80-Bus News


April–June 1982, Volume 1, Issue 2

Page 36 of 55

Colour your Computer Green (for envy?) by D. R. Hunt

What to date has been much speculated upon, is highly colourful, and so far has remained completely invisible? What is it, that by the time you get round to reading this, you should just about be able to rush out and buy? (At least don’t blame me if you can’t.) What is he talking about? (Does he ever know what he’s talking about?) Of course, it’s the Nascom Advanced Video Card!!

At a recent Nascom dealer meeting it was there in all it’s glory, and for the first time we were allowed to poke it a little bit. No, they weren’t generous enough to donate one to a worthy cause (me) so what follows is a description of what it is and what it does rather than a review of how good (or bad) it is.

First impressions are certainly good, and it could prove itself extremely useful in the educational field and, to a lesser extent (as far as the colour graphics is concerned), in the business field. The business area will be much more interested in it’s 80 x 25 screen format. For the home user, well I don’t know. I still maintain that any colour graphics is a facility that most could do without and on balance, will remain without. Although the initial cost is modest, there are hidden overheads. For instance, in the average home there is only one colour display device, the TV, and the only reason that your home computer is allowed in the house at all is because the ‘Mrs’ is able to sit in front of the box watching Coronation Street, whilst you play with your toys. Suggest that the colour TV should be connected to the computer instead of the aerial, and you, the computer, the newly acquired colour board and any other bits of your assorted iron-mongery, are liable to find themselves out in the street. The alternative, buy another colour TV. But if you are going to do that, then, realising that the rotten performance of a colour TV connected to a colour computer is almost all down to the poor bandwidth of the TV colour demodulating circuits (for a TV picture it doesn’t need to be better than about 1.5MHz), then, why not a colour monitor. You then find that what was once a relatively cheap colour facility has turned into a wallet depleting demon, at a cost of several times the original cost of the computer. Some home users will do this. But I think the majority will stay with playing Space Invaders and Galaxians in black and white. Perhaps I’m wrong, it remains to be seen.

What does it consist of and how does it work. Firstly it should be made clear that except when used with CP/M business type systems, Nascom consider the AVC as a peripheral rather than the main display device. This means that two monitors are really required, although the Nascom video output can be directed through the AVC if required. If two monitors are used, a B & W one would be used for displaying the programming details from the standard Nascom video, and a colour one to display the colour results. This is a useful scheme and one commonly adopted in colour software development. Anyway, the AVC itself is best considered as three planes of dynamic RAM arranged one above the other. The video controlling being achieved by Motorola MC6845 CRTC processor. Each plane is 16K and each plane deals with one of the three primary colours. Each plane is identically memory mapped to the screen in a similar fashion to the existing video RAM is mapped to the screen on the Nascom. The only difference that need concern you is that there is 16K of video RAM in each plane as opposed to the 1K of RAM in the Nascom. Using 16K of RAM allows a resolution of some 390 dots horizontally and 256 dots vertically. For full colour use, the three planes are effectively placed in parallel, providing three outputs one representing RED, one representing GREEN and one representing BLUE. If these output were fed directly to what is known as an RGB monitor (that’s one with three inputs, one red, one green and one blue), then that’s all there is to it. On the Nascom card further options are allowed, the RGB signals may be fed to an optional PAL encoder and then to an optional high bandwidth UHF modulator to provide a composite UHF TV signal. There is also a monochrome (B & W) monitor output.

On the monitor screen, the RED output lights up the red dots, the GREEN output lights the green dots, etc. Additive colour mixing takes place on the screen, so that if, for instance, the RED and BLUE signals were on together then a sort of

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Page 36 of 55