224 lines
6.2 KiB
HTML
224 lines
6.2 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<HTML
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><HEAD
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><TITLE
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>How Video Displays Work</TITLE
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HREF="tools.html"><LINK
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TITLE="Basic Things to Know about your Display and Adapter"
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>X.org/XFree86 Video Timings HOWTO</TH
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><H1
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><A
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NAME="video"
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></A
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>5. How Video Displays Work</H1
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><P
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>Knowing how the
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display works is
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essential to understanding what numbers to put in the various fields
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in the file Xconfig. Those values are used in the lowest levels of
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controlling the display by the X server.</P
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><P
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>The display generates a picture from what you could consider to be a
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series of raster dots. The dots are arranged from left to right to form
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lines. The lines are arranged from top to bottom to form the picture.
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The dots emit light when they are struck by the electron beams inside
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the display, one for each primary color. To make the beams strike
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each dot for an equal amount of time, the beams are swept across the
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display in a constant pattern, called a raster.</P
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><P
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>We say "what you could consider to be a series of dots" because
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these raster dots don't actually correspond to physical phosphor dots.
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The physical phosphor dots are much smaller than raster dots -- they
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have to be, otherwise the display would suffer from severe
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moir<EFBFBD>-pattern effects. The raster dots are really samples of
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the analog driver signal, and display as a grid of dots only because
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the peaks and valleys in the signal are quite regularly and finely
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spaced.</P
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><P
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>The pattern starts at the top left of the screen, goes across
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the screen to the right in a straight line, moving ever so slightly
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"downhill" (the downhill slope is too small to be perceptible). Then
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the beams are swept back to the left side of the display, starting at
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a new line. The new line is swept from left to right just as the
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first line was. This pattern is repeated until the bottom line on the
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display has been swept. Then the beams are moved from the bottom
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right corner of the display (sweeping back and forth a few times) to
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the top left corner, and the pattern is started over again.</P
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><P
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>There is one variation of this scheme known as
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interlacing: here
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only every second line is swept during one half-frame and the others
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are filled in during a second half-frame.</P
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><P
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>Starting the beams at the top left of the display is called the
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beginning of a frame. The frame ends when the beams reach the the top
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left corner again as they come from the bottom right corner of the
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display. A frame is made up of all of the lines the beams traced from
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the top of the display to the bottom.</P
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><P
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>If the electron beams were on all of the time they were sweeping
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through the frame, all of the dots on the display would be
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illuminated. There would be no black border around the edges of the
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display. At the edges of the display the picture would become
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distorted because the beams are hard to control there. To reduce the
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distortion, the dots around the edges of the display are not
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illuminated by the beams (because they're turned off) even though the
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beams, if they were turned on, would be pointing at them. The
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viewable area of the display is reduced this way.</P
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><P
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>Another important thing to understand is what becomes of the beams
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when no spot is being painted on the visible area. The time the beams
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would have been illuminating the side borders of the display is used
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for sweeping the beams back from the right edge to the left. The time
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the beams would have been illuminating the top and bottom borders of
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the display is used for moving the beams from the bottom-right corner
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of the display to the top-left corner.</P
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><P
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>The adapter card generates the signals which cause the display to turn
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on the electron beams (according to the desired color) at each dot to
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generate a picture. The card also controls when the display moves the
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beams from the right side back to the left by generating a signal
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called the horizontal sync (for synchronization) pulse. One
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horizontal sync pulse occurs at the end of every line. The adapter
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also generates a vertical sync pulse which signals the display to move
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the beams to the top-left corner of the display. A vertical sync
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pulse is generated near the end of every frame.</P
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><P
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>The display requires that there be short time periods both before and
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after the horizontal and vertical sync pulses so that the position of
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the electron beams can stabilize. If the beams can't stabilize, the
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picture will not be steady.</P
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><P
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>For more information, see <A
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HREF="http://fribble.cie.rpi.edu/~repairfaq/REPAIR/F_deflfaq.html"
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TARGET="_top"
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>TV
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and Monitor Deflection Systems</A
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>.</P
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><P
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>In a later section, we'll come back to these basics with definitions,
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formulas and examples to help you use them.</P
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></DIV
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>Prev</A
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>Home</A
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>Tools for Automatic Computation</TD
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><TD
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WIDTH="34%"
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VALIGN="top"
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> </TD
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VALIGN="top"
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>Basic Things to Know about your Display and Adapter</TD
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> |