Teletext


Teletext

Teletext (or "broadcast teletext") is a television information retrieval service developed in the United Kingdom in the early 1970s. It offers a range of text-based information, typically including national, international and sporting news, weather and TV schedules. Subtitle (or closed captioning) information is also transmitted in the teletext signal, typically on page 888[1] or 777.

Contents

History

Development

Around 1970 the BBC had a brainstorming session in which Stephen Earley decided to start researching ways to send closed captioning information to the audience.[citation needed] As the so-called Teledata research continued they became increasingly interested in using the same system for delivering other kinds of information, and not just closed captioning.

Teletext is a means of sending text and diagrams to a properly equipped television screen by use of one of the "vertical blanking interval" lines that together form the dark band dividing pictures horizontally on the television screen.[2] Broadcasters who use the PAL system have more vertical-blanking-interval lines available, and can use several lines for teletext.

Transmitting and displaying subtitles was relatively easy. It requires limited bandwidth; at a rate of perhaps a few words per second. However, it was found that by combining even a slow data rate with a suitable memory, whole pages of information could be sent and stored in the TV for later recall.

Meanwhile the General Post Office (whose telecommunications division later became British Telecom) had been 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 ("see facts", the departmental stationery used the "Cx" logo), on various news shows. The Independent Television Authority (ITA) announced their own service in 1973, known as ORACLE (Optional Reception of Announcements by Coded Line Electronics). Not to be outdone, the GPO immediately announced a 1200/75 baud videotext service under the name Prestel.

The systems were originally incompatible; Ceefax displayed 24 lines of 32 characters each, while ORACLE offered 22 lines of 40 characters each. In other ways the standards overlapped; for instance, both used 7-bit ASCII characters and other basic details. In 1974 all the services agreed a standard for displaying the information. The display would be a simple 40 × 24 grid of text, with some graphics characters for constructing simple graphics. 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).

Rollout

Following test transmissions in 1973–74, towards the end of 1974 the BBC news department put together an editorial team of nine, including and led by Editor Colin McIntyre, to develop a news and information service. Initially limited to 30 pages, the Ceefax service was later expanded to 100 pages and was launched formally in 1976. It was followed quickly by ORACLE and Prestel. Wireless World magazine ran a series of articles between November 1975 and June 1976 describing the design and construction of a teletext decoder using mainly TTL devices; however, development was limited until the first TV sets with built-in decoders started appearing in 1977.

By 1982 there were two million such sets, and by the mid-80s they were available as an option for almost every European TV set, typically by means of a plug in circuit board. It took another decade before the decoders became a standard feature on almost all sets over 15" (teletext is still usually only an option for smaller "portable" sets). From the mid-80s both Ceefax and ORACLE were broadcasting several hundred pages on every channel, slowly changing them throughout the day.

The "Broadcast Teletext Specification" was published in September 1976 jointly by the IBA, the BBC and the British Radio Equipment Manufacturers' Association. The new standard also made the term "teletext" generic, describing any such system. The standard was internationalised as World System Teletext (WST), formalised as an international standard by CCIR in 1986 as CCIR Teletext System B.

North America

In the 1980s a similar system called Telidon was developed in Canada by the Department of Communications. It used a simple graphics language that would allow a more complex circuit in the TV to decode not only characters, but graphics as well. To do this, the graphic was encoded as a series of instructions (graphics primitives) like "polyline" which was represented as the characters PL followed by a string of digits for the X and Y values of the points on the line. This system was referred to as PDI (Picture Description Instructions). Later improved versions of Telidion were developed into NAPLPS.

Although there were numerous attempts to introduce NAPLPS services in North America, none of these were successful and eventually shut down. A number of special-purpose systems lived on for some time, similar to Prestel's lingering death, but the widespread rollout of internet access in the 1990s ended these efforts.

Technology

Teletext information is broadcast in the vertical blanking interval between image frames in a broadcast television signal. It is closely linked to the PAL broadcast system. Other teletext systems have been developed to work with the SECAM and NTSC systems, but teletext failed to gain widespread acceptance in North America and other areas where NTSC is used. In contrast, teletext is nearly ubiquitous across Europe as well as some other regions, with most major broadcasters providing a teletext service. Common teletext services include TV schedules, regularly updated current affairs and sport news, simple games (such as quizzes) and subtitling for deaf people or in different languages.

Teletext is broadcast in numbered "pages." For example a list of news headlines might appear on page 110; a teletext user would type "110" into the TV's remote control to view this page.

Teletext allows up to eight 'magazines' to be broadcast, identified by the first digit of the three-digit page number (1-8). Within each magazine there may theoretically be up to 256 pages at a given time, numbered in hexadecimal and prefixed with the magazine number - for example magazine 2 may contain pages numbered 200-2FF. In practice, however, non-decimal page numbers are rarely used as domestic teletext receivers will not have options to select hex values A-F, with such numbered pages only occasionally used for 'special' pages of interest to the broadcaster and not intended for public view.

The broadcaster constantly sends out pages in sequence in one of two modes: Serial mode broadcasts every page sequentially whilst parallel mode divides VBI lines amongst the magazines, enabling one page from each magazine to be broadcast simultaneously. There will typically be a delay of a few seconds from requesting the page and it being broadcast and displayed, the time being entirely dependent in the number of pages being broadcast in the magazine (parallel mode) or in total (serial mode) and the number of VBI lines allocated. In parallel mode, therefore, some magazines will load faster than others.

More sophisticated systems use a buffer memory to store some or all of the teletext pages as they are broadcast, allowing instant display from the buffer.

This basic architecture separates from other digital information systems, such as the internet, whereby pages are 'requested' and then 'sent' to the user - a method not possible given the one-way nature of broadcast teletext.

Because of its presentation of user-requested graphic information, teletext can be seen as a predecessor of the World Wide Web. Unlike the internet, teletext is broadcast, so it does not slow down further as the number of users increase, although the greater number of pages, the longer one is likely to wait for each to be found in the cycle. For this reason, some pages (e.g. common index pages) are broadcast more than once in each cycle.

It has proved to be a reliable text news service during events such as the September 11, 2001 terrorist attacks, during which the webpages of major news sites became inaccessible because of the high demand.[citation needed] Teletext is also used for carrying special packets interpreted by TVs and video recorders, containing information about channels, programming, etc. (see "Other teletext-related services").

Although the term "teletext" tends to be used to refer to the PAL-based system, or variants, the recent availability of digital television has led to more advanced systems being provided that perform the same task, such as MHEG-5 in the UK, and Multimedia Home Platform.

Data transmission

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 Vertical Blanking Interval (VBI) television lines which do not carry picture information. The teletext signal is digitally coded as 45-byte packets, so resulting data rate is 7,175 bits per second per used lines (41 7-bit 'bytes' per line, on each of 25 frames per second).

A standard PAL signal contains 625 lines of video data per screen, broken into two "fields" containing half the lines of the whole image. Lines near the top of the screen are used to synchronize the display to the signal, and are not seen on-screen. CEPT1 hides the data in these lines, where they are not visible, using lines 6–22 on the first field and 318–335 on the second field. The system does not have to use all of these lines; a unique pattern of bits allows the decoder to identify which lines contain data. Some teletext services use a great number of lines, others, for reasons of bandwidth and technical issues, use fewer.

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, although teletext broadcasters can control the speed and priority with which various pages are broadcast.

Modern television sets, however, usually 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, 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. Characters are sent using a 7-bit codec, with an 8th bit employed for error detection.[3] 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 (in-vision services from Ceefax & ORACLE did use it at various times however, though even this was ceased by the BBC in 1996), although transmission rates were doubled from two to four lines a frame.

Decoders

The type of decoder circuitry is sometimes marked on televisions as CCT (Computer Controlled Teletext), or ECCT (Enhanced Computer Controlled Teletext).

Other systems

A number of similar teletext services were developed in other countries, some of which attempted to address the limitations of the British-developed system, with its simple graphics and fixed page sizes.

The Acorn BBC Micro's default graphics mode (mode 7) was based on Teletext display, and the computer could be used to create and serve Teletext-style pages over a modem connection. With a suitable adapter, the computer could receive and display teletext pages, as well as software over the BBC's Ceefax service, for a time.

Later developments

While the basic teletext format has remained unchanged in more than 30 years, a number of improvements and additions have been made.

  • Standard Electronic Programme Guides (EPG), like NexTView, are based on teletext, using a compact binary format instead of preformatted text pages.
  • Various other kinds of information are sent over the teletext protocol. For instance, Programme Delivery Control signals—used by video recorders for starting/stopping recording at the correct time even during changes in programming—are sent as teletext packets. A similar, but different, standard Video Programming System is also used for this purpose.
  • Teletext pages may contain special packages allowing VCRs to interpret their contents. This is used in relation to the Video Programming by Teletext (also known as startext) system which allows users to program their videos for recording by simply selecting the program on a teletext page with a listing of programs.
  • Other standards define how special teletext packets may contain information about the name of the channel and the program currently being shown.

Video Program System

A closely related service is the Video Program System (VPS), introduced in Germany in 1985. Like teletext, this signal is also broadcast in the vertical blanking interval. It consists only of 32 bits of data, primarily the date and time for which the broadcast of the currently running TV programme was originally scheduled. Video recorders can use this information (instead of a simple timer) in order to automatically record a scheduled programme, even if the broadcast time changes after the user programmes the VCR. VPS also provides a PAUSE code; broadcasters can use it to mark interruptions and pause the recorders, however advertisement-financed broadcasters tend not to use it during their ad breaks. VPS (line 16) definition is now included in the PDC standard from ETSI.

Prestel

Prestel was a British information-retrieval system based on teletext protocols. However, it was essentially a different system, using a modem and the phone system to transmit and receive the data, comparable to systems such as France's Minitel. 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 and a limited amount of online banking.

Interactive teletext

Some TV channels offer a service called interactive teletext to remedy some of the shortcomings of standard teletext. To use interactive teletext, the user calls a special telephone number with a regular telephone. A computer then instructs the user to go to a certain teletext page which has been assigned to the customer for that session. Usually the page initially contains a menu with options and the user chooses among the options using the buttons on the telephone. When a choice has been made, the selected page is immediately broadcast and can be viewed by the user. This is in contrast with usual teletext where the customer has to wait for the selected page to be broadcast, because the pages are broadcast sequentially. This technology enables teletext to be used for games, chat, access to databases etc. It allows one to overcome the limitations on the number of available pages. On the other hand, only a limited number of users can use the service at the same time, since one page is allocated per user. Some channels solve this by taking into account where the user is geographically calling from and by broadcasting different teletext pages in different geographical regions. In that way, two different users can be assigned the same page number at the same time as long as they don't receive the TV signals from the same source. Another drawback to the technology is the privacy concerns in that many users can see what a user is doing because the interactive pages are received by all viewers. Also, the user usually has to pay for the telephone call to the TV station. For these reasons, these services have since been superseded by the World Wide Web.

Levels

Several levels are defined for teletext in ascending order of rendering complexity, although no TV sets currently implement the two most sophisticated levels.[4][5]

Level Description
1 The initial specifications set out by the BBC, IBA, BREMA in September 1976.
1.5 Used in Spain. Created by TVE (Televisión Española), is a extended version of the level 1, created to support Spanish special characters and other ASCII-like characters, and colouring modes.
2 Multi-language text, wider range of display attributes that may be non-spacing, wider range of colours and an extended mosaic pictorial set.
3 Dynamically Redefined Character Set (DRCS) allowing the display of non-Roman characters (e.g. Arabic and Chinese). Pictorial Graphic characters can also be defined.
4 Full geometric graphics. It needs computing power to generate the display from a sequence of drawing instructions. The colour palette has 250,000 shades.
5 Full-definition still pictures allows better quality than video cameras. Modulated onto a carrier. No noise added to the picture during transmission.

Level 2.5 teletext / Hi-Text

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 or HiText. 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.

However, 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

Digital teletext

NRK 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. However, standard teletext remains very popular. Some digital television platforms such as Sky Digital in the UK and Ireland, and the Saorview DTT platform in Ireland incorporate separate teletext streams (used by the BBC from 1998 to 2004, and still used by Irish broadcasters RTÉ, TV3 and TG4), which are provided to the television set in the normal analogue TV manner. Such emulation of analogue teletext on digital TV platforms may ensure its continued use for some time (particularly as there are no plans for an immediate transition to digital terrestrial transmission in some countries, such as Ireland). This emulation is only possible due to the DVB-TXT and DVB-VBI sub-standards of DVB, which allow a set-top box or integrated DVB TV to reproduce the vertical blanking interval data in which Teletext is carried.

Other teletext-related services

Various other kinds of information are sent over the teletext protocol. For instance, Programme Delivery Control signals—used by video recorders for starting/stopping recording at the correct time even during changes in programming—are sent as teletext packets. A similar, but different, standard Video Programming System is also used for this purpose.

Teletext pages may contain special packages allowing VCRs to interpret their contents. This is used in relation to the Video Programming by Teletext (also known as startext) system which allows users to program their videos for recording by simply selecting the program on a teletext page with a listing of programs.

Other standards define how special teletext packets may contain information about the name of the channel and the program currently being shown.

Cessation of service

A number of broadcast authorities have recently ceased the transmission of teletext services, notably CNN International.[6] Most pages are still available, although they have not been updated since 31 October 2006.

The BBC has also announced that Ceefax is to be phased out in the run-up to the UK Digital Switchover in 2012. The full service is no longer carried on any digital services, although many channels on Sky still broadcast teletext subtitles and may still have a small number of active pages.[7] Teletext will end in each region after analogue broadcasts finish. See Digital switchover dates in the United Kingdom.

In Australia, the Seven Network shut down the Austext service on 30 September 2009. They claimed the technology has come to the end of its useful service life and is not commercially viable to replace. Closed captioning services still continue however.

See also

References

  1. ^ Kawamoto, Kevin (2003). Digital Journalism: Emerging Media and the Changing Horizons of Journalism. Rowman & Littlefield. p. 32. ISBN 074252681X. 
  2. ^ Sterling, Christopher H.; Kittross, John M. (1990). Stay tuned : a concise history of American broadcasting. Wadsworth Pub. Co. ISBN 9780534119041. 
  3. ^ Brice, Richard (2003). Newnes guide to digital TV. Newnes. p. 41. ISBN 9780750657211. 
  4. ^ "Teletext Transmission". ExamPointer. 1993. http://exampointers.com/teletext/teletext.phtml. Retrieved 2009-07-14. 
  5. ^ Graziplene, Leonard R. (2000). Teletext: its promise and demise. Lehigh University Press. ISBN 9780934223645. 
  6. ^ "CNNText". CNN. http://edition.cnn.com/cnntext/. Retrieved 2010-01-30. 
  7. ^ "Teletext Gallery: Ceefax: The beginning of the end". Teletext Then and Now. 2001-09-19. http://teletext.mb21.co.uk/gallery/ceefax/end.shtml. Retrieved 2010-01-30. 

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