Date   : Fri, 10 Dec 2004 11:28:19 +0000
From   : Richard Gellman <splodge@...>
Subject: Re: Video ULA shenanigans

>>Another potential mode is high clock rate, teletext -
>>I haven't tried this though.
>>    
>>
>
>This one I hadn't considered.  Has anyone ever got any results out of this?
>An 80-column teletext mode would be great, but as possible as it seems in
>my head, I bet there's gotchas...
>  
>
One or two...

Firstly, such a thing is impossible, not because of rates, etc., but 
because of the way the chip is designed. The chip generates a 40x25 
display. No arguments. If you feed weird clock rates at it, at best you 
*might* get 80x12.5, but my guess is after 40 chars the teletext chip 
just idles until the next line sync pulse.

The net effect is you get 40x25, but squashed on the left side of the 
screen.

IIRC, the internal circuitry uses a separate clock for the teletext 
which is controlled by the teletext bit in the Video ULA. Thus Acorn 
have effectively prevented such a mode. :(

-- Richard Gellman