Date   : Sun, 13 Jun 2004 11:57:45 +0100 (BST)
From   : Pete Turnbull <pete@...>
Subject: Re: network-aware scrolling message thingy

On Jun 12, 18:50, Jules Richardson wrote:

> Well the video wall's looking more realistic project all the time.
Other
> people seem keen, plus I got a lead on another school full of
unwanted
> BBCs and Cubs earlier, so a source of hardware may have been found.

> One approach would be for the controlling system to take an image,
split
> it into chunks according to how many screens/BBC's are hooked up,
send
> each chunk to an off-screen image on each BBC over Econet, and then
tell
> each BBC on the network to display it's off-screen image at once.
>
> That method relies a) on Econet being fast enough to transfer the
data
> in a reasonable amount of time, and b) for the BBC to have enough
memory
> to buffer an entire local image off-screen in memory. Likely not a
prob;
> my memory's a bit hazy on both transfer speeds of Econet and how much
> memory the various BBC video modes need.

I think the suggestion of a line of monitors on Beebs, with something
scrolling across, is more realistic.  A short(ish) Econet with Beebs
will typically run at 150kHz - 200kHz clock speed; a DFS floppy at
125kb/s.  Allowing for the overheads, that makes them roughly
comparable -- and indeed that's what you'll find in practice.

That means you won't have time to transfer much data between frames, so
you'll either have to stick to something fairly chunky,
resolution-wise, or something each Beeb can do on its own, and just be
synchronised by the master unit.  Synchronising probably won't need to
be frequent; once or twice a second should do because Beebs all run at
pretty closely the same speed.

You're not limited to left-right scrolling, of course.  In graphics
modes, you can easily have each Beeb do some palette switching to fade
things in and out, or in Mode 7 you might achieve a similar effect by
switching changing the graphics characters in your 1-kilobyte image to
switch off alternate sixths of the graphics characters, and so on, and
you can scroll things up (and down).  If the screen mode is such that
the screen memory is small, you can do bank-switching by reprogramming
the start-of-screen address.  Stick to really simple techniques and
small amounts of data, and add imagination.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York