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There are other types of graphic display systems available, the vector scan being the beat known. However, vector scan devices and controllers, offer less colour shades and are considerably more expensive.

The display produced by the AVC can best be considered as a two dimensional grid, the grid being divided into small squares (called pixels – picture elements) each of which can be independently controlled by the computer. When viewed from a distance these combinations of pixels merge together to form shapes. This is similar to the way that newspaper photographs are produces – if you look closely at a newspaper photograph the individual picture elements can be seen.

The AVC uses the raster scan principle. In this an electron beam scans across the face of the display screen from left to right, then moves down 2 line and repeats the left, right, down sequence until it reaches the bottom of the screen. This can be compared with how a person reads or writes a page of text. As the beam traverses the screen its energy is converted into light by the special coating on the inside of the screen. The intensity of the light produced can be varied by changing the energy of the beam. Colour displays are produced by having three coatings (phosphors) to give the three primary colours. The whole screen is scanned fifty tines a second, which is just fast enough for the human eye to average out the light levels. Thus a stable picture is seen, and not just a moving spot of light. This is the way in which television pictures are formed.

Whereas with raster scan systems all the possible pixels are traversed by the beam, in vector scan systems the beam is moved only to the pixels which require ilumination. This provides very fast line generation, but does not give constant illumination.

The larger the number of pixels used the better will be the definition of the picture formed. The AVC has two resolution modes:

1. Single Density, in which 392 horizontal and 256 vertical pixels are used.
2. Double Density, in which 784 horizontal and 256 vertical pixels are used.

The AVC uses three separate bit-mapped memory planes to store the pixels in. Bit-mapped means that each pixel on the display screen is controlled by a known bit (or bits for colour) in the memory of the AVC. Each byte of the memory controls the status of wight horizontally adjacent pixels. The use of the three planes of memory allows the status of the three primary colour planes to be controlled by the data inserted into the AVC memory. Because colour is mixture of the three primary hues red, green and blue, any colour can be produced by controlling these three primary colours. Each of the three memory planes is used to control one of the primary colours.

These memory planes are all on the AVC board, and do not