Teletext

     TELETEXT is a one-way, or non-interactive, system for transmission of text and graphics via broadcasting or cable for display on a television set. A decoder or microchip resident in the TV set is needed to extract the teletext information.
     Teletext can be transmitted over one-way cable or over-the-air broadcasting via radio or television. In the case of TV, it can occupy a full channel or be encoded in the vertical blanking interval, or VBI. If you've ever mistuned a TV set and noticed the wide, black line that appears between "frames," you've seen the VBI. Before the development of teletext, the VBI was an unused portion of the television broadcast.
     Teletext can be liked to a sort of "Rolodex in the sky." It is a closed loop of pages of information that are transmitted one after the other, over and over again. The viewer uses an on-screen index or directory to choose the pages of information to be viewed. A page number is then entered and, after a slight delay, the page is displayed on the television screen.
     Although teletext may appear to the viewer to be interactive, it is not. When one punches in a page number on a teletext decoder, the machine simply waits for that page to be broadcast, captures it and displays it on the television set. The delay varies depending on the number of "frames" in the teletext "loop" or set, but it generally averages 5 to 30 seconds.

• Usually about 100-150 frames in a series, sometimes with 2 or 3 frames to a page.
• Service limited to a few hundred pages to make delays tolerable to viewers.
• User selects subsections from the main menu using a hand-held keypad similar to a TV remote control.
• Access time for a particular frame can be up to 30 seconds, a key factor.
• Many believe teletext has the best potential to become a mass medium because of the almost universal presence of television sets in many nations.

     In Britain, where teletext was developed, two systems competed. CEEFAX was owned and developed by the British Broadcasting Corp., a government entity, and ORACLE was operated by Philips and Associated Newspapers, a British newspaper company.

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