Hardware Review

Archimedes Video Digitiser
Watford Electronics
£286.35 (January 1989)

Party tricks for the Archimedes

Mike Cook puts Watford's Video Digitiser for the Archimedes through its paces

ANYONE who has been to an Electron and BBC Micro User Show since the launch of the Archimedes will have seen the Watford Video Digitiser doing its thing. I am sure that all who have seen it have been impressed by its results, especially when demonstrated by its creator Mike Harrison. He has exploited the speed and power of the Risc chip to produce some real time image manipulation routines that have previously only been possible on equipment costing about a hundred times the price of the Archimedes.

The trick lies in the fact that the Archimedes can replace some of the necessary hardware needed for a digitiser of this type by simple interrupt-driven background tasks. This helps keep the cost to reasonable proportions and produces a very compact system.

The monochrome image is digitised to a resolution of 512 by 256 pixels with six bits being allocated to each pixel. Having six bits per pixel allows 64 different grey shades much better than can be seen using the standard colour monitor that comes with the Archimedes. The best this can do is 16 shades of grey which looks a lot better than it sounds but for the best quality work you need a monochrome monitor.

The digitiser plugs into the expansion backplane, and as these are not fitted as standard on the A300 series you will need one in order to get the system going. As well as the digitiser the unit also contains an eprom containing many support commands needed to drive it. Once digitised, an image can be used in many other programs like Artisan, Arctist and ARM Paint. You can set up a key trigger combination so that you can grab an image from inside another application and this makes it compatible with most pixel based graphics programs. The digitiser's automatic gain control eliminates the need for input level adjustments and ensures that the video signal is always being digitised over the full dynamic range of the system. Also a colour sub carrier notch filter allows the use of colour signal inputs without excessive patterning. Signals can be taken from a camera, video recorder or TV tuner.

As the digitised image resolution is fixed, the software scales the image to achieve the correct aspect ratio in the many different Archimedes modes. This is done by zero order interpolation or pixel repeating.

The digitiser will work in real time, grabbing an image in the time taken for one TV frame scan. This it stores in its own memory, so to get another frame it has to transfer the first image to the micro's memory. As this takes some time, you can only display full screens of digitised images in the 16 shade mode at a rate of 12.5 a second. If you are using a quarter screen size this is boosted to 25 frames a second, which is fast enough for all but the most demanding research projects.

All the VDU calls, options and utilities are adequately described in the 77 page manual. It was especially refreshing to see that this also contained an index. However, I will briefly describe a few of the tricks the digitiser can perform.

The demo program on the accompanying disc which puts the digitiser through its paces, has been the talking point of many shows. Images are rotated, scaled and bounced about the screen. There are many different ways to wipe from one image to the next and the centre screen shows a live image while all around are smaller pictures of past samples. Pictures are poured into ovals, rectangles and all manner of shapes as well as sequences being repeatedly replayed backwards and forwards - a sort of video rap.

The images can be given false colours and threshold sliced. The demo continues at a cracking pace, but there is also a mode to allow you to select just the effects you want.

The Watford Digitiser has more tricks up its sleeve. If the image is stationary it can grab an interlaced image of 512 by 512 and there is a routine to allow you to see it on normal non-multi sync monitors. If you have coloured filters you can make three successive frame grabs through a red, green and blue filter to make a colour image. Other routines allow you to correct the colour balance to produce a most acceptable result.

There are two image processing commands: *SMOOTH produces a nearest neighbour low pass filter to reduce noise and give a soft feel to the picture, while the amusingly named *ZIT will eliminate impulse noise in the form of isolated pixels.

Those who want to use a more extensive range of image processing operations will be pleased to know that the public domain AIM package also supports the Watford digitiser directly. It was produced by the Department of Physics at Delft University in the Netherlands and, best of all, it is free. Although this software will only run with a Mode 20 monitor, it offers a range of powerful operations like histogram equalisation, filtering and edge detection.

Grabbed images may be printed on Epson FX, MX and LQ type printers with the grey scale represented by dot patterns which you can select.

If you are at all interested in digitised images on the Archimedes this is a must. The power and versatility of the Archimedes coupled with Watford's Digitiser are very impressive. There is potential both for fun and serious applications and a set of built-in utilities allow you easily to explore the world of digital image processing.

avideodigitiser-1.jpg avideodigitiser-4.jpg
Sample screens from the digitiser


Image processing with the digitiser