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 page 100 of German public broadcaster ARD

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.

It is closely linked to the PAL broadcast system used in Europe. 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 subtitles (or closed captioning).


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. The broadcaster constantly sends out pages in sequence. 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. More sophisticated receivers 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.[2] Teletext is also used for carrying special packets interpreted by TVs and video recorders, containing information about channels, programming, etc. (see Latter developments).

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.

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. In the UK the decline of teletext has been hastened by the introduction of digital television, though an aspect of teletext continues in closed captioning. On other countries the system is still widely used on standard-definition DVB broadcasts.

History

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.[3] 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.

In the early 1970s work was in progress in Britain to develop such a system. 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 U.K. agriculture. The original idea was the brainchild of Philips (CAL) Laboratories in 1970.

In 1971, CAL engineer John Adams created a design and proposal for UK broadcasters. 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.

A major objective for Adams during the concept development stage was to make Teletext affordable to the home user. In reality, there was no scope to make an economic Teletext system with 1971 technology. However, as low cost was essential to the project's long term success, this obstacle had to be overcome.

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.

Development

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 24 × 40 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.

Levels

Comparison between teletext Level 1.0 and teletext Level 2.5.

In the early 1980s a number of higher extension levels were envisaged for the specification, based on ideas then being promoted for worldwide videotex standards (telephone dial-up services offering a similar mix of text and graphics).

The most common implementation is Level 1.5, that supports languages other than English. Virtually any TV sold in Europe since the 1990s has support for this level. Around the year 2000 some station adopted Level 2.5 teletext / Hi-Text, that allowed for a larger color pallette and higher resolution graphics.

The proposed higher content levels included geometrically-specified graphics (Level 4), and higher-resolution photographic-type images (Level 5), to be conveyed using the same underlying mechanism at the transport layer. No TV sets currently implement the two most sophisticated levels.[4][5]

Decoders

Main article: Mullard SAA5050

The Mullard SAA5050 was a character generator chip used in the UK teletext-equipped television sets. In addition to the UK version, several variants of the chip existed with slightly different character sets for particular localizations and/or languages. These had part numbers SAA5051 (German), SAA5052 (Swedish), SAA5053 (Italian), SAA5054 (Belgian), SAA5055 (U.S. ASCII), SAA5056 (Hebrew) and SAA5057 (Cyrillic).

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

Besides the hardware implementations, it's also possible to decode teletext using a PC and video capture or DVB board.

North America

Screenshot of an Electra teletext page.

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. Throughout the period of analogue broadcasting, teletext or other similar technologies in the US were practically non-existent, with the only technologies resembling such existing in the country being closed captioning, TV Guide On Screen, and Extended Data Services (XDS).

North American Broadcast Teletext Specification

Main article: NABTS

NABTS was originally developed as a protocol by the Canadian Department of Communications, with their industry partner Norpak, for the Telidon system. It was similar to the European World System Teletext (WST, aka CCIR Teletext System B), but differences between the European and North American television standards and the greater flexibility of the Telidon standard led to the creation of a new delivery mechanism that was tuned for speed.

First demonstrated in the United States in 1978, NABTS was the standard used for both CBS's ExtraVision and NBC's very short-lived NBC Teletext services in the mid-1980s.

World System Teletext

Main article: World System Teletext

Station KSL in Salt Lake City, Utah premiered a teletext service using Ceefax. They were followed by television network CBS, which carried out preliminary tests on both the British Teletext and the rival French Antiope system.[6][7]

One of the most prominent providers was the Electra teletext service, using World System Teletext (WST), broadcast from the early 1980s on American cable channel WTBS. Electra also carried another teletext service on its higher-numbered pages, a service called Tempo. Tempo mainly carried sports (and other miscellaneous) information on its pages. At the time of Electra's closing in 1993, it was the only existing teletext service in the USA.

A few other services were offered by some large-market TV stations in the US throughout the 1980s, such as Metrotext from KTTV in Los Angeles and KeyFax from WFLD in Chicago. 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.

Telidon

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.

Other systems

Besides the US and UK developments, a number of similar teletext services were developed in other countries, some of which attempted to address the limitations of the initial British-developed system.

Antiope

Main article: Antiope (teletext)

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.

BBC Micro Teletext Adapter

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.

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.

Technology

Teletext information is broadcast in the vertical blanking interval between image frames in a broadcast television signal, in numbered "pages."

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.

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.


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[8] 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 bit/s[8] using binary NRZ line coding.[9][8] The amplitude for a "0" is black level ±2% and a "1" is 66±6% of the difference between black and peak white level.[8] The clock run in consist of 8 times of "10" and the framing code is "11100100".[8] 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.[8]

The 6.9375 Mbit/s rate is 444 × nominal fH, i.e. the TV line frequency.[8] 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.[8] 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.[8]

Like EIA-608 bits are transmitted in the order of LSB to MSB with odd parity coding of 7-bit character codes.[8] 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.[8]

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[9][10]
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.[11] 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.

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

Main article: 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.

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.

Digital teletext

NRK digital teletext

With the advent of digital television some countries adopted the misnomer "digital teletext" which, despite the previous teletext standard's digital nature, uses an interpreted binary language, such as MHEG-5 and Multimedia Home Platform (MHP).

Others use the same teletext streams as before on DVB transmissions, due to the DVB-TXT and DVB-VBI sub-standards. Those allow the emulation of analogue teletext on digital TV platforms, directly on the TV or set-top box, or via analog output, reproducing the vertical blanking interval data in which Teletext is carried.

Cessation of informational service

A number of broadcast authorities have recently ceased the transmission of teletext services.

(Subtitling still continues to use Teletext in these three 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. ↑ "The Evening Independent – Google News Archive Search". google.com.
  2. ↑ http://www.theinquirer.net/inquirer/feature/1566350/the-internet-kills-teletext
  3. ↑ Sterling, Christopher H.; Kittross, John M. (1990). Stay tuned : a concise history of American broadcasting. Wadsworth Pub. Co. ISBN 978-0-534-11904-1.
  4. ↑ "Teletext Transmission". ExamPointer. 1993. Retrieved 2009-07-14.
  5. ↑ Graziplene, Leonard R. (2000). Teletext: its promise and demise. Lehigh University Press. ISBN 978-0-934223-64-5.
  6. ↑ Teletext: Soon You'll Be Punching Buttons And Talking Back To Your TV, Associated Press (syndicated), 16 May 1979
  7. ↑ 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
  8. 1 2 3 4 5 6 7 8 9 10 11 ETS 300 706 – Enhanced Teletext specification, May 1997
  9. 1 2 ee-techs.com – What is Teletext?, 2004
  10. ↑ https://www.itu.int/dms_pubrec/itu-r/rec/bt/R-REC-BT.653-3-199802-I!!PDF-E.pdf
  11. ↑ Brice, Richard (2003). Newnes guide to digital TV. Newnes. p. 41. ISBN 978-0-7506-5721-1.
  12. ↑ "CNNText". CNN. Retrieved 2010-01-30.
  13. ↑ "Teletext Gallery: Ceefax: The beginning of the end". Teletext Then and Now. 2001-09-19. Retrieved 2010-01-30.
  14. ↑ 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.
  15. ↑ Archived September 5, 2013, at the Wayback Machine.
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