5. How Video Displays Work

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 what you could consider to be a series of raster 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 beams inside the display, one for each primary color. To make the beams strike each dot for an equal amount of time, the beams are swept across the display in a constant pattern, called a raster.

We say "what you could consider to be a series of dots" because these raster dots don't actually correspond to physical phosphor dots. The physical phosphor dots are much smaller than raster dots -- they have to be, otherwise the display would suffer from severe moiré-pattern effects. The raster dots are really samples of the analog driver signal, and display as a grid of dots only because the peaks and valleys in the signal are quite regularly and finely spaced.

The pattern starts at the top left of the screen, goes across the screen to the right in a straight line, moving ever so slightly "downhill" (the downhill slope is too small to be perceptible). Then the beams are swept back to the left side of the display, starting at a new 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 beams are moved from the bottom right corner of the display (sweeping back and forth a few times) to the top left corner, and the pattern is started over again.

There is one variation of this scheme known as interlacing: here only every second line is swept during one half-frame and the others are filled in during a second half-frame.

Starting the beams at the top left of the display is called the beginning of a frame. The frame ends when the beams reach the the top left corner again as they come from the bottom right corner of the display. A frame is made up of all of the lines the beams traced from the top of the display to the bottom.

If the electron beams were on all of the time they were 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 beams are hard to control there. To reduce the distortion, the dots around the edges of the display are not illuminated by the beams (because they're turned off) even though the beams, if they were turned on, would be pointing at them. The viewable area of the display is reduced this way.

Another important thing to understand is what becomes of the beams when no spot is being painted on the visible area. The time the beams would have been illuminating the side borders of the display is used for sweeping the beams back from the right edge to the left. The time the beams would have been illuminating the top and bottom borders of the display is used for moving the beams 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 beams (according to the desired color) at each dot to generate a picture. The card also controls when the display moves the beams from the right side back to the left 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 beams 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 beams can stabilize. If the beams can't stabilize, the picture will not be steady.

For more information, see TV and Monitor Deflection Systems.

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