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
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
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.
Sample screens from the digitiser
processing with the digitiser