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 main 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.

Table of contents

1 History
2 Description
3 See also
4 External Links

4.1 Teletext content on the Internet

History

The BBC started an engineering project in 1970 or 1971 on what would become teletext, after considering ways to display closed captioning information on a TV. Closed captioning 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. Feeling left out, 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 a separate company formed by Philips and Associated Newspapers. At that point the Post Office decided it wanted in as well, with their system called Viewdata (later to be renamed as Prestel). The three then agreed on a common format, which was standardized in 1974.

Following test transmissions in 1973-74 the Ceefax system went live in 1975 with thirty pages on offer. ORACLE followed in 1977, and Prestel (phone based) in 1979. 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 TV. Both Ceefax and ORACLE broadcast several hundred pages on every channel, slowly changing them throughout the day. ORACLE was later replaced on ITV by another similar service offered by Teletext Ltd. at the beginning of 1993.

Description

In the case of the Ceefax and Oracle systems and their successors, 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 scan line (only lines 6 - 22 and 318 - 335 are used). The data rate is about 600bps.

Each page is comprised of 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 that page to be sent, and then captures it for display. In order to keep the delays short, enough to not be too bothersome, services typically only include a few hundred frames in total. Even with this limited number of frames, waits can be up to 30 seconds.

The original standard supported 24 rows of information with 40 characters a row. The standard was improved in 1976 to allow for improved appearance and color support. The proposed high resolution Level 2 (1981) was not adopted in Britain, although transmission rates were doubled from two to four lines a frame in 1981. The service extended to British commercial television through Teletext Ltd. Again, as an early adopter, Britain also rejected Level 2.5 (HighText).

The text can be displayed instead of the television image (but standard with the sound), or through it.

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 75bps, and received 1200bps. 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 is actually has entirely different standards, such as MHEG-5 and Multimedia Home Platform.