How Video Displays Work

# 2 How Video Displays Work

Contents of this section

Knowing how the display works is essential to understanding what numbers to put in the various fields in the file Xconfig. Those values are used in the lowest levels of controlling the display by the XFree86 server.

The display generates a picture from a series of dots. The dots are arranged from left to right to form lines. The lines are arranged from top to bottom to form the picture. The dots emit light when they are struck by the electron beam inside the display. To make the beam strike each dot for an equal amount of time, the beam is swept across the display in a constant pattern.

The pattern starts at the top left of the screen, goes across the screen to the right in a straight line, and stops temporarily on the right side of the screen. Then the beam is swept back to the left side of the display, but down one line. The new line is swept from left to right just as the first line was. This pattern is repeated until the bottom line on the display has been swept. Then the beam is moved from the bottom right corner of the display to the top left corner, and the pattern is started over again.

Starting the beam at the top left of the display is called the beginning of a frame. The frame ends when the beam reaches the the top left corner again as it comes from the bottom right corner of the display. A frame is made up of all of the lines the beam traced from the top of the display to the bottom.

If the electron beam were on all of the time it was sweeping through the frame, all of the dots on the display would be illuminated. There would be no black border around the edges of the display. At the edges of the display the picture would become distorted because the beam is hard to control there. To reduce the distortion, the dots around the edges of the display are not illuminated by the beam even though the beam may be pointing at them. The viewable area of the display is reduced this way.

Another important thing to understand is what becomes of the beam when no spot is being painted on the visible area. The time the beam would have been illuminating the side borders of the display is used for sweeping the beam back from the right edge to the left and moving the beam down to the next line. The time the beam would have been illuminating the top and bottom borders of the display is used for moving the beam from the bottom-right corner of the display to the top-left corner.

The adapter card generates the signals which cause the display to turn on the electron beam at each dot to generate a picture. The card also controls when the display moves the beam from the right side to the left and down a line by generating a signal called the horizontal sync (for synchronization) pulse. One horizontal sync pulse occurs at the end of every line. The adapter also generates a vertical sync pulse which signals the display to move the beam to the top-left corner of the display. A vertical sync pulse is generated near the end of every frame.

The display requires that there be short time periods both before and after the horizontal and vertical sync pulses so that the position of the electron beam can stabilize. If the beam can't stabilize, the picture will not be steady.

In a later section, we'll come back to these basics with definitions, formulas and examples to help you use them.

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