Date   : Wed, 07 Jul 1993 02:17:00 +1200
From   : David Andrew Sainty <David.Sainty@...>
Subject: Colour Graphics

   Date: Tue, 6 Jul 93 09:03:55 +0200
   From: stjohn@... (Chris StJohn)

   Re: David.Sainty's message about Beeb Colour

   Yes. Over the past couple of months I've been looking into what sort of colour
   support could be put on the side of a beeb. The options are:

   (1) Piggy-back the 6845 with an additional memory board to give true colour.
   This runs into problems mostly because of the way the 6845 is organised and
   the bandwidth required. Might have to array 32x16k DRAMs which would be
   fastiduous (not least as 16k Ds are comparitively expensive these days!).

   (2) Build a board which goes into the 1 MHz bus or some other port and which
   has a controller on-board. There are a few (very few) single-chip solutions
   for display circuitry. If anybody is interested in details, watch
   sci.electronics where soon I'll summarize the question I posted a week
   ago.

I was hopeing to avoid messing about inside the computer itself, but have
come to the conclusion that unless a complete seperate CRTC circuit is used,
there's no other feasable way of locking onto the original video signal.

But for anything serious, I'd throw idea 1. The colour generation is the
job of the video ULA, so at the very least that'd come into the picture too.


   The bandwidth required is 1 pixel every 150ns, which with modern memory is
   easy. I have designed a tentative circuit which will display 320x204x256colours
   using very cheap 4x64k DRAMs. (L2 each - need 2 of 'em). This circuit didn't
   arbitrate the bus, but with modern RAM accesses at 80ns there's no reason
   why it would be impossible to build a zero-contention video card.

Hmmm, how are you looking at accessing the video memory? If through the 1MHz
bus, accesses will be far and few.... What I was playing about with today
(at the paper stage) was as bad as updating the external video ram during the
horizontal sync! :-)

(That's obviously too slow for high res, one update per scan line)

   The biggest headache is the sync stuff. Digital electronics I can do,
   but it's this analog sync lark that freaks me.

Anything cooperating with the old circuitry is going to be messy with the sync
bit! With proper colour/high res in mind, I think a whole new circuit is
needed, which avoids the messy sync problems (And leaves your machine intact!)

   Recommendations for books would be appreciated.
   Any hardware gurus out there who can help me/us ??

Ditto!