World System Teletext

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World System Teletext (or WST) is the name of a standard for encoding and displaying teletext information, which is used as the standard for teletext throughout Europe today.

Development[edit]

It originally stems from the UK standard developed by the BBC and the UK Independent Broadcasting Authority in 1974 for teletext transmission, extended in 1976 as the Broadcast Teletext Specification. With some tweaks to allow for alternative national character sets, and adaptations to the NTSC 525-line system as necessary, this was then promoted internationally as "World System Teletext". It was accepted by CCIR in 1986 under international standard CCIR 653 (now ITU-R BT.653) as one of four recognised standards for teletext worldwide, and may now most commonly be referred to as CCIR Teletext System B.

Almost all television sets sold in Europe since the early ’80s have built-in WST-standard teletext decoders as a feature.

WST is used for all teletext services in Europe & Scandinavia, including Ceefax from the BBC and services from Teletext on ITV in the United Kingdom, ZDFtext from ZDF and ARDText from ARD in Germany, and Tekst-TV from NRK in Norway, among many other teletext services offered by other television networks throughout the European continent.

Levels[edit]

In the early 1980s a number of higher extension levels were envisaged for the specification, based on ideas then being promoted for worldwide videotex standards (telephone dial-up services offering a similar mix of text and graphics). The proposed higher content levels included geometrically-specified graphics (Level 4), and higher-resolution photographic-type images (Level 5), to be conveyed using the same underlying mechanism at the transport layer. No TV sets currently implement the two most sophisticated levels.[1][2]

Level 1 (1976)[edit]

The initial specifications set out by the BBC, IBA, BREMA in September 1976:[3]

  • Alphamosaic (drawn using a 2x3 block matrix) characters[4]
  • spacing attributes
  • fixed colour palette (red, green, yellow, blue, magenta, cyan, white)
  • support for double height or flash effect[5]
  • 40 columns x 24 rows

Level 1.5 (1981)[edit]

Comparison between Teletext level 1 and 1.5

An extended version of level 1, with support for 13 extended character sets and other ASCII-like characters.

  • Czech & Slovak
  • English
  • Estonian
  • French
  • German
  • Italian
  • Latvian & Lithuanian
  • Polish
  • Portuguese & Spanish
  • Rumanian
  • Serbian & Croatian & Slovenian
  • Swedish & Finnish
  • Turkish

Level 2 (1988[6])[edit]

New features:

  • Multi-language support
  • 32 colour mode.[7]
  • Non-spacing attributes
  • Allows re-definable characters

(Level 2 was replaced by level 2.5)

Level 2.5 teletext / Hi-Text (1995)[7][edit]

Comparison between teletext Level 1.0 and teletext Level 2.5.
Comparison between teletext Level 1.0 and teletext Level 2.5.

A new graphic standard found its way to the European market around 2000: Level 2.5[7] or HiText.[3] With Level 2.5 it is 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.

New features:

  • Multi-language support
  • Wider colour palette with re-definable colours (4016 colour palette[7])
  • Non-spacing attributes
  • Allows re-definable characters
  • Provides side panels for additional text or graphics in 16:9 TVs
  • nexTView EPG
  • Adopted initially by ARTE, ARD, ZDF, Bayern 3 and SwissTXT

The system has not been widely implemented, with only a handful of European state broadcasters supporting it. Television stations which are known to transmit teletext in Level 2.5 include:

  • the Dutch public broadcaster NOS (background colour on all pages, and a test page with hi-res graphics),
  • the French France 3 and
  • the German
    • ZDF (some pages),
    • 3sat (some pages) and
    • SWR Fernsehen (completely backwards-compatible Level 2.5 teletext, with higher quality text and graphics on nearly all pages), as well as
    • Bayerisches Fernsehen,
    • BR-alpha
    • Phoenix (on some pages) and
    • Bürgerfernsehen Gera (background-colour on all pages, test pages 460 to 485).

Level 3[edit]

New features:

  • Dynamically Redefined Character Set (DRCS) allowing the display of non-Roman characters (e.g. Arabic and Chinese)
  • Pictorial Graphic characters can also be defined

(Level 3 was replaced by level 3.5)

Level 3.5 (1997)[edit]

Extends the number of re-definable characters and their complexity and introduces different font styles and proportional spacing.

New features:

  • Dynamically Redefined Character Set (DRCS) allowing the display of non-Roman characters (e.g. Arabic and Chinese)
  • Pictorial Graphic characters can also be defined
  • Different font styles
  • Proportional spacing.

Level 4 (1981)[8][edit]

Tested by IBA

  • Vector graphics in resolutions of 320x256
  • Needs computing power to generate the display from a sequence of drawing instructions
  • 250,000 colours palette

Level 5 [9][edit]

Full-definition still pictures allows better quality than video cameras

  • Modulated onto a carrier
  • No noise added to the picture during transmission
  • Image compression used

WST in the United States[edit]

ExtraVision index page, 1984

WST also saw some use in the United States in the 1980s, for the Electra service, which was carried on SuperStation WTBS (now TBS). It was also used for other teletext services on other television stations and networks in the USA as well.

Zenith in the US also included built-in WST teletext decoders in their higher-end models of TV sets, such as their Digital System 3 line throughout the 1980s. Also, Dick Smith Electronics offered through their American distributors a WST teletext decoder in the form of a set-top box, which was sold as a kit.

This was all in competition to another teletext standard developed exclusively in North America, NABTS (North American Broadcast Teletext Standard). It was developed in Canada by Norpak, and was used by CBS for their ExtraVision service and for a very short time by NBC in the mid-1980s. However, NABTS never became as successful as WST in the American continent, since NABTS was a more advanced technology, which required a much more complicated and expensive decoder (even though it had improved graphics capability over WST).

Further reading[edit]

  • World System Teletext Technical Specification, UK Department of Trade and Industry, 1985. Also retitled as World System Teletext and Data Broadcasting System Technical Specification, various revisions until 1989.
  • CCIR 653 (now ITU-R BT.653) set of four standards for teletext systems worldwide. Adopted 1986. Revisions 2 and 3 were published in 1993 and 1998. WST was formalised by this standard as CCIR Teletext System B.
  • Enhanced Teletext specification ETS 300 706 (ETSI, 1997); Version 1.2.1, April 2003. Current European standard for CCIR Teletext System B.

References[edit]