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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