Teletext
Teletext is an information retrieval service provided by television broadcast companies. Teletext pages can be viewed on television sets with suitable decoders. They offer a range of text-based information, usually including national, international and sporting news, weather and TV schedules. Subtitle (or closed caption) information is also transmitted in the teletext signal.
Teletext is widely used across Europe, with every major television station having its own teletext service. In some commercial stations the teletext is also used as a publicity channel, advertising products such as travel destinations. Common teletext services include TV schedules, regularly updated current affairs and sport news, simple games (like quizzes) and subtitling for deaf people or in different languages.
History
The BBC started an engineering project in 1970 or 1971 on what would become teletext, after considering ways to display subtitles (known as closed captioning in the US) on a TV. Display subtitles requires limited bandwidth and would be unsuitable for more than a few words per second. However by combining even a slow data rate with a suitable memory, pages of information could be sent and stored in the TV for later recall.
The system was announced to the public by the BBC as Ceefax (originally known as Teledata internally) in October 1972. The Independent Television Authority (ITA) announced their own service in 1973, known as ORACLE (Optional Reception of Announcements by Coded Line Electronics). Unlike Ceefax, ORACLE content was provided by a separate company formed by Philips and Associated Newspapers. Both systems used a common format, CEPT1 which was standardized in 1974. Recent versions of this standard are called World System B and commonly known as European teletext.
Following test transmissions in 1973–74 the Ceefax and ORACLE systems went live, Development was limited until the first sets with decoders started appearing in 1977, but by 1982 there were two million such sets, and by the mid-80s they were a standard feature of almost every European TV. Both Ceefax and ORACLE were now broadcasting several hundred pages on every channel, slowly changing them throughout the day.
Meanwhile, The Post Office had also decided it wanted a videotex system, and designed Viewdata (later renamed Prestel), launched in 1979. While using the same CEPT1 system as the existing systems, Viewdata operated over the telephone system rather than being broadcast on TV.
European teletext replaced the Antiope system in France at the beginning of the 1990s.
In 1993 ORACLE was replaced as content provider by Teletext Ltd, now owned by Daily Mail and General Trust and Media Ventures International. Branding themselves simply as "Teletext", they operate on ITV, Channel 4 and, more recently, Five.
Description
In the case of the Ceefax and Oracle systems and their successors in the UK, the teletext signal is transmitted as part of the ordinary analogue TV signal but concealed from view in the VBI (vertical blanking interval). The teletext signal is digitally coded as 45-byte packets at the end of each of lines 6–22 and 318–335. The resulting data rate is about 600 bps.
A teletext page comprises one or more frames, each containing a screen-full of text. The pages are sent out one after the other in a continual loop. When the user requests a particular page the decoder simply waits for it to be sent, and then captures it for display. In order to keep the delays reasonably short, services typically only transmit a few hundred frames in total. Even with this limited number, waits can be up to 30 seconds.
The text can be displayed instead of the television image (but usually with the sound), or superimposed on it (a mode commonly called mix). Some pages, such as subtitles (closed captioning) are in-vision, meaning that text is displayed in a block on the screen covering part of the television image.
The original standard provides a monospaced 40×24 character grid. The standard was improved in 1976 to allow for improved appearance and the ability to individually select the color of each character from a palette of 8. The proposed higher resolution Level 2 (1981) was not adopted in Britain, although transmission rates were doubled from two to four lines a frame in 1981. Britain also rejected Level 2.5 HiText.
Although it used the same page encoding and display methods, Prestel was quite a different system, using a modem and the phone system to transmit and receive the data. The modem was asymmetric, with data sent at 75 bps, and received 1200 bps. This two-way nature allowed pages to be served on request, in contrast to the TV-based systems' sequential rolling method. It also meant that a limited number of extra services were available such as booking event or train tickets. Prestel was in some ways similar to the French Minitel system.
Digital television introduced "digital teletext" which, despite the previous teletext standard's digital nature, has entirely different standards, such as MHEG-5 and Multimedia Home Platform. Despite the age of the technology, analogue teletext (with its now quaint looking 1980s-style computer graphics) remains very popular; although the service will stop upon the cessation of analogue TV broadcasting, sometime before 2012.
See also
- PDC / StarText.
- Videotex Telephone line based information system using Teletext graphics.
