Raster Scan Display

All video cards operate on the same principle: They store an image in video memory (also called video RAM or VRAM for short) and generate the appropriate signals to display the image on the monitor's screen.

The monitor is the physical device that contains the display screen where the graphic and text output appears. The display screen is typically a phosphor-coated glass tube on which an electron beam traces the output image. On laptop computers, the display screen is a liquid crystal display (LCD). More expensive laptops use active-matrix LCD-display screens.

The image that appears on the monitor is made up of many horizontal lines, known as raster lines. An electron beam in the monitor generates the raster lines by sweeping back and forth on a phosphor-coated screen, as illustrated in Figure 3-1.

Raster line

Raster line

Figure 3-1: A Typical Raster-Scan Display.

The phosphor on the screen glows in proportion to the intensity of the electron beam. The glowing dot on the screen represents a picture element, or pixel. Thus, a line of the image is generated by controlling the intensity of the beam as it scans across the screen. The phosphor fades in a while, but if the lines are redrawn repeatedly, our persistence of vision creates an illusion of a steady image. Most PC monitors redraw an entire screen full of raster lines 50 to 90 times per second.

As Figure 3-1 shows, the electron beam scans an area larger than the actual view area of the display screen, but the electron beam is active only when the beam is in the viewable area. Also, after reaching the end of a line, the beam has to return to the start of the next raster line. This part of the beam's motion is known as the horizontal retrace. Similarly, when the beam reaches the bottom of the screen, it has to return to the first line to start another cycle of drawing. This period is known as the vertical retrace. The beam's intensity is reduced (the beam is blanked) during horizontal and vertical retrace so that those lines do not appear on the screen.

The video card generates the signals necessary to sweep the electron beam across the display and refresh the display at a rapid rate. There are two types of refreshing: interlaced and noninterlaced. In an interlaced refresh, each screen is drawn in two steps. First, the electron beam sweeps across the screen, drawing all the odd-numbered raster lines. Then the electron beam goes back to the beginning of the screen and draws all the even-numbered raster lines. Broadcast television uses interlaced refreshing. A noninterlaced refresh involves drawing all raster lines in a single step. Most video cards refresh the screen 60 times or more per second and use a noninterlaced refresh.

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