Teletext - Technology

Technology

Teletext information is broadcast in the vertical blanking interval between image frames in a broadcast television signal. It is closely linked to the PAL broadcast system. 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 subtitling for deaf people or in different languages.

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.

Teletext allows up to eight 'magazines' to be broadcast, identified by the first digit of the three-digit page number (1–8). Within each magazine there may theoretically be up to 256 pages at a given time, numbered in hexadecimal and prefixed with the magazine number – for example magazine 2 may contain pages numbered 200-2FF. In practice, however, non-decimal page numbers are rarely used as domestic teletext receivers will not have options to select hex values A-F, with such numbered pages only occasionally used for 'special' pages of interest to the broadcaster and not intended for public view.

The broadcaster constantly sends out pages in sequence in one of two modes: Serial mode broadcasts every page sequentially whilst parallel mode divides VBI lines amongst the magazines, enabling one page from each magazine to be broadcast simultaneously. 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 in the magazine (parallel mode) or in total (serial mode) and the number of VBI lines allocated. In parallel mode, therefore, some magazines will load faster than others.

More sophisticated systems 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.

Because of its presentation of user-requested graphic information, teletext can be seen as a predecessor of the World Wide Web. 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.

It has proved to be a reliable text news service during events such as the September 11, 2001 terrorist attacks, during which the webpages of major news sites became inaccessible because of the high demand. Teletext is also used for carrying special packets interpreted by TVs and video recorders, containing information about channels, programming, etc. (see "Other teletext-related services").

Although the term "teletext" tends to be used to refer to the PAL-based system, or variants, the recent availability of digital television has led to more advanced systems being provided that perform the same task, such as MHEG-5 in the UK, and Multimedia Home Platform.