GB1513179A - Data display apparatus - Google Patents

Abstract

1513179 Cathode-ray tube displays BRITISH BROADCASTING CORP 16 Nov 1976 [17 Nov 1975] 47338/75 Heading H4T In a display system such as CEEFAX (RTM) decoding for display on a TV screen digital character codes indicating standard characters (Fig. 1, not shown), one or more character codes can be redefined as modified characters by including in the data a control code followed by the character code (S) and for each code a set of bytes defining successive lines in the 10 x 6 dot matrix of the modified character; until cancelled the redefined character codes are then translated by the modified characters (e.g. ¢ in Fig. 2, not shown, is replaced by 'A' as in Fig. 3, not shown). A "page" RAM 16 stores 40 character codes forming a row for each of 24 rows per page, a character ROM 38 holds the bytes defining the ten slices of the matrix of 96 standard characters, and a character RAM 44, loaded from the data highway 11 stores the bytes of the alternative display for up to 96 characters. After the occurrence of a character redefinition code, e.g. three special codes usually in a page address, detected by 48 which then inhibits recording for display, ensuring character codes each with ten associated bytes define the address at which these bytes are to be stored in character RAM 44, serving to redefine those same codes when they occur in the display data until cleared, e.g. at the page end or otherwise. After the page address, received as a dummy row before the display rows, has been recognized 12 as one selected, other dummy rows containing character redefinitions are received, followed not necessarily in order by the 24 rows for display. After the complete page has been stored in the page RAM 16, rows in turn are extracted, and as successive scan lines are traversed by scanner 46 the bytes for successive slices of the matrix for each character in the row are read from character ROM 38 or RAM 44, for display as dots on the screen. Each of the three memories holds 1024 bytes of 8 bits; since 5 bits of row address are required to define 24 rows and 6 bits to define 40 characters in a row, the 960 character codes to be stored in the page RAM 16 can be accommodated if the combined 11 bits are reduced to 10 bits by a transcoder 24; similar requirements with transcoders 26, 62 apply to the other two memories. In storing the character codes in the character RAM 44, the eighth (parity) bit is used to indicate whether redefinition has occurred, and thus whether the character RAM or ROM is to be read. In an alternative system (Fig. 7, not shown) the bytes redefining some or all the characters are stored temporarily in the page RAM 16 and subsequently transferred to the character RAM.