Patent GB-1291556-A (GB)
Title: Apparatus for code conversion
IPC: G09G5/00
Filed: 1970-01-20
Abstract: 1291556 Code converters BUNKER-RAMO CORP 20 Jan 1970 [13 Feb 1969] 2686/70 Heading G4H A code converting system comprises a first recirculating register 15 for storing data to be converted from a first code such as ASC11 code to a second code such as a video code, the data consisting of groups of bits each group being separated from the next group by a number of bit positions sufficient to store a group of bits of the second code, a second recirculating register 16 for storing a dictionary of all possible code groups of the first code, each code group being followed by a code group of the second code, a comparator 17 for comparing code groups in the first register with code groups in the second register and a gate 18 for transferring to the first register a code group from the second register each time a correlation is detected by the comparator. A magnetic drum which comprises a file storage unit 12 and a display storage unit 11 stores pages of ASC11 coded data in a series of files. Each file comprises the interleaved ASC11 coded characters of six pages, each page consisting of fifteen lines of sixty-four characters plus eight character positions which enable the beam of a cathode-ray tube display unit 10 to retrace after each line. The drum also stores a dictionary of the ASC11 and video codes of all the characters. When an operator, by means of a keyboard 13, requests data to be transferred from a file and displayed on the cathode-ray tube, the ASC11 and video codes for each character are read from the drum into the register 16. The data requested for display is also read from the drum and fed to the register 15 and since the data is stored on the drum in an interleaved manner, each line of requested data is accompanied by the corresponding line data of five other pages. The ASC11 coded characters in the dictionary register are then successively compared by the comparator with the ASC11 coded characters of the selected line in the data register and each correlation causes the comparator to open the gate 18 so that the corresponding video code in the dictionary register is transferred to the data register. Thus the video coded character occupies the bit positions of the ASC11 characters of the five non-selected pages and is therefore positioned adjacent the corresponding ASC11 coded character of the selected page. The ASC11 code comprises seven bits plus a parity bit for each character whilst the video code comprises a matrix of 8 x 5 bit positions of which five bits enable the return of the cathode-ray tube beam after its sweep of each row of seven bit positions, a "1" bit in any position causing a dot to be displayed. The register 15 stores XP characters where X equals the number of character positions in a line (i.e. 72) and P equals the number of pages (i.e. 6) whereas the register 16 stores X-1 characters. A line of ASC11 coded characters circulating in the register 15 therefore requires a maximum of 71 recirculations before all the correlations have been detected. A counter 22 counts the number of correlations and when the count equals 64, a decoder 23 stops the code conversion and causes erasure of the ASC11 coded characters in the register 15 before the video coded characters are transferred from the register 15 to the storage unit 11. A counter 20 counts the number of circulations of the register 15 and if all 64 correlations have not been detected when the counter reaches a count of 71 the erasure is not effected and an error signal is fed to the unit 12. The keyboard 13 enables editing of the display and the insertion of characters or a cursor. Conversion of the video coded data back to ASC11 coded data is also described.