system single lines of pages of text are transmitted during the frame flyback interval of a television signal, each line carrying a line identifying circle and the signals of each page a page identifying code. Each character is coded by 7 bits, but some sets of the 128 possible are reserved to indicate, combined with SPACE, such enhancements as "Display in a window in the TV picture (W)", "Flash on and off at, e.g. once per sec. (F)", and/or "Display in colour". In the TV set a normal picture may be displayed by closing switch 2, or a page may be selected for display by a 22 bit signal from unit 7. The successive lines from the selected page are stored in memory 8 and are led in turn to character generator 10, where a read-only memory constructs 5Î7 dot displays of the required characters, and unit 11 arranges the appropriate set of 5 dots for each in turn of the seven raster lines making one line of type, brightening the marked dots. If a colour CRT is available the characters will be in white until unit 14 recognizes the special interword "SPACE" code for colour, continued until a code word for "cancel colour" is encountered. Similarly code words for "Flash" (F) and "Cancel Flash" allow specified words to be intermittently brightened by a multivibrator 19 feeding an AND gate 20, e.g. at 1 Hz. If switch 2 is closed and a "window" code is received, switches 17, 22 are also closed, allowing blanking pulses to cause unit 3 to produce a dark window over part of the TV picture with the selected words alone displayed in the window. The units 6, 7, 8 are controlled by clock pulses 12 synchronized with the time base locked to the transmitted signals. The special control and SPACE codes take no more transmission time than the standard SPACE code. To avoid false switching two code words in succession are necessary to activate the "window", and a single code, ignoring the second of two in succession, to cancel "window". Other codes may be used to display special symbols, segments of diagrams or even special pages inserted between sections of transmitted text.