Teletext

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.

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 palette 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.[9][10]

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.

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.

Station KSL-TV 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.[11][12]

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.

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.

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.

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 home computer's video logic was also based on a chip designed to provide Teletext services in TVs.

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 events or train tickets and a limited amount of online banking.

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 use the VBI lines 6–22 in first half image and 318–334 in the other[13] 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[13] using binary NRZ line coding.[14][13] The amplitude for a "0" is black level ±2% and a "1" is 66±6% of the difference between black and peak white level.[13] The clock run in consist of 8 times of "10" and the framing code is "11100100".[13] 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.[13]

The 6.9375 Mbit/s rate is 444 × nominal fH, i.e. the TV line frequency.[13] 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.[13] If the horizontal sync pulse during the vertical synchronization starts in the middle of the 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.[13]

Like EIA-608 bits are transmitted in the order of LSB to MSB with odd parity coding of 7-bit character codes.[13] However unlike EIA-608, the 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.[13]

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.

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 analog 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 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 decoder updates the page in memory.

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.[16] 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.

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.

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 push-button telephone. A computer then instructs them to go to a Teletext page which is assigned to them for that session.

Usually, the page initially contains a menu of options, from which the user chooses using the telephone keypad. When a choice has been made, the selected page is immediately broadcast for viewing. This is in contrast with usual Teletext where the user has to wait for the selected page to be broadcast.

This technology enables Teletext to be used for games, chat, access to databases, etc. It overcomes the limitations on the number of available pages. On the other hand, only a limited number of users can be serviced at the same time, since one page number is allocated per user. Some channels solve this by taking into account where the user is 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.

These services have since been superseded by the World Wide Web.

NRK digital teletext

With the advent of digital television, some countries adopted the name "digital Teletext" for newer standards, despite the older Teletext standards' digital nature. "Digital Teletext" is encoded with standards including MHEG-5 and Multimedia Home Platform (MHP).

Other countries 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 by recreating analog output, reproducing the vertical blanking interval data in which Teletext is carried.

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

• International broadcasters: A live teletext is no longer available on CNN International.[18] Although many pages are still available, they have not been updated since 31 October 2006.
• United Kingdom: the founder of the world's first Teletext service, the BBC, closed its Ceefax service in 2012 when Britain adopted a fully digital television broadcast system. The BBC maintains a Red Button service on digital TV which includes access to the latest text news; that text news service is accessible on the BBC News Channel and during BBC One newscasts. Plans to shut it down in 2020 were changed and a reduced service is planned into 2021.[19] Many channels on Sky still broadcast Teletext subtitles and may still have a small number of active pages.[20] Analog teletext ended in each region after analog broadcasts finished: see Digital switchover dates in the United Kingdom.
• Australia: the Seven Network shut down the Austext service on 30 September 2009. They said that the technology has come to the end of its useful service life and is not commercially viable to replace.
• 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.[21]
• Italy: State-owned RAI was the first Italian broadcaster to launch its Teletext service, called Televideo, in 1984. Mediaset, the main commercial broadcaster, launched its Mediavideo Teletext in 1997. Both services are still active (as of 2020), as are those of some smaller local broadcasters. Other nation-wide Teletext services were instead switched off; for example, MTV Video was active between 2000 and 2010, while "LA7 Video", the Teletext service of La7, was launched in 2001 but was discontinued in 2014.
• Singapore: MediaCorp announced that they will discontinue its eponymously titled service Teletext, with effect from 30 September 2013.[22]

(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.)