The idea of a video digitiser is a simple one - it is a device for taking a picture from some video source, and converting it into a series of discrete pixels which can be displayed by the computer. However, in practice the situation is more involved. Firstly, the sampling of the picture needs to be done quickly, which necessitates a good deal of electronics, and secondly, the sampling must be coordinated with the sync pulses produced by the video source, otherwise the picture will be jumbled up randomly, and will appear as nonsense. These reasons, together with the limited screen resolution offered by the Beeb have meant that video digitisers for the Beeb have been almost non-existent. However, there is one, produced by Watford Electronics, which has been available for some time now, and it is this which we will look at here.

The digitiser comes in the form of a grey plastic box about six inches by four inches, with a sloping front that is two inches high at the back, and one inch at the front. For some reason, having seen the picture of the device in Watford's advert, I expected it to be about twice the size that it actually is. Protruding from the back of the unit is a 20-way ribbon cable that connects to the computer's user port, and which is about a metre and a half long. Also on the back is a BNC-type socket for connecting the video source (more of which later). On the front of the case are two switches, two knobs, and an LED. The left hand switch has three positions marked mode 0, mode 1 and mode 2, and not surprisingly these correspond to screen modes 0, 1 and 2 on the computer.

The other switch controls the signal levels used to distinguish between black and white. In 'auto' mode the system will attempt to work out the levels automatically, while with the switch in the 'manual' position you have to set the black and white levels using the two knobs. Finally, the LED indicates Sync pulses from the video source, and should flash on and off about once every two seconds provided a video source is connected to the digitiser, and the software is installed in the computer.

The software is supplied on a 16K EPROM which will need to be installed in the computer. If you are tight on ROM sockets then I suspect that an image of the ROM could be transferred to disc and loaded into sideways RAM, though I have not tried this. Having installed the software, the next stage is to connect a suitable video source. The digitiser needs to be connected to the video-out socket on either a camera or a video recorder. The system is compatible with both 75 ohm monochrome and composite colour (PAL) signals, though the images are always grabbed in black and white.

Initially, I tried to use a Ferguson Videostar camera, but the results obtained were far from acceptable. The manual does state that there can be problems with some cameras, and this was evidently one. As no other camera was available, the video output of an Archimedes 310 was used to feed the digitiser. You may experience difficulty in finding the correct lead to connect the video source and the digitiser, in which case a video connecting kit will prove invaluable. These are available from most video shops. Once everything is connected and powered up, the Sync light should flash reassuringly.

The digitiser is controlled by a number of star commands, though these can be packed up into a more usable form, as shown by the program elsewhere in this issue. The most important command is *IMAGE which grabs a screen from the digitiser. For this to work, the computer must be in the same mode as the switch on the digitiser, and the screen must not have been scrolled. The only parameter that *IMAGE can take is an 'N' to grab a negative of the image. It takes a couple of seconds to grab a single image, and therefore when using a camera it is best to mount it on a tripod. Similarly, the picture should be frozen if a video recorder is being used.

The commands IMPRNT and OPRINT dump the grabbed image to an Epson or Acorn Sparkjet printer respectively. An optional aspect ratio can be specified to ensure that the dimensions of the dump match those of the original picture. One very annoying feature of these commands is that they assume that the printer is setup to produce linefeeds each time a carriage return is received. If this is not the case, which it won't be for many printers, then the entire dump is printed on a single line. You therefore have to open up the printer and change the DIP switch settings.

IMLOAD and IMSAVE load and save grabbed images in a compressed format. Watford claim that a 20K screen will be reduced to between 1 and 12K, depending on complexity. Finally, the commands IMART, MASK and *IMMIX are used to convert a grabbed image into a format suitable for transfer to the AMX art packages and Stop Press.

In use, the only problem I found with the digitiser was setting it up. The 'auto' mode proved to produce a picture which was too dark, and lacking in contrast. I therefore had to resort to using 'manual' mode, and adjusting the two threshold controls in order to obtain a suitable image. The best way to do this is to use a simple program which repeatedly grabs images. Once setup, the controls should not need adjusting unless the video source is changed. The manual suggests that the setting up is performed in mode 2, however this is only of use on a monochrome monitor, as the colours prove to make the image almost unrecognisable. However, with a monochrome system, the colours in mode 2 are used to good effect to provide grey shades.

The documentation is in the form of a twenty-four page manual, with an additional section covering ROM installation. The manual explains what a digitiser is, gives details of all the available commands, and describes how to set up the system. There are also sections on using the commands within your own programs, and fault-finding.

Product: Beeb Video Digitiser
Supplier: Watford Electronics, Jessa House, 250 Lower High Street, Watford, Herts WD1 2AN. Tel: (0923) 37774
Price: £130.35 (inc. VAT and p&p)

Having found a suitable video source, the digitiser proved to perform very well, and the results produced are probably as good as is possible on the Beeb. My only niggle is that the printer dumps assume a particular printer setting, but apart from that I cannot fault the system. However, I would advise anyone who is considering buying the system to check if it is compatible with the camera they intend to use. The price of the unit also seems reasonable for what it offers.