Date   : Sun, 07 Jun 2009 22:19:46 +0100
From   : mfirth@... (Michael Firth)
Subject: Teletext Display Error

Ian Wolstenholme wrote:
> I've noticed a problem with certain Teletext pages being displayed
> incorrectly via the Teletext adapter.
>
> Actually, I have been aware of this problem for several years but I put
> it down to bad tuning on my Acorn Teletext adapter, but now I've noticed
> the same problem using the Morley Teletext adapter on a different aerial.
>
> It occurs on lines where there is a coloured background, and the text
> colour does not display, leaving a "blank" line of colour.
>
> If anyone is still using Teletext and can go to page 704 on BBC-1, this
> is an index of the local government results between pages 707 and 712.
> On my Teletext adapters, the two lines for page 707 are blank, the first
> line is white and the second line is yellow.  The second line for page
> 709 and the first line for page 710 are both white.
>
> (I can probably take some pictures to illustrate if it will help.)
>
> Is there a way to cure this problem?
>
>   
I think this problem is likely to be being caused by the BBC Micro 
having an early version teletext display chip (the SAA5050), which 
doesn't support some of the features used in modern teletext pages.

It isn't directly caused by the teletext adapter, or the software, just 
by the design of the BBC Micro Teletext system, which put the detection 
and decoding functions in the external adapter, but left the display to 
the BBC Micro itself.

I doubt there's an easy fix, as you'd need a compatible replacement for 
the 5050 that supports the new features, and I expect that most teletext 
systems were more integrated by the time the new features became 
commonplace.

Regards

Michael