varied from over 6µs early to over 3µs late relative to the mixed syncs input reference. Digital multipliers are used to key the map onto the foreground and background data streams. These are full 8 x 8 bit devices, and correct scaling is incorporated to ensure unity gain where necessary. The two keys are processed to prevent any excess amplitude after combining. The two data streams are added digitally, before being blanked. Normally digital blanking needs to be shaped to conform to PAL system I, but since all the signals are generated internally, correct shaping is naturally included within the data. (In any case the start and end of all lines are black.) A new, triple-video, analogue-to- digital converter has been designed to provide the YUV outputs and an analogue matrix used for two sets of RGB outputs. Both these units employ close tolerance components to minimise drift, and hence regular alignment. A test waveform is included within the system for checking output levels and matrix accuracy. The system also includes a large power supplier, based on a commercial unit, and a BBC designed clock generator which is common with other digital equipments. The main store of the system is known as the 'sequence' memory. This holds the data for the map of the world for each of the 600 fields that are displayed. This data is compressed by a coding system which combines the benefits of traditional run-length coding, with the advantages of pixel definitions. There is physical space for up to 7.5 Mbytes of memory, although the addressing can access up to 16 Mbytes. For this application a field of data is stored in less than 8 kbytes of memory space, as opposed to over 400 kbytes for a full field store. To the right of these cards are five unused slots and to the right of those is a card with a red handle, which is the processor card. The player can be made to display variants of images by changing some switch settings on the processor card. Software The starting point for the sequence data was a purchased data-base of a Mercator's projection map of the world. This was edited by Computer Graphics to remove all political boundaries, and transferred from their VAX-11 computer into the Quantel Paintbox. This was used for a 2:1 size reduction which incorporated the anti-aliasing algorithms. The data was then transferred