Date   : Tue, 06 Jul 1993 09:03:55 +0200
From   : stjohn@... (Chris StJohn)
Subject: Colour Graphics

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.

(3) Do what everyone else does and build using TTL or PLAs.

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.

The biggest headache is the sync stuff. Digital electronics I can do, but it's
this analog sync lark that freaks me. I've got a circuit of a video board which
uses only TTL and another which uses the 6845 (1983 vintage!) so I'm hoping to
be able to work out that part. Recommendations for books would be appreciated.

Any hardware gurus out there who can help me/us ??

  ,--,             Chris StJohn, LIX, Ecole Polytechnique, 91128
 /    /_  __' _    Palaiseau, Cedex, FRANCE. (+33) 1.69.33.45.93
(____/ (_/ (_/_)     "He that breaks a thing to find out what it