broadcast videotex
Teletext is a TV-based information service, without the possibility of interactive communication. With teletext, text and graphic messages are transmitted along with the normal TV program. With analog TV technology, videotext is transmitted in several lines of the vertical blanking interval. With digital TV according to DVB- TXT, videotext is transmitted via a separate transport stream( TS) using MPEG-TS.
Teletext dates back to the 1970s and had extremely limited display capabilities of text and graphics in the early years. With the analog TV signal, the videotext signals are inserted in the blanking intervals of the two fields. In 9 lines each.
The resolution was sufficiently poor. Only 25 lines with 40 characters could be displayed. The letters, digits, special characters and graphics are displayed with alpha mosaic. The number of displayable characters is 96 letters, digits and special characters and 128 graphic characters.
The teletext pages are called up in a loop. There is a basic scheme for the content overviewwith page numbering, sorted by areas of interest such as politics, sports, news, economy, science, TV program, etc., where each teletext page contains a page number, which is called up via the remote control. Since the textual and graphical representations were extremely crude, several variants were developed with improved extended character sets, improved typography and color graphics with higher resolution.