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 and the UK, 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 – 1970's
Development of Teletext
In about 1970 the BBC had a brainstorming session in which it was decided to start researching ways to send closed captioning information to audience. As the Teledata research continued they became increasingly interested in using the same system for delivering any sort of information, not just closed captioning. Displaying subtitles requires limited bandwidth, at a rate of perhaps 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.
Meanwhile the General Post Office (soon to become British Telecom) was researching a similar concept since the late 1960s, known as Viewdata. Unlike Ceefax which was a one-way service carried in the existing TV signal, Viewdata was a two-way system using telephones. Since the Post Office owned the telephones, this was considered to be an excellent way to drive more customers to use the phones.
In 1972 the BBC demonstrated their system, now known as Ceefax, on various news shows. Not to be outdone by the BBC, the GPO immediately announced their service, under the name Prestel. 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.
In 1974 all of the services sat down and created a standard for displaying the information. The display would be a simple 40x24 grid of text, with some graphics characters for constructing simple graphics. This standard was called CEPT1. The standard did not define the delivery system, so both Viewdata-like and Teledata-like services could at least share the TV-side hardware (which at that point in time was quite expensive). The standard also introduced a new term that covered all such services, teletext. Recent versions of this standard are called World System B (also known as WST , or World System Teletext) and commonly known as European teletext.
Introduction of Teletext
Following test transmissions in 1973–74 Ceefax went live in 1976 with 30 pages of information. They were followed quickly by ORACLE and Prestel. Development was limited until the first TV sets with built-in 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.
History – 1980's
North America
A North American standard of teletext, called NABTS (North American Broadcast Teletext Standard) was also developed in the early 80's by Norpak , a Canadian company, which provided improved graphic and text capability over WST, but was quite short-lived. This was mainly due to the expensive cost of NABTS decoders, costing in the thousands of dollars upon their release to the public. NABTS, however, was adopted for a short while by American TV networks NBC & CBS throughout the early-to-mid 80's, CBS using it for their short-lived ExtraVision teletext service.
WST was also used for a short time in the USA as well, with services provided throughout the late 70s and early 80s by several regional American TV networks & large-market US TV stations, but the most prominent of American teletext providers was the Electra teletext service, which was broadcast starting in the early 1980's on the VBI of the famed American cable channel Superstation WTBS (now TBS Superstation). Electra was owned and operated by Taft Broadcasting and Satellite Syndication Services (SSS). Electra ran up until 1993, when it was shut down due to a lack of funding, and lagging interest in American teletext by the consumer.
Zenith's higher-end televisions manufactured throughout the 80's for the USA had WST teletext decoders built-in, as well as Dick Smith Electronics (DSE) offering in the US a set-top WST teletext decoder kit, using as its core the same teletext decoding module (manufactured by UK electronics company Mullard) installed in most British TV sets, with additional circuitry to adapt it for American NTSC video, and to utilize it in a separate set-top box.
History – 1990's
North America
The NABTS protocol received a revivial of sorts in the late 90s, when it was used for the datacasting features of WebTV for Windows under Windows, and for Intel's now-defunct InterCast service (also for Windows as well), using a proper TV card (such as the ATI All-In-Wonder).
InterCast was a modern teletext system created by Intel in 1996, using a TV card installed in a desktop PC running Windows with the InterCast Viewer software. The software would receive data via the VBI (Vertical Blanking Interval) of a television channel's video, while displaying in a window in the InterCast software the TV channel itself. The data received would then be displayed as HTML in another window in the Intercast software. It usually was extra supplemental information relevant to the the TV program being viewed, such as extra clues for the viewer during a murder mystery show, or extra news headlines or extended weather forecasts during a newscast.
NBC, as well as The Weather Channel, CNN and M2 (now MTV2), utilized InterCast technology to complement their programming. InterCast, however, fell into disuse, and Intel discontinued support of InterCast a few years later.
Europe
European teletext replaced the Antiope system in France at the beginning of the 1990s.
In 1993 ORACLE in the United Kingdom 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 at the end of lines that are not visible on the screen. 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 bit/s.
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.
Modern television sets however have a built-in memory, often for a few thousand different pages. This way, the teletext decoder captures every page sent out and stores it in memory, so when a page is requested by the user it can be loaded directly from memory instead of having to wait for the page to be transmitted. When the page is transmitted again, the television checks if the page in memory is still up-to-date and updates it if necessary.
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 bit/s, and received 1200 bit/s. 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.
Level 2.5 teletext
Because teletext looks a bit outdated with its 80's-style graphics, a new standard found its way to the European market around 2000: Level 2.5 or HiText. With Level 2.5 it's possible to set a background colour and have higher resolution text and images. However, very few television stations transmit their teletext in this new standard. One of the problems with Level 2.5 is that it often takes several transmission cycles before the higher resolution items show on the screen. In order to watch Level 2.5 teletext, a rather recent television set with a special decoder chip is required.
Television stations which are known to transmit teletext in Level 2.5 include the Dutch public television (background colour on all pages, and a test page with hi-res graphics) and the German ZDF (completely backwards compatile Level 2.5 teletext, with higher quality text and graphics on nearly all pages).
Digital Teletext
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.
Electronic Programme Guides based on teletext
Standard Electronic Programme Guides, like NexTView, are based on teletext, using a compact binary format instead of preformatted text pages.
See also
External links
Teletext content on the Internet
Last updated: 10-24-2005 10:10:51
