Teletext

Teletext (or broadcast teletext) is a television information retrieval service created in the United Kingdom in the early 1970s by the Philips Lead Designer for VDUs, John Adams. Teletext is a means of sending pages of text and simple geometric shapes from mosaic blocks to a VBI decoder equipped television screen by use of a number of reserved vertical blanking interval lines that together form the dark band dividing pictures horizontally on the television screen. It offers a range of text-based information, typically including news, weather and TV schedules. Paged subtitle (or closed captioning) information is also transmitted within the television signal.

It is closely linked to the PAL broadcast system used in Europe. Other teletext systems have been developed to work with the SECAM and NTSC systems, but teletext failed to gain widespread acceptance in North America and other areas where NTSC is used. In contrast, teletext is nearly ubiquitous across Europe as well as some other regions, with most major broadcasters providing a teletext service. Common teletext services include TV schedules, regularly updated current affairs and sport news, simple games (such as quizzes) and subtitles (or closed captioning).

Teletext is broadcast in numbered "pages." For example, a list of news headlines might appear on page 110; a teletext user would type "110" into the TV's remote control to view this page. The broadcaster constantly sends out pages in sequence. 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. More sophisticated receivers use a buffer memory to store some or all of the teletext pages as they are broadcast, allowing instant display from the buffer.

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 Internet, teletext is broadcast, so it does not slow down further as the number of users increase, although 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.

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