406 lines
10 KiB
HTML
406 lines
10 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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>Interpreting the Basic Specifications</TITLE
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NAME="GENERATOR"
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TITLE="Basic Things to Know about your Display and Adapter"
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TITLE="Tradeoffs in Configuring your System"
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>X.org/XFree86 Video Timings HOWTO</TH
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><TR
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><TD
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WIDTH="10%"
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>Prev</A
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><DIV
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CLASS="sect1"
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><H1
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CLASS="sect1"
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><A
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NAME="specs"
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></A
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>7. Interpreting the Basic Specifications</H1
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><P
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>This section explains what the specifications above mean, and some
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other things you'll need to know. First, some definitions. Next to
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each in parentheses is the variable name we'll use for it when doing
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calculations</P
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><P
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></P
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><DIV
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CLASS="variablelist"
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><DL
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><DT
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>horizontal sync frequency (HSF)</DT
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><DD
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><P
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>Horizontal scans per second (see above).</P
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></DD
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><DT
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>vertical sync frequency (VSF)</DT
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><DD
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><P
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>Vertical scans per second (see above). Mainly
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important as the upper limit on your refresh rate.</P
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></DD
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><DT
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>dot clock (DCF)</DT
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><DD
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><P
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>More formally, `driving clock frequency'; The
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frequency of the crystal or VCO on your adaptor --- the maximum
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dots-per-second it can emit. </P
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></DD
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><DT
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>video bandwidth (VB)</DT
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><DD
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><P
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> The highest frequency you can feed into your monitor's video
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input and still expect to see anything discernible. If your adaptor
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produces an alternating on/off pattern (as in an interlaced
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mode), its lowest frequency is half the DCF, so in theory
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bandwidth starts making sense at DCF/2. For tolerately crisp
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display of fine details in the video image, however,
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you don't want it much below your highest DCF, and preferably
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higher.
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</P
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></DD
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><DT
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>frame length (HFL, VFL)</DT
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><DD
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><P
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> Horizontal frame length (HFL) is the number of dot-clock ticks
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needed for your monitor's electron gun to scan one horizontal line,
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<EM
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>including the inactive left and right borders</EM
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>. Vertical
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frame length (VFL) is the number of scan lines in the
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<EM
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>entire</EM
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> image, including the inactive top and bottom
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borders.
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</P
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></DD
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><DT
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>screen refresh rate (RR)</DT
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><DD
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><P
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> The number of times per second your screen is repainted (this is
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also called "frame rate"). Higher frequencies are better, as they
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reduce flicker. 60Hz is good, VESA-standard 72Hz is better.
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Compute it as</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="screen"
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> RR = DCF / (HFL * VFL)
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</PRE
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></FONT
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></TD
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></TR
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></TABLE
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><P
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>Note that the product in the denominator is <EM
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>not</EM
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> the same
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as the monitor's visible resolution, but typically somewhat larger.
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We'll get to the details of this below.</P
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></DD
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></DL
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></DIV
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><P
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>The rates for which interlaced modes are usually specified (like 87Hz
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interlaced) are actually the half-frame rates: an entire screen seems
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to have about that flicker frequency for typical displays, but every
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single line is refreshed only half as often.</P
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><P
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>For calculation purposes we reckon an interlaced display at its
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full-frame (refresh) rate, i.e. 43.5Hz. The quality of an interlaced
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mode is better than that of a non-interlaced mode with the same
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full-frame rate, but definitely worse than the non-interlaced one
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corresponding to the half-frame rate.</P
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><DIV
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CLASS="sect2"
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><H2
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CLASS="sect2"
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><A
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NAME="AEN216"
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></A
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>7.1. About Bandwidth</H2
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><P
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>Monitor makers like to advertise high bandwidth because it
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constrains the sharpness of intensity and color changes on the screen.
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A high bandwidth means smaller visible details.</P
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><P
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>Your monitor uses electronic signals to present an image to your
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eyes. Such signals always come in in wave form once they are
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converted into analog form from digitized form. They can be
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considered as combinations of many simpler wave forms each one of
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which has a fixed frequency, many of them are in the Mhz range, eg,
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20Mhz, 40Mhz, or even 70Mhz. Your monitor video bandwidth is,
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effectively, the highest-frequency analog signal it can handle without
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distortion.</P
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><P
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>For our purposes, video bandwidth is mainly important as an approximate
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cutoff point for the highest dot clock you can use.</P
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></DIV
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><DIV
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CLASS="sect2"
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><H2
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CLASS="sect2"
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><A
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NAME="AEN223"
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></A
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>7.2. Sync Frequencies and the Refresh Rate:</H2
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><P
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>Each horizontal scan line on the display is just the visible
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portion of a frame-length scan. At any instant there is actually only
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one dot active on the screen, but with a fast enough refresh rate your
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eye's persistence of vision enables you to "see" the whole
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image.</P
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><P
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>Here are some pictures to help:</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="screen"
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> _______________________
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| | The horizontal sync frequency
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|->->->->->->->->->->-> | is the number of times per
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| )| second that the monitor's
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|<-----<-----<-----<--- | electron beam can trace
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| | a pattern like this
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|_______________________|
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_______________________
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| ^ | The vertical sync frequency
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| ^ | | is the number of times per
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| | v | second that the monitor's
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| ^ | | electron beam can trace
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| | | | a pattern like this
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| ^ | |
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| | v |
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| ^ | |
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|_______|_v_____________|
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</PRE
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></FONT
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></TD
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></TR
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></TABLE
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><P
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>Remember that the actual raster scan is a very tight zigzag
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pattern; that is, the beam moves left-right and at the same time
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up-down.</P
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><P
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>Now we can see how the dot clock and frame size relates to
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refresh rate. By definition, one hertz (hz) is one cycle per second.
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So, if your horizontal frame length is HFL and your vertical frame
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length is VFL, then to cover the entire screen takes (HFL * VFL)
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ticks. Since your card emits DCF ticks per second by definition, then
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obviously your monitor's electron gun(s) can sweep the screen from
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left to right and back and from bottom to top and back DCF / (HFL *
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VFL) times/sec. This is your screen's refresh rate, because it's how
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many times your screen can be updated (thus
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<EM
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>refreshed</EM
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>) per second! </P
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><P
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>You need to understand this concept to design a configuration
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which trades off resolution against flicker in whatever way suits your
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needs.</P
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><P
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>For those of you who handle visuals better than text, here is one:</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="screen"
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> RR VB
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| min HSF max HSF |
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| | R1 R2 | |
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max VSF -+----|------------/----------/---|------+----- max VSF
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| |:::::::::::/::::::::::/:::::\ |
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| \::::::::::/::::::::::/:::::::\ |
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| |::::::::/::::::::::/:::::::::| |
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| |:::::::/::::::::::/::::::::::\ |
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| \::::::/::::::::::/::::::::::::\ |
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| \::::/::::::::::/::::::::::::::| |
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| |::/::::::::::/:::::::::::::::| |
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| \/::::::::::/:::::::::::::::::\|
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| /\:::::::::/:::::::::::::::::::|
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| / \:::::::/::::::::::::::::::::|\
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| / |:::::/:::::::::::::::::::::| |
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| / \::::/::::::::::::::::::::::| \
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min VSF -+----/-------\--/-----------------------|--\--- min VSF
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| / \/ | \
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+--/----------/\------------------------+----\- DCF
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R1 R2 \ | \
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min HSF | max HSF
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VB
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</PRE
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></FONT
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></TD
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></TR
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></TABLE
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><P
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>This is a generic monitor mode diagram. The x axis of the diagram
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shows the clock rate (DCF), the y axis represents the refresh rate
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(RR). The filled region of the diagram describes the monitor's
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capabilities: every point within this region is a possible video
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mode.</P
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><P
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>The lines labeled `R1' and `R2' represent a fixed resolutions
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(such as 640x480); they are meant to illustrate how one resolution can
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be realized by many different combinations of dot clock and refresh
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rate. The R2 line would represent a higher resolution than R1.</P
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><P
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>The top and bottom boundaries of the permitted region are simply
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horizontal lines representing the limiting values for the vertical sync
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frequency. The video bandwidth is an upper limit to the clock rate and
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hence is represented by a vertical line bounding the capability region on
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the right.</P
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><P
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>Under <A
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HREF="cplot.html"
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>Plotting Monitor
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Capabilities</A
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> you'll find a program that will help you plot a
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diagram like this (but much nicer, with X graphics) for your
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individual monitor. That section also discusses the interesting part;
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the derivation of the boundaries resulting from the limits on the
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horizontal sync frequency.</P
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></DIV
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>Basic Things to Know about your Display and Adapter</TD
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>Tradeoffs in Configuring your System</TD
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