Teletext

This article is about the teletext system as a whole. For the UK company whose teletext services are also branded as "Teletext", see Teletext Ltd. For the obsolete communications system, see Teletex.

Teletext (or broadcast teletext) is a television information retrieval service created in the United Kingdom in the early 1970s by the Philips Lead Designer for VDUs, John Adams. Teletext is a means of sending pages of text and simple geometric shapes from mosaic blocks to a VBI decoder equipped television screen by use of a number of reserved vertical blanking interval lines that together form the dark band dividing pictures horizontally on the television screen.[1] It offers a range of text-based information, typically including news, weather and TV schedules. Paged subtitle (or closed captioning) information is also transmitted within the television signal. The first test transmissions were made by the BBC in 1973, known as Ceefax ("see facts"). After adoption in the UK the standards became international as the European Teletext standards and as the World System Teletext (WST). The World Wide Web began to take over some of the functions of teletext from the late 1990s, and many broadcasters have ceased broadcast of teletext—CNN in 2006 and the BBC in 2012. The decline of teletext has been hastened by the introduction of digital television, though an aspect of teletext continues in closed captioning.

History

Invention & Design

In the early 1970s work was in progress in Britain that would transmit paged information within a television broadcast. While engineers in several organisations were developed the equipment that would be used to transmit and receive the information. The work progressed at relatively the same pace as the U.S. developed EIA-608 caption only format. The goal was to provide UK rural homes with electronic hardware that could download pages of up-to-date news, reports, facts and figures targeting the U.K. farming communities. The original idea was the brainchild of Philips (CAL) Laboratories in 1970 and would be later standardised for worldwide use by the European Broadcasting Union (EBU) as Teletext. In 1971, CAL engineer John Adams created a design and proposal for UK broadcasters, which became accepted universally as the basis for all future Teletext systems and standards. Eventually it would be established across the globe.

The invention and design that became the basis of all Teletext and similar systems was created in 1971 by the Philips Lead Designer for VDUs, John Adams. John Adams was a highly creative engineer with a passion for computer electronics, he was a specialist in digital systems architecture and had an expertise in text generating hardware. In the same year he produced a technical proposal for a Teletext system, which was released to UK broadcasting authorities. His configuration contained all the fundamental elements of classic Teletext including pages of 24 rows with 40 characters each, page selection, sub pages of information and vertical blanking interval data transmission. The information he supplied laid out the format and standards for Teletext - an essential step to enable the first test transmissions and their successful reception.

A major objective for Adams during the concept development stage was to make Teletext affordable to the home user. In reality, there was not the slightest chance of making an economical Teletext system with 1971 technology. However, as low cost was essential to the project's long term success, this obstacle had to be overcome.

Adams built a fully functional Teletext prototype in 1971 and the first test transmissions were made by the BBC in 1973. His invention enabled the world's first widely used implementation of the information revolution. In the UK, his format and standards for Teletext were eventually adopted by the BBC as Ceefax and by the Independent Broadcasting Authority - IBA (Oracle). They also formed the basis of British Telecom Prestel and other similar telephone text services in many other organisations and countries. In addition, they were the basis of the British Teletext standards and of the "Broadcast Teletext Specification" which was published in September 1976 jointly by the IBA, the BBC and the British Radio Equipment Manufacturers' Association. The standards became international as the European Teletext standards and as the "World System Teletext" (WST) and formed the base of all Teletext systems built throughout the world for the rest of the century. The highly successful Philips Teletext chip sets, of which many millions were made, were also based on Adams' original design and concepts.

Development

Teletext is a means of sending text and simple geometric shapes 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 (GPO), whose telecommunications division later became British Telecom, had been researching a similar concept since the late 1960s, known as Viewdata. Unlike Teledata 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 pages of 24 lines with 32 characters each, while ORACLE offered pages of 22 lines with 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-1980s 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 with a screen size above 15 inches (teletext is still usually only an option for smaller "portable" sets). From the mid-1980s 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

Screenshot of an Electra teletext page.

World System Teletext

Main article: World System Teletext

NABTS, 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. Station KSL in Salt Lake City, Utah, premiered a teletext service using Ceefax. They were followed by American television network CBS, which carried out preliminary tests on both the British Teletext and the rival French Antiope system.[3][4]

One of the most prominent providers was the Electra teletext service, using WST, broadcast from the early 1980s and 1990s until 1993 on American cable channel WTBS.

Despite this, the system never caught on in the USA partly due to EIA-608 being deployed for captioning before Teletext was introduced and the higher cost of Teletext receivers.

Rival Telidon system

Main article: Telidon

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 Telidon were developed into NAPLPS.

Although there were numerous attempts to introduce NAPLPS services in North America, none of these was 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.

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 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.

Data transmission

A standard PAL signal contains 625 lines of video data per screen, broken into two "fields" containing half the lines of the whole image, divided as every odd line, then every even line number. 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.

Teletext in the PAL B system can make use the VBI lines 6–22 in first half image and 318–334 in the other[5] to transmit 360 data bits including clock run-in and framing code during the active video period at a rate of 6.9375 Mbit/s 25 ppm[5] using binary NRZ line coding.[6][5] The amplitude for a "0" is black level ±2% and a "1" is 66±6% of the difference between black and peak white level.[5] The clock run in consist of 8 times of "10" and the framing code is "11100100".[5] The two last bits of the clock-run in shall start within 12+0.4
−1.0 µs from the negative flank of the line synchronization pulse.[5]

The 6.9375 Mbit/s rate is 444 × nominal fH, i.e. the TV line frequency.[5] Thus 625 * 25 * 444 = 6 937 500 Hz. Each bit will then be 144 ns long. The bandwidth amplitude is 50% at 3.5 MHz and 0% at 6 MHz.[5] If the horizontal sync pulse during the vertical synchronization starts in the middle of horizontal scan line. Then first interlace frame will be sent, otherwise if vertical synchronization let the full video line complete the second interlace frame is sent.[5]

Like EIA-608 bits are transmitted in the order of LSB to MSB with odd parity coding of 7-bit character codes.[5] However unlike EIA-608, the digital DVB version is transmitted the same way. For single bit error recovery during transmission, the packet address (page row and magazine numbers) and header bytes (page number, subtitle flag, etc.) use hamming code 8/4 with extended packets (header extensions) using hamming 24/18, which basically doubles the bits used.[5]

The commonly used standard B uses a fixed PAL subtitling bandwidth of 8600 (7680 without page/packet header) bits/s per field for a maximum of 32 characters per line per caption (maximum 3 captions - lines 19 - 21) for a 25 frame broadcast. While the bandwidth is greater than EIA-608, so is the error rate with more bits encoded per field. Subtitling packets use a lot of non-boxed spacing to control the horizontal positioning of a caption and to pad out the fixed packet. The vertical caption position is determined by the packet address.

Teletext binary NRZ encodings[6]
Standard Color
system 		Informational
CVBS Lines 		Bit rate
[Mbit/s] 		Waveform Bits per line
(including run-in) 		Max. Characters
(per page row)
A (France) SECAM 7–18 6.203 squared Sine wave 320 35
B (global) NTSC 10–18 5.727 Symmetrical about 1/2 bit rate 296 32
PAL 7–18 6.938 Symmetrical about 1/2 bit rate 360 40
C (NABTS) NTSC 10–18 5.727 Raised cosine 100% roll-off 288 31
PAL-60 5.734
D (Japan) NTSC 10–18 5.727 Controlled cosine roll-off of 0.6 296 32
PAL-60 5.642 100% cosine roll-off

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 the resulting rate is 7,175 bits per second per used lines (41 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 mono spaced 40×24 character grid. Characters are sent using a 7-bit codec, with an 8th bit employed for error detection.[7] 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. The Philips P2000 homecomputer's video logic was even based on a chip designed to provide teletext services in TVs.

Later developments

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

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

Main article: 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 at 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 do not 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.[8][9]

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 an 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;

Digital teletext

NRK digital teletext

Digital television introduced the misnomer "digital teletext" informational replacement which, despite the previous teletext standard's digital nature, uses an interpreted binary language, such as MHEG-5 and Multimedia Home Platform (MHP). However, standard teletext still remains the de facto way of delivering captioning in SD PAL markets. 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 (There are plans for an immediate transition to digital terrestrial transmission in some countries, such as Ireland on 24 October 2012). 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 informational service

A number of broadcast authorities have recently ceased the transmission of teletext services, notably CNN International.[10] Most pages are still available, although they have not been updated since 31 October 2006.

The BBC phased out Ceefax in October 2012 as part of 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.[11] Teletext ended in each region after analogue broadcasts finished. See Digital switchover dates in the United Kingdom.

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

In New Zealand, TVNZ Access Services announced the discontinuation of the service on April 2, 2013. A claim about equipment failures and that web sites have been used instead has been given as the reason.[12]

In Singapore, MediaCorp announced that they will discontinue its eponymously titled service Teletext, with effect from 30 September 2013.[13]

Subtitling still continues to use Teletext in these countries with some providers switching to using image based DVB subtitling for HD broadcasts. New Zealand solely uses DVB subtitling on terrestrial transmissions despite Teletext still being used on internal SDI links.

See also

References

  1. ↑ Teletext: Soon You'll Be Punching Buttons And Talking Back To Your TV
  2. ↑ Sterling, Christopher H.; Kittross, John M. (1990). Stay tuned : a concise history of American broadcasting. Wadsworth Pub. Co. ISBN 978-0-534-11904-1.
  3. ↑ Teletext: Soon You'll Be Punching Buttons And Talking Back To Your TV, Associated Press (syndicated), 16 May 1979
  4. ↑ KSL-TV's Teletext Testing Shows No Reception Problems, Infoworld, 18 February 1980. ůAlso includes a prescient analysis of the likely economics of teletext and videotex
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10 ETS 300 706 – Enhanced Teletext specification, May 1997
  6. ↑ 6.0 6.1 ee-techs.com – What is Teletext?, 2004
  7. ↑ Brice, Richard (2003). Newnes guide to digital TV. Newnes. p. 41. ISBN 978-0-7506-5721-1.
  8. ↑ "Teletext Transmission". ExamPointer. 1993. Retrieved 2009-07-14.
  9. ↑ Graziplene, Leonard R. (2000). Teletext: its promise and demise. Lehigh University Press. ISBN 978-0-934223-64-5.
  10. ↑ "CNNText". CNN. Retrieved 2010-01-30.
  11. ↑ "Teletext Gallery: Ceefax: The beginning of the end". Teletext Then and Now. 2001-09-19. Retrieved 2010-01-30.
  12. ↑ The Dark At The Top Of The Stairs (2012-12-13). "About TVNZ | Television New Zealand | Entertainment | TV One, TV2". Tvnz.co.nz. Retrieved 2014-05-01.
  13. ↑

External links

Look up teletext in Wiktionary, the free dictionary.