television receiver with additional circuits, is levels. For international transmission via satellite or for satellite capable of reconstructing written information and displaying it on news gathering bit rate reduction can be to the Mbit/s level. A rate the screen. The system allows the transmission of very many bul- of 30Mbit/s to 45Mbit/s is practical for international TV links since letins of information, and the viewer can choose any one by select- two TV programmes, of better quality than with FM transmission, ing a three figure number on a set of controls, usually push buttons. Television transmission 57/15 100%- White level- Data fills active line Data fills active line 66%+6% Signal levels 40 display characters (8,8,8, (8,8,...) Page header Remaining lines Colour burst Figure 57.16 Teletext data organisation. (The figures in brackets are the number of binary digits) After a short interval the information appears and remains for as from these lines is then stored, usually in a semiconductor memory, long as it is needed. SO that the page can be displayed at the same rate as a normal A major use of teletext is as an information service, but it can also picture. The binary number codes are then translated to their corre- supplement normal television programmes with subtitles or linked sponding characters or graphics patterns. Finally a video raster scan pages. The entire signal is accommodated within the existing 625 representation of the page is switched onto the screen. line television allocation, and so costs nothing in terms of radio frequency spectrum space. It was, effectively, the first broadcasting 57.2.8 Digital audio standards system to transmit information in digital form. Teletext pages look rather like pages of typescript, except that they A number of different sampling rates have emerged for digital audio can also include large sized letters and sinple drawings. The stand- systems: ard sized words can use upper or lower case, and be in any one of six colours, red, green, blue, yellow, cyan and 