Teletext
Encyclopedia
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 or 777.
had a brainstorming session in which Stephen Earley decided to start researching ways to send closed captioning
information to the audience. 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. 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 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).
. 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.
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.
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. 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
.
(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. 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.
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.
VPS (line 16) definition is now included in the PDC
standard from ETSI.
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.
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
.
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
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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.
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.
. 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. 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.
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
information retrieval service developed in the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
in the early 1970s. It offers a range of text-based information, typically including national, international and sporting news, weather and TV schedules. Subtitle
Subtitle (captioning)
Subtitles are textual versions of the dialog in films and television programs, usually displayed at the bottom of the screen. They can either be a form of written translation of a dialog in a foreign language, or a written rendering of the dialog in the same language, with or without added...
(or closed captioning
Closed captioning
Closed captioning is the process of displaying text on a television, video screen or other visual display to provide additional or interpretive information to individuals who wish to access it...
) information is also transmitted in the teletext signal, typically on page 888 or 777.
Development
Around 1970 the BBCBBC
The British Broadcasting Corporation is a British public service broadcaster. Its headquarters is at Broadcasting House in the City of Westminster, London. It is the largest broadcaster in the world, with about 23,000 staff...
had a brainstorming session in which Stephen Earley decided to start researching ways to send closed captioning
Closed captioning
Closed captioning is the process of displaying text on a television, video screen or other visual display to provide additional or interpretive information to individuals who wish to access it...
information to the audience. 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. 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
Bandwidth (computing)
In computer networking and computer science, bandwidth, network bandwidth, data bandwidth, or digital bandwidth is a measure of available or consumed data communication resources expressed in bits/second or multiples of it .Note that in textbooks on wireless communications, modem data transmission,...
; 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
General Post Office
General Post Office is the name of the British postal system from 1660 until 1969.General Post Office may also refer to:* General Post Office, Perth* General Post Office, Sydney* General Post Office, Melbourne* General Post Office, Brisbane...
(whose telecommunications division later became British Telecom) had been researching a similar concept since the late 1960s, known as Viewdata
Viewdata
Viewdata is a Videotex implementation. It is a type of information retrieval service in which a subscriber can access a remote database via a common carrier channel, request data and receive requested data on a video display over a separate channel. Samuel Fedida was credited as inventor of the...
. 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
Ceefax
Ceefax is the BBC's teletext information service transmitted via the analogue signal, started in 1974 and will run until April 2012 for Pages from Ceefax, while the actual interactive service will run until 24 October 2012, in-line with the digital switchover.-History:During the late 60s, engineer...
("see facts", the departmental stationery used the "Cx" logo), on various news shows. The Independent Television Authority
Independent Television Authority
The Independent Television Authority was an agency created by the Television Act 1954 to supervise the creation of "Independent Television" , the first commercial television network in the United Kingdom...
(ITA) announced their own service in 1973, known as ORACLE
ORACLE (teletext)
ORACLE was a commercial teletext service first broadcast on ITV in 1974 and later on Channel 4 in the United Kingdom, finally ending on both channels at 23:59 GMT on 31 December 1992....
(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
Prestel
Prestel , the brand name for the UK Post Office's Viewdata technology, was an interactive videotex system developed during the late 1970s and commercially launched in 1979...
.
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
ASCII
The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
characters and other basic details. In 1974 all the services agreed a standard for displaying the information. The display would be a simple 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 PrestelPrestel
Prestel , the brand name for the UK Post Office's Viewdata technology, was an interactive videotex system developed during the late 1970s and commercially launched in 1979...
. Wireless World
Wireless World
Wireless World was the pre-eminent British magazine for radio and electronics enthusiasts. It was one of the very few "informal" journals which were tolerated as a professional expense.- History :...
magazine ran a series of articles between November 1975 and June 1976 describing the design and construction of a teletext decoder using mainly TTL
Transistor-transistor logic
Transistor–transistor logic is a class of digital circuits built from bipolar junction transistors and resistors. It is called transistor–transistor logic because both the logic gating function and the amplifying function are performed by transistors .TTL is notable for being a widespread...
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
1980s
File:1980s decade montage.png|thumb|400px|From left, clockwise: The first Space Shuttle, Columbia, lifted off in 1981; American President Ronald Reagan and Soviet leader Mikhail Gorbachev eased tensions between the two superpowers, leading to the end of the Cold War; The Fall of the Berlin Wall in...
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
1980s
File:1980s decade montage.png|thumb|400px|From left, clockwise: The first Space Shuttle, Columbia, lifted off in 1981; American President Ronald Reagan and Soviet leader Mikhail Gorbachev eased tensions between the two superpowers, leading to the end of the Cold War; The Fall of the Berlin Wall in...
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
World System Teletext
World System Teletext is the name of a standard for encoding and displaying teletext information, which is used as the standard for teletext throughout Europe today....
(WST), formalised as an international standard by CCIR
ITU-R
The ITU Radiocommunication Sector is one of the three sectors of the International Telecommunication Union and is responsible for radio communication....
in 1986 as CCIR Teletext System B.
North America
In the 1980s a similar system called TelidonTelidon
Telidon was a videotex/teletext service developed by the Canadian Communications Research Centre during the late 1970s and early 1980s...
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
NAPLPS
NAPLPS is a graphics language for use originally with videotex and teletext services. NAPLPS was developed from the Telidon system developed in Canada, with a small number of additions from AT&T...
.
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 intervalVertical blanking interval
The vertical blanking interval , also known as the vertical interval or VBLANK, is the time difference between the last line of one frame or field of a raster display, and the beginning of the first line of the next frame. It is present in analog television, VGA, DVI and other signals. During the...
between image frames in a broadcast television signal. It is closely linked to the PAL
PAL
PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...
broadcast system. Other teletext systems have been developed to work with the SECAM
SECAM
SECAM, also written SÉCAM , is an analog color television system first used in France....
and NTSC
NTSC
NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
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
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
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
Hexadecimal
In mathematics and computer science, hexadecimal is a positional numeral system with a radix, or base, of 16. It uses sixteen distinct symbols, most often the symbols 0–9 to represent values zero to nine, and A, B, C, D, E, F to represent values ten to fifteen...
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
World Wide Web
The World Wide Web is a system of interlinked hypertext documents accessed via the Internet...
. Unlike the internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
, teletext is broadcast
Broadcasting (networks)
right|250pxIn telecommunication and information theory, broadcasting refers to a method of transferring a message to all recipients simultaneously...
, 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. 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
Digital television
Digital television is the transmission of audio and video by digital signals, in contrast to the analog signals used by analog TV...
has led to more advanced systems being provided that perform the same task, such as MHEG-5
MHEG-5
MHEG-5, or ISO/IEC 13522-5, is part of a set of international standards relating to the presentation of multimedia information, standardised by the Multimedia and Hypermedia Experts Group...
in the UK, and Multimedia Home Platform
Multimedia Home Platform
Multimedia Home Platform is an open middleware system standard designed by the DVB project for interactive digital television. The MHP enables the reception and execution of interactive, Java-based applications on a TV-set. Interactive TV applications can be delivered over the broadcast channel,...
.
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 IntervalVertical blanking interval
The vertical blanking interval , also known as the vertical interval or VBLANK, is the time difference between the last line of one frame or field of a raster display, and the beginning of the first line of the next frame. It is present in analog television, VGA, DVI and other signals. During the...
(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
Closed captioning
Closed captioning is the process of displaying text on a television, video screen or other visual display to provide additional or interpretive information to individuals who wish to access it...
), 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. 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
BBC Micro
The BBC Microcomputer System, or BBC Micro, was a series of microcomputers and associated peripherals designed and built by Acorn Computers for the BBC Computer Literacy Project, operated by the British Broadcasting Corporation...
'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
Telesoftware
The word Telesoftware was coined by W J G Overington who first proposed the idea; it literally means “software at a distance” and it refers to the transmission of programs for a microprocessor or home computers via broadcast Teletext...
over the BBC's Ceefax
Ceefax
Ceefax is the BBC's teletext information service transmitted via the analogue signal, started in 1974 and will run until April 2012 for Pages from Ceefax, while the actual interactive service will run until 24 October 2012, in-line with the digital switchover.-History:During the late 60s, engineer...
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.- StandardStandardizationStandardization is the process of developing and implementing technical standards.The goals of standardization can be to help with independence of single suppliers , compatibility, interoperability, safety, repeatability, or quality....
Electronic Programme GuideElectronic program guideElectronic program guides and interactive program guides provide users of television, radio, and other media applications with continuously updated menus displaying broadcast programming or scheduling information for current and upcoming programming...
s (EPG), like NexTViewNexTViewNexTView is an electronic program guide for the analog domain. It is used by TV programme listings for all of the major networks in Germany, Austria, France and Switzerland...
, are based on teletext, using a compact binaryBinary fileA binary file is a computer file which may contain any type of data, encoded in binary form for computer storage and processing purposes; for example, computer document files containing formatted text...
format instead of preformatted text pages. - Various other kinds of information are sent over the teletext protocol. For instance, Programme Delivery ControlProgramme Delivery ControlProgramme delivery control is specified by the standard ETS 300 231, published by the European Telecommunications Standards Institute...
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
Programme Delivery Control
Programme delivery control is specified by the standard ETS 300 231, published by the European Telecommunications Standards Institute...
standard from ETSI.
Prestel
PrestelPrestel
Prestel , the brand name for the UK Post Office's Viewdata technology, was an interactive videotex system developed during the late 1970s and commercially launched in 1979...
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
Minitel
The Minitel is a Videotex online service accessible through the telephone lines, and is considered one of the world's most successful pre-World Wide Web online services. It was launched in France in 1982 by the PTT...
. 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 telephoneTelephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...
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
Online chat
Online chat may refer to any kind of communication over the Internet, that offers an instantaneous transmission of text-based messages from sender to receiver, hence the delay for visual access to the sent message shall not hamper the flow of communications in any of the directions...
, 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
World Wide Web
The World Wide Web is a system of interlinked hypertext documents accessed via the Internet...
.
Levels
Several levels are defined for teletext in ascending order of rendering complexity, although no TV sets currently implement the two most sophisticated levels.| 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
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 NOSNederlandse Omroep StichtingThe Nederlandse Omroep Stichting , English: Netherlands Broadcasting Foundation, is one of the broadcasters in the Netherlands Public Broadcasting system...
(background colour on all pages, and a test page with hi-res graphics), - the French France 3France 3France 3 is the second largest French public television channel and part of the France Télévisions group, which also includes France 2, France 4, France 5, and France Ô....
and - the German
- ZDFZDFZweites Deutsches Fernsehen , ZDF, is a public-service German television broadcaster based in Mainz . It is run as an independent non-profit institution, which was founded by the German federal states . The ZDF is financed by television licence fees called GEZ and advertising revenues...
(some pages), - 3sat3sat3sat is the name of a public, advertising-free, television network in Central Europe. The programming is in German and is broadcast primarily within Germany, Austria and Switzerland .3sat was established for cultural...
(some pages) and - SWR FernsehenSWR FernsehenSWR Fernsehen is a German regional television channel targeting the states of Baden-Württemberg and Rhineland-Palatinate. It is produced by Südwestrundfunk and is one of eight regional "third channels" broadcast by the ARD members....
(completely backwards-compatible Level 2.5 teletext, with higher quality text and graphics on nearly all pages), as well as - Bayerisches Fernsehen,
- BR-alphaBR-alphaBR-alpha is a German television station owned by regional broadcaster Bayerischer Rundfunk BR-alpha broadcasts 24 hours a day, 7 days a week, with programmes from BR, as well as from ARD and Austrian broadcaster ORF.-Programmes:...
, and - PhoenixPhoenix (German TV station)Phoenix is a publicly-funded television station in Germany which is produced jointly by public broadcasting organizations ARD and ZDF. Its programming consists of documentaries, news broadcasts, special events coverage, and discussion programmes...
on some pages.
- ZDF
Digital teletext
MHEG-5
MHEG-5, or ISO/IEC 13522-5, is part of a set of international standards relating to the presentation of multimedia information, standardised by the Multimedia and Hypermedia Experts Group...
and Multimedia Home Platform
Multimedia Home Platform
Multimedia Home Platform is an open middleware system standard designed by the DVB project for interactive digital television. The MHP enables the reception and execution of interactive, Java-based applications on a TV-set. Interactive TV applications can be delivered over the broadcast channel,...
. However, standard teletext remains very popular. Some digital television platforms such as Sky Digital in the UK and Ireland, and the Saorview
Saorview
Saorview is the national free-to-air digital terrestrial television service in Republic of Ireland.The service began operation on 29 October 2010 on a trial basis with full launch on 26 May 2011. By legislation it was required to be available to approximately 90% of the population by end of...
DTT platform in Ireland incorporate separate teletext streams (used by the BBC from 1998 to 2004, and still used by Irish
Republic of Ireland
Ireland , described as the Republic of Ireland , is a sovereign state in Europe occupying approximately five-sixths of the island of the same name. Its capital is Dublin. Ireland, which had a population of 4.58 million in 2011, is a constitutional republic governed as a parliamentary democracy,...
broadcasters RTÉ
RTE
RTÉ is the abbreviation for Raidió Teilifís Éireann, the public broadcasting service of the Republic of Ireland.RTE may also refer to:* Recep Tayyip Erdoğan, 25th Prime Minister of Turkey...
, 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
Set-top box
A set-top box or set-top unit is an information appliance device that generally contains a tuner and connects to a television set and an external source of signal, turning the signal into content which is then displayed on the television screen or other display device.-History:Before the...
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 ControlProgramme Delivery Control
Programme delivery control is specified by the standard ETS 300 231, published by the European Telecommunications Standards Institute...
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 InternationalCNN International
CNN International is an international English language television network that carries news, current affairs, politics, opinions, and business programming worldwide. CNN is one of the world's largest news organizations. It is owned by Time Warner, and is affiliated with CNN, which is mainly...
. Most pages are still available, although they have not been updated since 31 October 2006.
The BBC has also announced that Ceefax
Ceefax
Ceefax is the BBC's teletext information service transmitted via the analogue signal, started in 1974 and will run until April 2012 for Pages from Ceefax, while the actual interactive service will run until 24 October 2012, in-line with the digital switchover.-History:During the late 60s, engineer...
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. Teletext will end in each region after analogue broadcasts finish. See Digital switchover dates in the United Kingdom.
In Australia, the Seven Network
Seven Network
The Seven Network is an Australian television network owned by Seven West Media Limited. It dates back to 4 November 1956, when the first stations on the VHF7 frequency were established in Melbourne and Sydney.It is currently the second largest network in the country in terms of population reach...
shut down the Austext
Austext
Austext is the former Australian teletext service based in Brisbane, Queensland. The production was carried and operated by the Seven Network and its affiliates over most of Australia. It carried news, finance information, weather, lottery results, TV guide and other information...
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
- List of teletext services
- Digital terrestrial televisionDigital terrestrial televisionDigital terrestrial television is the technological evolution of broadcast television and advance from analog television, which broadcasts land-based signals...
- DigitiserDigitiserDigitiser was a video games magazine that was broadcast on the Teletext service on Channel 4 in the UK from 1993 to 2003, and was updated from Monday to Saturday...
- Electronic program guideElectronic program guideElectronic program guides and interactive program guides provide users of television, radio, and other media applications with continuously updated menus displaying broadcast programming or scheduling information for current and upcoming programming...
- Multimedia Home PlatformMultimedia Home PlatformMultimedia Home Platform is an open middleware system standard designed by the DVB project for interactive digital television. The MHP enables the reception and execution of interactive, Java-based applications on a TV-set. Interactive TV applications can be delivered over the broadcast channel,...
- NAPLPSNAPLPSNAPLPS is a graphics language for use originally with videotex and teletext services. NAPLPS was developed from the Telidon system developed in Canada, with a small number of additions from AT&T...
- NABTSNABTSNABTS, the North American Broadcast Teletext Specification, is a protocol used for encoding NAPLPS-encoded teletext pages, as well as other types of digital data, within the vertical blanking interval of an analog video signal...
- Park AvenuePark Avenue (teletext soap)Park Avenue was a daily teletext based soap opera on ITV's ORACLE Teletext service, which was written by Robbie Burns. It was launched in 1988, and 1,445 episodes were written during its time on air...
- Programme Delivery ControlProgramme Delivery ControlProgramme delivery control is specified by the standard ETS 300 231, published by the European Telecommunications Standards Institute...
- SubRipSubRipSubRip is a software program for Windows which "rips" subtitles and their timings from video. It is free software, released under the GNU GPL...
- Text semigraphicsText semigraphicsText semigraphics is a primitive method used in early video hardware to emulate per pixel addressable graphics without having to implement the logic for a true "high resolution" mode....
- VideotexVideotexVideotex was one of the earliest implementations of an "end-user information system". From the late 1970s to mid-1980s, it was used to deliver information to a user in computer-like format, typically to be displayed on a television.In a strict definition, videotex refers to systems that provide...
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)
- Enhanced Teletext specification ETS 300 706 (ETSI, 1997); Version 1.2.1, April 2003
- History of Teletext
- History of Ceefax
- KCET Teletext Trial (1980) sample screens
- Teletext online - Spain and South America
- 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
- Teletext Decoder Certification
- Teletext Browser
- Dutch Teletext
- Czech Teletext
- Finnish Teletext