8 Oct
2004
8 Oct
'04
10:50 a.m.
In the U.S., the "closed caption" system,
which is primarily for
text for hard of hearing folk, puts two bytes of ASCII into line 21
of the VBI (vertical blanking interval). It CAN be used for other
purposes.
This sounds very similar to the UK Teletext system (which is, indeed,
used to provide subtitles for deaf people, along with many 'pages' of
information). As I mentioned last night, this was used to distribute
software for the BBC micro.
Interesting thing : The page number is a 3 digit BCD number, but nowhere
in the hardware are the invald BCD nybbles (1010..1111) ever prohibited.
Pages with such numbers can't be selected on a normal TV, but can be
decoded and displayed by a computer-controller teletext decoder. And such
pagesd certainly exist...
-tony