This text is a slightly edited version of part of my third year project report. It deals with the initial development of the teletext and viewdata systems, up to about the early 1980's, and explains the reasons why the two systems are so similar, despite their different origins.
The teletext system was initially devised in the early 1970's by engineers from the BBC and the Independent Broadcasting Authority (IBA), at first working independently but later as a combined force. Teletext allows a viewer to display a large number of pages containing information about the news, current events, weather, traffic, travel information, stock market, etc. It is transmitted along with standard TV signals. This makes used of the fact that about 30 of the 625 scan lines on a TV screen are not actually used to allow time for the scanning beam to return to the top of the screen when it has reached the bottom. Initially, some of these unused lines were used to broadcast test signals, but it was soon realised that they could also be used to provide a public information service available on the TV. The initial BBC and IBA systems were slightly different. The BBC version originally called Teledata but later changed to Ceefax (See Facts) was based on a page containing 24 rows of 32 characters per row whilst the IBA version, called ORACLE (for Optional Reception of Announcements by Coded Line Electronics), allowed 22 rows of 40 characters per row.
Once it had been shown that transmission of data in this way was feasible, the two different standards were combined to create the system now known as teletext. The best features of both systems were employed, resulting in a page consisting of 24 rows with 40 characters per row. The speed at which pages were transmitted was also increased (from about one per second with ORACLE and two per second with Ceefax) to four per second. The maximum number of pages available was also increased to 800, and provision made for colour and graphics to be used. The teletext system finally became operational in 1976, following some alterations to the standard to add more facilities. One of the relatively recent innovations to teletext is the introduction of fastext. On a standard TV-based teletext receiver, the fastext system normally consists of four buttons on the handset, corresponding to four 'links', displayed at the bottom of the page on the TV screen. The links and buttons are normally colour coded, as red, green, yellow and blue. For example, the red link could correspond to News Headlines, the green link to Sport, the yellow to Weather and the blue to Horoscopes. Pressing the green button will take the viewer directly to one of the main Sport pages. There is sometimes a button marked Index on the teletext handset, which takes the viewer to the index page directly preceding the page the user is currently viewing.
Viewdata (sometimes referred to as videotex) is similar to teletext in providing a public information system. The system was developed by the Post Office, using the name Viewdata. Later the name was changed to Prestel, with viewdata becoming the name for the method of transmission. It became operational experimentally in 1978. There are some significant differences in the way viewdata and teletext operate, although with the arrival of the teletext standard, it was decided to make the viewdata signals compatible with those for teletext. This included making the same range of characters available for pages. The most important difference is viewdata is not transmitted using TV signals. Instead, the information is transmitted using the standard telephone network. This allows for more two way interaction by the user, such as the transmission of messages (known as responses) to other people. There are also some other differences, mainly to do with the page numbering schemes. Teletext uses a 3 digit numeric identifier, while viewdata uses identifiers with up to 10 characters, with the last character always being a lower case letter and all other characters being numeric. This allows for a much greater range of pages. There are also some operational differences, concerned mainly with movement around the system. The viewer can jump directly to one of 11 different pages, by using keys 0 to 9, and the # key. The page that is linked with the # key is always the frame with the same 9 digit numerical part of the page number, but the next lower case letter in sequence. For example, if # was pressed on page 0a the system would attempt to move to 0b. This method of navigation uses a similar idea to the teletext fastext system, but allows a much greater number of pages to be linked. This is the main method of navigating around a typical viewdata system.
A large number of effects are available with both teletext and viewdata systems. Up to eight colours (including black and white) can be displayed, as either text, graphics or background colours. Flashing and double height text and graphics are also available.
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