TELETEXT (or BROADCAST TELETEXT) is a television information
retrieval service created in the
It is closely linked to the
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
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.
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 .
* 1 History
* 2 Development
* 2.1 Rollout * 2.2 Levels * 2.3 Decoders
* 3 North America
* 4 Other systems
* 5 Technology
* 5.1 Data transmission
* 6 Later developments
* 6.1 Video Program System * 6.2 Interactive teletext * 6.3 Digital teletext * 6.4 Cessation of informational service
* 7 See also * 8 References * 9 External links
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
In 1971, CAL engineer John Adams created a design and proposal for UK
broadcasters. His configuration contained all the fundamental elements
A major objective for Adams during the concept development stage was
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
The first teletext test transmissions were made by
Ceefax in 1973.
After proliferation of the
A number of teletext services have been syndicated to web viewers,
which mimic the look and feel of broadcast teletext.
In 2016, the Teefax teletext service was launched in the United Kingdom to coverage by the BBC, ITV and others. Using a Raspberry Pi computer card as a set-top box , it feeds its service to standard televisions. Teefax content is a mix of crowdsourcing , syndication and contributions from media professionals who contributed heavily to broadcast teletext services. Teefax is also syndicated to a web viewer.
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).
Following test transmissions in 1973–74, towards the end of 1974
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.
Comparison between teletext Level 1.0 and teletext Level 2.5.
Main article: World_System_
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
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.
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.
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
Telidon system. It was similar to the European World System
WORLD SYSTEM TELETEXT
Main article: World System Teletext
Station KSL in
Salt Lake City, Utah
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
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.
Main article: List of Teletext 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.
Main article: Antiope (teletext)
In France, where the
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.
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.
The 6.9375 Mbit/s rate is 444 × nominal fH, i.e. the TV line frequency. 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. 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.
Like EIA-608 bits are transmitted in the order of LSB to MSB with odd parity coding of 7-bit character codes. 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.
The commonly used standard B uses a fixed
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
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. 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 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 .
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
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
CESSATION OF INFORMATIONAL SERVICE
A number of broadcast authorities have recently ceased the transmission of teletext services.
* International broadcasters: A live teletext is also no longer
(Subtitling still continues to use
* List of teletext services * Teletext systems * Digital terrestrial television * Digitiser – video games magazine * Electronic program guide (EPG) * Multimedia Home Platform (MHP) – open middleware system standard designed by the DVB project * NAPLPS – graphics language * NABTS – protocol used for encoding NAPLPS-encoded teletext pages * Park Avenue – teletext based soap opera * Programme Delivery Control (PDC) – indicating when transmission of a programme starts and finishes * SubRip – optical character recognition of subtitles superimposed on video (or in DVDs) * Text semigraphics – primitive form of raster graphics * Videotex – interactive service using teletext graphics * Radio Data System (RDS) – 1 kbit/s data transmission over FM
This article NEEDS ADDITIONAL CITATIONS FOR VERIFICATION . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed. (January 2010) (Learn how and when to remove this template message )
* ^ "The Evening Independent – Google News Archive Search".
* ^ Sterling, Christopher H.; Kittross, John M. (1990). Stay tuned
: a concise history of American broadcasting. Wadsworth Pub. Co. ISBN
* ^ "
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