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Date   : Thu, 08 Jul 1993 03:33:27 +1200
From   : David Andrew Sainty <David.Sainty@...>
Subject: Colour Graphics 

   Date: Tue, 06 Jul 93 20:23:48 +0200
   From: stjohn@...

>>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.

>Ah, but the ULA can't cope with anything but the standard 8 colours (or
can it???)

Quite true, I misinterpreted what you were suggesting.

>>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! :-)

>So far the circuit I designed hooked up to the Vsync and Hsync outputs of the
>6845. Remember that (at least on my board) this is socketed, so inserting a
>board under it would be easy. Also, that way the beeb still generates the sync
>and you can use a few op-amps to sum the beeb screen with the new RGB. Like
>that you only need 1 monitor and no fiddling to switch modes.

I think any useful system must allow switching between the two screen
drivers. Summing is an interesting idea... But the old outputs are only TTL
level anyway, so switching isn't a big deal....

You must have a model B. The Master 6845 is soldered in. :-(

>The access would take place during frame blanking (which is about half the
>time) so it wouldn't be fast.
>
>Anyway, all this is academic as I'm looking at a chip solution from Intel/NEC -
>the 82716. A guy at ICL told me about it, and it seems to do everything more
>or less. It has minimal sprite and window functionality, outputs RGB (but I
>don't know what sort of standard yet) and just needs a data bus and some DRAM
>plugged into it. (this is what he's been telling me - the Ecole doesn't have
>an electronics lab so I can't get any data books to see for myself yet. Waiting
>'till I'm back in Cambridge next week)

Hmmm, what's the chance of my university having info on it! Not very likely!
:-( I'll have a look too though.

>To sum it all up, apparently a company in the UK is happily selling these
>things at L2 each (!!).

Hmmm, not bad!

>How would your idea go about getting the bandwidth out of the memory? I'm
looking
>to just pump the 8 bit wide data bus to a DAC and use the relatively high speed
>of the modern memory instead of the more conventional shift register approach.
>This is mainly because I remember what a pain programming raster blits on the
>Atari is due to its screen model (in fact, the beeb must be worse for non-aligned
>raster blits). Also, as I said it's far cheaper to buy a couple of 4464s than
>32 4116s.

Well, bandwidth really isn't a hassle yet with me, as I'm only wanting to
read data on a per-character basis.

>So... PLEASE tell me more about your ideas !!!

Well, character based attributes are first and foremost my priority. This
avoids most of your problems. I was also hopeing to design something that
most people could make relatively easilly.

There is an intensity line on my monitor, I was going to use that (This may
mean my monitor is TTL input, which would be bad news!).

Character addressing is easy, with (off the 6845) the Character clock line
to address characters, the Display enable line to both count
rastor lines and indicate the display start position, and Vsync to reset.
I could probably use Hsync instead of DE, and get both off the RGB port,
but as I need CCLK anyway off the 6845, I may as well get the other two!

Cost: a couple of ripple counters, a few latches, couple of multiplexers,
and a smidgeon of memory. Not bad for the pocket! Might even get it to fit
inside the computer!

I haven't yet put any thought to more complex design, but I think anything
serious should have its own controller, rather than trying to tack onto the
resident 6845. An external controller even allows the option of TWO
screens (Nice if you can afford it! :-)

Dave.
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