[BBC-Micro] CEEFAX short story contest

From: Rob O'Donnell <classiccmp.org_at_irrelevant.fsnet.co.uk>
Date: Wed Sep 22 07:38:53 2004

At 02:26 22/09/2004, Tony Duell wrote:


>I wonder if the stnadard video modulators would have enough video
>bandwidth for this. I know of at least one device which used such a
>modulator without an audio input, and generated the 6MHz sound
>intercarrier with a separate oscillator, mixed it with the video and fed
>it into the modulator. It worked. So you might get away with feeding in
>teletext data too.


Some years back, (about '89) I tried generating teletext signals with a BBC
micro itself - fiddled the video generation registers so that the
displayable portion of the screen actually started up in the VBI, and use
the bit-mapped graphics to create the relevant bit-stream on the right lines.

My reference on this was a library book "Teletext and Viewdata" (Steve A
Money, Newnes Technical books. I have the 1981 edition here...) It goes
into great detail on how the original teletext spec works, timings, block
diagrams and snippets of example schematics, etc.

I never actually got it working much further than getting the TV to
recognise that there was /something/ there, as the only teletext TV we had
was at the other end of the house from the computer, and what with
stringing the wire up, and the constant walking backwards and forwards to
see if it saw anything, I got quickly tired. I still believe it should be
quite possible to achieve it on that hardware though.

Of course, modern teletext has a lot more extra features now, but I am sure
most TVs can cope with an "original" specification signal if given it;
different countries use different implementations, after all, and most TVs
these days are universal.

Rob.
Received on Wed Sep 22 2004 - 07:38:53 BST