[BBC-Micro] CEEFAX short story contest

[Rob O'Donnell]

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.