Teletext is a one-way non-interactive information retrieval service. A fixed number of information pages are repetitively broadcast on unused portions of a TV channel bandwidth. With these broadcasts a decoder at the television set is used to select and display pages. The ability to send simple Text and Graphics as part of a television signal was discovered by BBC engineers in the mid 1970's. Teletext was first screened on Australian and New Zealand Television on the 1st of February 1984. Although Teletext was originally intended as a service for the deaf and hearing impaired, the popularity of the service has grown, and today there are more than 7 million viewers in Australia and New Zealand alone. The UK is recording figures as high as 18 million people using the teletext service every week, a figure surprising for a technology originally intended for the hearing impaired.
As mentioned earlier, teletext is broadcast as part of the Television signal. Australia uses the PAL Television standard (Phase Alternating Line). This standard is also used by New Zealand, the UK and Europe. The USA uses the NTSC standard (National Television System Committee).
The PAL Television signal contains 625 Lines in which information can be carried, however only 576 of these lines are actually required to carry a Television picture. The extra lines are used to carry Teletext information.
As the Teletext information is outside the viewable area of the Television picture, a special decoder is required to display the Teletext page on a TV set. This is also why a VCR can't record Teletext. However there are some high end VCRs that are capable of recording Teletext pages.
As mentioned above, these pages are transmitted one at a time utilising spare capacity in the television composite video signal. When the complete service has been transmitted, the cycle is repeated, although the broadcaster can choose to transmit some pages more frequently if required. A domestic television set equipped with a suitable teletext decoder can display any one of these pages at a time. The viewer selects the page for display by means of a remote handset.
The teletext service is one way; the user is unable to request a page directly and can only instruct the decoder to search for a particular page in the teletext data stream. There will usually be a delay before the requested page appears in the transmission cycle. When the page is detected, the decoder captures and displays the information contained in the page. Thus the more pages within the service, the longer the access item. For this reason, broadcasters usually adjust the size of their services to obtain a cycle time of around 30 seconds, and therefore an average access time of 15 seconds.
Although originally intended for the hearing impaired, teletext is still gathering a fan base 20 years after first screening here in Australia. In this digital age its services are still proving relevant to a growing public.
Peter Hawkins 01:48, 29 Oct 2004 (EST)
