Date : Sun, 19 Dec 2004 05:40:49 -0800 (PST)
From : "F. Haroon" <haroonnet2002@...>
Subject: Re: Video ULA shenanigans
You know what, I came across half-width teletext display at Sheffield
Interchange bus station, when used BBC Masters and CUB monitors to
display bus information etc. The display was in the main mall-area of
it showing all the destinations, times and bus services from A-Z. It
was an 80-column display and I wonder how they managed it? That was up
till 2002 though when they gave up on 8-bit technology for this
purpose. :-(
Fiaz.
--- Richard_Talbot-Watkins@... wrote:
> 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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