Patent GB-1370535-A (GB)
Title: Transmission of alphanumeric data by television
IPC: H04N7/085
Filed: 1973-05-09
Abstract: 1370535 Television; multiplex pulse signalling BRITISH BROADCASTING COBP 9 May 1973 [9 Feb 1972 18 July 1972] 6136/72 and 33651/72 Headings H4F and H4L A message comprising alphanumeric characters is added in coded digital form to a television video signal for transmission in such a manner as not to interfere with the picture content of the video signal, and is extracted at the receiver for temporary storage, the stored digital signal then being decoded for use in the receiver display. Although the message may be modulated on a subcarrier in the audio channel, the particularly described embodiment uses time division multiplexing of character bits from several messages in lines occurring during the vertical blanking interval. One exemplified use of the system is the provision of subtitles for television display for use by deaf persons. The characters are arranged in 32 "pages", 768 characters per page, and 8 bits are used in coding the characters. In the standard 625 line format lines 13 and 14 in field 1 and lines 326, 327 in field 2 are used for carrying the coded messages in one of three organizations. If CnPr represents the nth character on the rth page then each line carries 32 characters and the three organizations are:  Organization 1. Field 1; C 1 P 1 , C 1 P 2 , C 1 P 3  ... C 1 P 32 . Field 2; C 2 P 1 , C 2 P 2 , C 2 P 3  ... C 2 P 32 .... Field 768; C 768 P 1  ... C 768 P 32 . Organization 2. Field 1; C 1 P 1 , C 2 P 1 , C 3 P 1  ... C 32 P 1 . Field 2; C 33 P 1  ... C 64 P 1  ..... Field 24; C 737 P 1 ... C 768 P 1 . Field 25; C 1 P 2  .... Organization 3. Field 1; C 1 P 1  ... C 32 P 1 . Field 2; C 1 P 2  ... C 32 P 2  .... Field 32; C 1 P 32  ... C 32 P 32 . Field 33; C 33 P 1  ... C 64 P 1  .... Field 768; C 737 P 32  ... C 768 P 32 . To separate out the message date from the received signals, timing signals indicating the 13, 14, 326 and 327th line in a field are applied to a data separator 53, Fig. 2, receiving the combined video and message signals. 32 (pages) x 768 (characters) x 8 (bits) = 196,800 bits are thus extracted and stored in a series to parallel store 54. The parallel bit signals are fed via a page selector 55 to a decoder, "read only memory", character generator 56, the character being fed out as black or whiter than white modulations on seven successive lines of the display, each character being formed in a 7 by 5 matrix frame. The page selector may precede the store, so that the latter need only have capacity for one page-Fig. 3 (not shown). Random access stores can be used. In the embodiment of Figs. 4 and 5, a one line message of 32 characters, arranged according to the first organization, e.g. the characters occupy the fast position in 32 sucessive line 13's of 13 fields, are transmitted and received for use by the deaf. The message is fed on punched tape into a reader 10, each character code group (in ISO-7 code) being framed by a start group STX and in end group ETX. When the message is to be sent, a read signal on line 17 sets flip-flop 16 and another AND gate 15. A sync. pulse counter 11 determines the 13th line and enables gates 13 and 20. The signal from gate 13 is fed via gate 15 to cause the read out of the message in parallel form, into parallel/series register 19. It is then read-out at the correct time in series format by shift pulses provided by bit clock 22, counter 23 and decoder 24, and travels via gate 20 for combination with the video signal. At the receiver the combined signal is fed to a series/parallel register 31 only enabled during the 13th line of the field by means of sync. pulse counter 35, decoder 36, correct position within the line determining circuit 32, 33, 34 and gate 37. The six character bits are fed from the register in parallel to six recycling registers 38 via gates 46. Gates 46 are enabled one character interval later each field so that the characters when decoded at 48 and parallel/series circuit 49 are corrected spaced along the video signals making up the character display. This staggered enabling is produced by a comparator 41 receiving a field count at one input and a character count at the other.