Date   : Fri, 10 Dec 2004 11:52:59 +0000
From   : Richard_Talbot-Watkins@...
Subject: Re: Video ULA shenanigans

Richard Gellman wrote on 10/12/2004 11:28:19:

> 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.

Yeah I wondered if that might be true - since teletext attributes last to
the end of a 40 column line, there must be a degree of hardwiring for a 40
column display.

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

...though I'd still love to see half-width teletext characters on screen,
if it were possible...

> 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. :(

That was the other thing I wondered - if the teletext clock speed is fixed,
perhaps doubling the Video ULA clock speed would result in only half of
each teletext character being displayed in each half-width cell?  That is,
unless as you say, the Video ULA clock speed is effectively overridden by
setting the Teletext bit.  An easy way to find out I guess would be to type
*FX154,91 in Mode 7 and see if setting the higher clock rate has any
visible effect at all.  If it did, I would expect at worst an unsynced
scambled display.  If it had no effect, then it's probably safe to assume
that Teletext mode ignores the Video ULA clock speed.

When I next get my hands on a Beeb, maybe I'll try these things out and see
what happens.



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