Teletext
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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 caption) information is also transmitted in the teletext signal, typically on page 888.
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[edit] History
[edit] Development
In c.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 (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 curious "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).
In 1974 all the services agreed 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.[citation needed]
The "Broadcast Teletext Specification" was published in September 1976 jointly by the IBA, the BBC and the British Radio Equipment Manufacturers' Association. Not to be outdone the GPO immediately announced a 1200/75 baud videotext service under the name Prestel.
Recent versions of the CEPT1 standard are called World System B (also known as WST, or World System Teletext) and commonly known as European teletext.
In 1980 a similar system called Telidon was developed in Canada. It used a simple graphics language that would allow a more complex circuit in the TV to decode not only characters, but simple 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 called NAPLPS.
[edit] Introduction
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.
[edit] 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, and most PAL televisions include teletext decoders. 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 uses a numbered page metaphor to present its information, all of which is broadcast in sequence; when a viewer keys in a page number, the receiver waits until that information is broadcast again, typically within a few seconds, and retrieves it for display on-screen. More sophisticated systems use a buffer memory to store some or all of the teletext pages, for instantaneous display.
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. 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 due to unexpected demand. Teletext is 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.
[edit] Function
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) television lines which do not carry picture information. The teletext signal is digitally coded as 45-byte packets and can use lines 6–22 and 318–335. Some teletext services use a great number of lines, others, for reasons of bandwidth and technical issues, use less. The resulting data rate is 7000 bit/s per line. (40 usable 7-bit bytes per line, on each of 25 frames per second.)
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. 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 in 1981.
[edit] Other systems
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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.
[edit] France: Antiope
In France, where the SECAM standard is used in television broadcasting, a teletext system was developed in the late 1970s under the name Antiope. It had a higher data rate and was capable of dynamic page sizes, allowing more sophisticated graphics. It was phased out in favour of standard teletext in 1991.
[edit] Canada:Telidon
The CBC ran a teletext service, IRIS, accessible only in Calgary, Toronto and Montreal. It ran from 1983 until about 1986, and used the Canadian-developed Telidon system, which was developed in 1980. Like Antiope, Telidon allowed significantly higher graphic resolution than standard teletext.
[edit] United States
A version of the European teletext standard designed to work with the NTSC television standard used in North America was first demonstrated in the USA in 1978 by American television network CBS, which decided to try both the British Ceefax and French Antiope software for preliminary tryouts for a teletext service using station KMOX (now KMOV) in St. Louis, Missouri as a testing ground. CBS decided on Antiope, and the service premiered on station KNXT (now KCBS) in Los Angeles. Also in 1978, station KSL in Salt Lake City, Utah, also premiered a teletext service using Ceefax.
Adoption in the United States was hampered due to a lack of a single teletext standard and consumer resistance to the high initial price of teletext decoders.
A significant reason for the demise of American teletext was when Zenith introduced built-in closed captioning decoders in TVs in the early '90s, as mandated by the FCC. It was not practical for Zenith to re-design their TV chassis models that previously had teletext decoder support to have both teletext and closed captioning support. So Zenith decided to drop the teletext features, therefore ending teletext service in the US in the early 1990s, considering Zenith was the only major manufacturer of teletext-equipped sets in America.
Nowadays, teletext or other similar technologies in the USA are practically non-existent, with the only technologies resembling such existing in the country being closed captioning, TV Guide On Screen, and XDS (eXtended Data Services).
[edit] World System Teletext
WST was also used for a short time in the USA, with services provided throughout the late 1970s and early 1980s by several regional American TV networks (such as the University of Wisconsin-Madison's Infotext service in the mid-1980s, which was carried on several TV stations across Wisconsin, and Agtext, provided by Kentucky Educational Television and carried on KET's stations, both services providing agriculturally oriented information) and major-market U.S. TV stations (such as Metrotext, which was formerly carried on station KTTV in Los Angeles, and KeyFax, formerly on WFLD in Chicago).
Zenith manufactured models of television sets in the USA in the 1980s, most notably their Digital System 3 line, that had built-in WST teletext decoders as a feature, much like most British/European TV sets. Teletext services in the USA like Electra could be received with one of these sets, but these were mostly more expensive higher-end sets offered by Zenith, possibly causing Electra (and American teletext in general) to never catch on with the public.
Australian company Dick Smith Electronics (DSE) also offered through their USA distributors a set-top WST teletext decoder kit. The kit used 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.
The successor of WST in Europe and other countries is based on the Enhanced Teletext Specification from ETSI. ETSI EN300706 classifies Teletext in Level 1.5, Level 2.5 and Level 3.5. Level 1.5 covers all European languages plus Arabic and Hebrew. Level 2.5 provides display enhancements. Level 3.5 adds proportional fonts and high resolution graphics. Level 1.5 is the most popular Teletext standard in analogue and DVB transmissions in Europe. Sometimes it is called 'classical' Teletext. Level 3.5 was outperformed through the success of HTML and other modern standards.
Teletext differs in navigation methods provided by the broadcaster like FLOF (link information for a tree of pages) or TOP (Table of Pages).
[edit] NABTS
Later, an official North American standard of teletext, called NABTS (North American Broadcast Teletext Specification) was developed in the early 1980s by Norpak, a Canadian company. NABTS 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 80s, CBS using it for their short-lived ExtraVision teletext service, which premiered after the early Antiope & Ceefax trials by CBS & KNXT, and NBC, who had a NABTS-based service called NBC Teletext for a very short time in the mid-1980s. NBC discontinued their service in 1986 due to the cost of NABTS decoders not dropping to an affordable level for the consumer public.
The NABTS protocol received a revival of sorts in the late 90s, when it was used for the datacasting features of WebTV for Windows under Windows 98, and for Intel's now-defunct InterCast service (also for Windows as well), using a proper TV tuner card (such as the ATI All-In-Wonder or Hauppauge's Win-TV).
InterCast was a modern teletext-like system created by Intel in 1996, using a TV tuner card installed in a desktop PC running Windows with the InterCast Viewer software. The software would receive data representing HTML pages 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 HTML data received would then be displayed in another window in the Intercast software. It usually was extra supplemental information relevant to 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.
[edit] Electra
Perhaps the most prominent of American teletext providers was the Electra teletext service, which was broadcast starting in the early 1980s on the vertical blanking interval (VBI) of the American cable channel WTBS. Electra was owned and operated by Taft Broadcasting and Satellite Syndicated Systems (SSS). Electra ran up until 1993, when it was shut down due to Zenith, the prominent (and only) American TV manufacturer at the time offering teletext features in their sets decided to discontinue such features, as well as a lack of funding and lagging interest in teletext by the American consumer.
[edit] WaveTop
Another service in the USA similar in delivery and content to teletext was the WaveTop service, provided and operated by the Wavephore Corporation. It used the same types of InterCast-compatible TV tuner cards, and used an application that ran under Windows, like InterCast. In fact, WaveTop software was also bundled with TV tuner cards that had InterCast software bundled with them as well.
However, Wavetop was an independent service from InterCast, and wasn't a complementary service to a television program or channel like the latter In fact, viewing television with a TV card was not possible while the WaveTop software was running, since the software utilized the TV tuner card as a full-time data receiver.
WaveTop provided content from several different providers in the form of HTML pages displayed in the WaveTop software, such as news articles from the New York Times, weather information provided by The Weather Channel, and sports from ESPN. It also delivered short video clips, usually commercials, that could be viewed in the software as well.
When it was in operation, WaveTop's data was delivered on the VBI of local public TV stations affiliated with the PBS network through their PBS National Datacast [1] division, that the TV card & WaveTop software tuned into to receive the service.
[edit] Guide+
Yet another service in the U.S. that relied on data delivery via the VBI like teletext, was the Guide+ (Guide Plus, also referred to as GuidePlus+ as well) service provided and developed by Gemstar. There were several models of television sets made throughout the 90s by Thomson Consumer Electronics under the RCA and General Electric brands that had built-in Guide+ decoders. Guide+ was an on-screen interactive program guide that provided current TV schedule listings, as well as other information like news headlines. Some Guide+ equipped sets from RCA even had an IR-emitting sensor that could be plugged in to the back of the TV, to control a VCR to record programs which could be selected from the on-screen Guide+ listings. In some ways, this was very similar to the Video Programming by Teletext (VPT), Video Program System (VPS), and Programme Delivery Control (PDC) features of British/European teletext.
Guide+ was a free service, supported by advertisements displayed on-screen in the Guide+ menu and listing screens, not unlike banner ads displayed on web pages. Guide+ was delivered over the VBI of select local American TV stations.
Guide+ was discontinued by Gemstar in June 2004, and soon afterwards, Thomson dropped the Guide+ features from all RCA and GE television sets made afterward.
However, Guide+ in America has now been replaced by Gemstar with a similar service (delivered in the same fashion via VBI like Guide+), called TV Guide On Screen [2]. A small amount of televisions, DVD recorders, and digital video recorders are now being released with TV Guide On Screen capabilities. The Guide+ name & service is still used in Europe by Gemstar. (The same service is known in Japan as G-Guide).
[edit] Star Sight
Similar to Guide+ was Star Sight [3], with its decoders built in to TVs manufactured by Zenith, Samsung, Sony, Toshiba, Magnavox, and others. This was an electronic program guide service similar to Guide+, but was a service that relied on monthly subscription fees paid by the user, not from revenue gathered from on-screen advertisements like Guide+. Star Sight discontinued operations on July 21, 2003, due to a lack of subscribers to the service. Star Sight's data was also delivered on the VBI of local PBS stations through the PBS National Datacast division, much like how WaveTop was delivered as mentioned previously in this article.
[edit] 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 VCR's 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.
[edit] 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.
[edit] 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.
[edit] 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 largely been superseded by the World Wide Web.
[edit] Levels
These are taken from http://exampointers.com/teletext/teletext.phtml
| Level | Description |
|---|---|
| 1 | The initial specifications set out by the BBC, IBA, BREMA in September 1976. |
| 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. |
[edit] Level 2.5 teletext
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) and the German ZDF and Bayerisches Fernsehen (completely backwards-compatible Level 2.5 teletext, with higher quality text and graphics on nearly all pages), as well as Arte and 3sat on some pages.
[edit] 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 incorporate separate teletext streams (used by the BBC from 1998 to 2004, and still used by Irish broadcaster RTÉ), 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 emulate the vertical blanking interval data in which teletext is carried.
[edit] 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 VCR's 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.
[edit] Cessation of service
A number of broadcast authorities have recently found fit to cease the transmission of teletext services, notably CNN International [4]. 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 [5]. Teletext will end in each region after analogue broadcasts finish.
[edit] See also
- List of teletext services
- Digital terrestrial television
- Digitiser
- Electronic program guide
- Multimedia Home Platform
- NAPLPS
- NABTS
- Programme Delivery Control
- Videotex
[edit] References
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This article does not cite any references or sources. (August 2006) Please help improve this article by adding citations to reliable sources. Unverifiable material may be challenged and removed. |
[edit] External links
- Broadcast Teletext Specification, September 1976 (HTML; graphics missing)
- Broadcast Teletext Specification, September 1976 (scanned copy of original document, MS-Word and Postscript files)
- History of Teletext
- History of Ceefax
- Teletext Then and Now
- Teletext Then and Now Gallery - pictures of teletext services in the UK, Ireland and around the world
- Timeline of Telecommunications, with some entries on teletext
- The page for the now-defunct Star Sight service
- A page on teletext in the USA
- Downloadable "Enhanced Teletext specification" (ETSI.org)
- Teletext Decoder Certification
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