Teletext

Before Teletext

Your TV picture consists of a single spot of light created by an electron beam which sweeps quickly from left to right to form lines and more slowly from top to bottom to form frames. In the first half of the century, when the tv systems were designed, it took receivers a relatively long time to force the spot to “flyback” from the right-hand side of the screen to the left, and from the bottom to the top. Gaps were left in the transmitted signal that contained only black space and “blacker-than-black” synchronising pulses.

Modern sets have much shorter flyback periods, and more efficient blanking circuits, which means that the transmitted blanking period no longer needs to be black. In the sixties, broadcasters started to use the “spare” lines for test signals and then began considering what else they might put there.

In the early seventies, the BBC was experimenting with optional subtitles, and the IBA with source identification (SLICE), both of which have found their way into the present teletext services. They then realised the service would be more useful if it carried more general news and information for viewers to read. After some experiments, the BBC (CEEFAX, first broadcast as a thirty-page magazine on 23 September 1974) and IBA (ORACLE) in consultation with BREMA, the set manufacturers’ organisation, jointly produced the 1974 Broadcast Teletext Specification. In 1976 a second specification was published allowing for more display attributes and a modified character set, among other things.

The key to teletext is that the receiver contains a memory, to store data transmitted over a period of several frames, and a character generator to present the data on the screen. That means that data for a whole row of text (40 characters) can be transmitted in one tv line, and data for a whole page spread over several frames. At first, only two lines per field (four lines per frame) were allowed, and a 24-row page took about a quarter of a second to send. Nowadays twelve lines per field are the norm, and many more pages are transmitted.


 

Decoders

In 1975/76 Wireless World published a design for a teletext decoder that could be built by a home constructor with little or no test equipment. The design used about a hundred TTL (transistor-transistor logic) ICs on three printed circuit boards, and a kit was made available. The intention was to tap off a video signal from a colour tv and feed the decoded RGB text signal back to the set. Interfacing was quite tricky, especially as sets of that period did not have the smooth video response they do today. Later, other magazines published designs that took an aerial signal and modulated the teletext to be fed into the aerial socket of a tv set, as a VCR does. The displayed text quality was not as good as with an integral decoder, and certain features were not available is some designs, for example, coloured text and text displayed in a box within the picture, for newsflashes and subtitles.

In those days of valves, it was not usual to apply RGB to the cathodes of the display tube. Instead, a powerful video output valve fed luminance (Y) to the cathode, and smaller valves fed the colour difference signals (R-Y, G-Y, B-Y) to the grids. Because the colour difference signals were lower bandwidth than the luminance this saved on wideband power output stages. It also saved “matrixing” the colour signals to produce RGB – a process that’s all done on a chip these days.

Because the matrixing was done “in the tube” it meant there was no RGB path in which to insert the teletext, so it was necessary to apply an inverted signal to each colour difference circuit. Unfortunately, the restricted bandwidth led to fuzzy characters, and a circuit was included to widen the white parts of the letters so they became brighter and more distinct.


 

Character Rendering

The beauty of teletext is that text is transmitted as character codes which are economical with bandwidth and allows the decoder to deal with the rendering of the actual glyphs (character shapes) at whatever resolution the designer feels is appropriate. However, because character generator chips are mass-produced, only a few styles of character have been used during the thirty years of decoder production. Almost all use the dot-matrix method of rendering text, switching sharply between the foreground and background colours as determined by the character generator ROM. Some advanced decoders use a system called ‘anti-aliasing’ whereby some of the pixels are a blend of the foreground and background colours which results in slightly blurred, but smoother, less jagged glyphs.

There are several basic ways of rendering text on an interlaced display like a television set. The easiest is to use a 5 x 9 matrix and repeat the pattern on adjacent fields so that pairs of lines in the resulting 5 x 18 display are identical. A refinement of this is to interpolate the 5 x 9 matrix and add half-sized ‘fillets’ to smooth the diagonals.
This results in very slight flicker because alternate fields are different. Another way is to remove interlace altogether and superimpose the lines of alternate fields so that a 5 x 9 flicker-free display is produced. Only the full-screen display can be treated that way. Subtitles and mixed screens are rendered over the normal interlaced raster. Many of the decoders using this technique have higher-resolution 10 x 9 glyphs.

Finally, it is possible for the decoder to rasterise fonts using anti-aliasing software which results in different patterns on alternate lines, giving higher definition glyphs that are subject to a degree of flicker, though this is minimised by the use of shades in between the foreground and background colours. Digital tv receivers and of course modern computers use this method almost exclusively.


 

DVB Text and subtitling

DVB Subtitles on Channel 4

Analogue satellite transmissions, which came along in the 1980s, can carry the same teletext magazines as an ordinary terrestrial service. The data rides on the spare lines just above the top of the picture as usual.

Digital transmissions (DVB, for ‘Digital Video Broadcasting’) can also carry exactly the same Teletext pages, including subtitles, but they are encoded separately and sent mixed in with the audio, video and ‘housekeeping’ data streams. A digital decoder can do one or both of two things: it can convert the data to standard Teletext format using the spare lines for your telly to decode and display as before, or it can decode the data itself and display it on the screen – useful for recording subtitles etc.

But digital transmissions may contain a new kind of text and/or subtitling service. Teletext was designed to be extremely simple to decode and sparing of bandwidth – consumer digital electronics was still in its infancy in the 1970s and there is a limited amount of free space in the analogue television signal. The solution adopted was that each character on the screen took one data byte to transmit, and its position in the data stream corresponded directly with its position on the screen.

By contrast, the digital text uses a so-called ‘mark-up’ language – just like pages on the Web – and the decoder generates pretty lettering according to the verbose instructions it receives. Consequently, more colours can be used, the text is smoother, and graphics (including photographs etc) can be displayed. However, a lot more data has to be sent per page than with Teletext.

Unfortunately, the text systems at present used are not universal – in the UK alone, a different system is used for terrestrial and satellite – and a third system is being introduced on the continent of Europe. However, the DVB subtitling system is more standardised. In it, the shape and colour of the lettering are defined by the broadcaster and so subtitles on the various channels can appear very (or just subtly) different from each other. In general, however, they are all generated ‘on the fly’ by the broadcaster from the same original data as the Teletext subtitles.

Unfortunately, because of the extra data transmitted and the complexity required in the decoder, DVB subtitles often do not work nearly as well as their Teletext counterparts, arriving on the screen too late, in the wrong position or colours, or not at all. Sometimes, having arrived, they are reluctant to be dismissed.

Often, both DVB and Teletext subtitles are included on digital transmissions, and the viewer may select which ones to watch, but in the UK only DVB subtitles have ever been used on terrestrial, while on satellite Teletext ‘888’ subtitles have been the norm, with DVB ones being only recently introduced for the benefit of the new ‘Freesat’ satellite decoders which operate independently of the proprietory Sky Electronic Programme Guide and text system.

Sky digital satellite receivers ignored the DVD subtitles, and instead decode the Teletext ones, displaying them in a ‘fancy’ proportional font rather than the old Teletext dot-matrix style fixed-width font.


 

The End of Teletext

Teletext in the UK and Ireland all but vanished with the completion of the closedown of the analogue network on 23/24 October 2012. The public services magazines from Teletext Limited, 4-Tel, Sbectel and FiveText on the various UK commercial channels had already closed several years before, and Ceefax ended when the analogue transmitters at Divis in Northern Ireland were shut down for the last time. The Irish analogue network closed on the same day, along with its Aertel Teletext service. Subtitles are still transmitted by Teletext on satellite, alongside the new-fangled DVB ones, which are the only sort available on terrestrial television, but very few, if any, services at 28°E now carry actual full-page Teletext magazine pages.

In Europe, the situation is rather different. The majority of German public service stations still have comprehensive magazines covering radio and tv programmes, if not news, weather and all the other content they used to carry, as do the services of many other countries on both Astra at 19.2°E and Hotbird at 13°E.

The teletext service on BBC World on all its satellite outlets closed in October 2012, a few days after the date advertised on its front page.

TV5MONDE, the French international public service channel, closed its service of News, weather, sport and comprehensive tv schedule details on 10 June 2013, while still proclaiming it to be ‘an ultramodern medium’. The page shown here mentions that all the contents of the teletext service are now available on the TV5MONDE website, while subtitles in several languages continue to be sent as VBI Teletext and/or DVB format.

Teletext magazines on the public service channels of France Télévisions either vanished or stalled (showing the same outdated pages) several years ago without any announcement.


 

After Teletext

Although in the UK and France almost all public service broadcasters have stopped offering a broadcast information service of any kind, in the rest of Europe teletext still flourishes for the time being, and some broadcasters are offering their Teletext service online, alongside, or instead of, the broadcast version.

See also:

The information on this page is compiled from the archive of Pembers’ Ponderings

 

 

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