Teletext is the generic name to text-based information retrieval services that were accessible through analogue TV in many countries in the late-20th century. In the early-1970s, technicians experimented with the idea of using the spare lines in TV pictures to send text information to TV viewers which was separate from the actual TV picture and could be flicked through using a remote control. Initially developed to provide subtitles for programmes for the deaf or hard-of-hearing, teletext services expanded to provide news summaries, sports results, recipes, stock market prices and more … They were an early restrictive top-down form of internet (without the hyper-links). These services continued to serve TV viewing populations in many places until the late-2000s when analogue TV transmitters were switching off to make way for digital broadcasting. By then, though, digital versions of teletext were in use (as well as the web). The system been depicted here is based on the world’s first teletext service – Ceefax (accessed through BBC television in the UK). All analogue teletext services around the world worked in the same way as Ceefax, but vary by spec.
http://en.wikipedia.org/wiki/Teletext
http://www.bbc.co.uk/news/uk-20032882
http://myweb.tiscali.co.uk/themicrouser/issues/05-03/howteletextworks.htm
“CEEFAX
broadcasts not only News and Information about national and international
events, but Sports Results and Stock Market prices, Recipes and Shopping Guides
for housewives, Weather and Travel Reports, Gardening Tips and TV Programme
Guides, Theatre Reviews and the Top Twenty records.” – from Ceefax press release (1977)
Ceefax
The first
ever teletext service was Ceefax (a play of the words “see facts”).
The idea
began in the late-1960s as a means to fax farming and stock market prices to
printers during close-down, devised by BBC engineers Geoff Larkby and Barry
Pyatt. Nicknamed BEEBFAX, this system remained experimental until it was
scrapped in 1970.
A fully-electronic
version (which became Ceefax) was announced on 23rd October 1972
followed by two years of test transmissions, finally going live on 23rd
September 1974. Starting off with 30pages, it expanded to over 600pages by
1985. By the 1990s 22million people were using Ceefax at least once a week.
Ceefax remained in operation until the last UK analogue TV transmitter was shut
down at 11:32pm (UK time) 23rd October 2012.
https://en.wikipedia.org/wiki/Ceefax
A good
use of gaps
The heart
of teletext is that all analogue TV systems leave gaps between frames when
transmitting pictures. These are the “vertical blanking interval” lines (which form the black horizontal line you’ll see if you alter the “horizontal hold” on an old analogue TV set). They’re there to help give the
electronics in early TV sets a chance to reset themselves for the next frame of
footage and prevent the electron beam drawing an annoying stripe on the screen.
It’s in these empty
gaps in the signal where the data that make up teletext pages are encoded.
Ceefax was
originally encoded on just two lines per frame. This was doubled to four in
1981. By the late-1980s this was increased to 6 lines.
It takes 24
lines to encode one whole teletext page. As BBC TV (using PAL) transmits pages
6lines per interlacing frame, it takes 4 frames to transmit a whole page. With
50interlacing frames been transmitted per second, 100 pages can be transmitted
every 8seconds.
Decoder
To view
teletext services, your TV had to have a special decoder. This decoder captures
the data sent in-between the frames in the TV signal and converts it to
viewable pages. To begin with, these decoders were aftermarket devices you connect
to your TV. Many early adopters even built their own decoders from sets and
instructions from electronics magazines. TVs with built-in decoders first
appeared in 1977. By 1984 1.5million sets with decoders were sold in the UK. By
the mid-1980s decoders were an optional extra for almost all new TVs in Europe.
A decade later they were a standard feature in TVs with screens bigger than
15in (they remained an option in smaller TVs).
Requesting
a Page
Every
page is sent out one after the other in a continuous loop. When the user
requests a particular page (by dialling a page number by remote control) the
decoder simply waits for it to be sent, and then captures it for display. In
order to keep delays reasonably short, most teletext services limit themselves
to a few hundred pages. 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.
Memory
Later
TVs (1980s onwards) usually had 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.
Pages
Each page is constructed out of a grid of 960 “character rectangles,” 24 by 40 blocks. Each block contains one “character,” each represented by a 8-bit binary code. These characters can be one from three overlapping sets of 96 characters, which include a set of 64 combinations of 6 blocks of two colours (for constructing graphics). Originally in black and white, Ceefax later upgraded to an 8-colour palette, which became the standard for all teletext systems.
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.
Because the data is attached to the actual TV signal teletext services were always up to date (providing if someone was inputting new information on the pages at HQ).
Subtitles
The original idea that led to Ceefax was to use the spare lines in TV signals to provide subtitles for shows for the deaf or hard-of-hearing. The first TV show to be subtitled through Ceefax was This is Ceefax, a film demonstrating the service that aired in 1975. This didn’t become a major thing on Ceefax until 1979, with the subtitling of Quietly in Switzerland, a documentary about deaf children. Due to tech limitations (no one could type fast enough), to begin with the subtitles were written in advance of broadcast, which limited their use to pre-recorded shows. Later on, the idea of using stenograph machines to transcribe live dialogue and a program that can translate their input into normal text was developed, making the subtitling of live shows possible. Ronald Reagan’s inauguration in 1981 was the first to be subtitled by such means. From September 1984 live subtitling became a regular thing. In the 21st century, voice recognition systems are regularly used to provide live subtitling.
It has been noted since the early days that subtitling services not always get it 100% right.
Speed
Teletext is
noted for their relative slowness. So slow that it has led to many jokes,
including this anonymous quote – “Oh, if I want to know what's on
television I could look on Ceefax, but actually it's a lot quicker to go out of
the house, go down to the newsagents, buy the Radio Times and come back
and look at it.”
However, this is a huge exaggeration. For those wondering, the actual
transmission rate of Ceefax was 6.9375 Mbits/s. But remember, the stream is
interrupted regularly by TV pictures.
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