Functions of Video Controller in Raster Scan System

Functions of Video Controller in Raster Scan System are explained following below.

  1. Deflection Voltage :
    In the output circuitry, it is video controller who who generates horizontal and vertical deflection voltage with the help of which monitor can sweep its beam across screen during raster scan. The various location in frame buffer and their corresponding screen positions are referenced in Cartesian coordinates in raster scan system. In graphics monitor, coordinates are started from the left corner of screen. In some of personal computers, origin of coordinate is at upper left corner of screen. in this type of system, positive values of x are increases towards right side and positive values for y are increases towards bottom to top as it is shown in following figure:


    Figure – Origin at the top left corner



    Working :
    To store coordinates of screen pixels, two register namely, X-register and Y-register are used. At the start of refresh cycle, X-register is set to 0 and Y-register is set to ymax. The address of (x, y) is translated to memory address where color value for this pixel position is stored. The role of video controller in this processor is to retrieve this color value for we can set binary number from frame buffer and breaks up into 3 separate parts and then send each of part to separate digital-to-analog converter. After conversion we obtain the voltages which are used to control intensity of three electron guns which are at (x, y) screen position by horizontal and vertical deflections.

    This procedure is as shown in the following figure:




    Figure – Operation of Video Controller


    After this, register-X is get increment by 1 and same procedure follows for next pixels along same line until x reaches xmax. On this scan line, when last pixel has been processed, x-register get reset to 0 and Y-register is start getting decrement by 1 for scanning of next line.

    Again same procedure is followed by y-axis which is that to scan lines until pixels on last scan line are generated. To display one pixel on screen, very little time is available. To display all frame on screen it consumes more time than given time. So to solve this issue, in single access multiple adjacent pixels are fetched from frame buffer. When this pixels get processed, other group of pixels are picked up from buffer for processing.

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