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<A HREF="VideoModes.html"><EM>XFree86 Video Timings HOWTO</EM></A>
 <b>:</b> <EM>Interpreting the Basic Specifications</EM><BR>
<b>Previous:</b> <A HREF="VideoModes4.html"><EM>Basic Things to Know about your Display and Adapter</EM></A><BR>
<b>Next:</b> <A HREF="VideoModes6.html"><EM>Tradeoffs in Configuring your System</EM></A>


<HR NOSHADE>
  <H2><A NAME="7"></A>5. Interpreting the Basic Specifications<A NAME="specs"></A></H2>

<p>This section explains what the specifications above mean, and some other
things you'll need to know.  First, some definitions.  Next to each in parens
is the variable name we'll use for it when doing calculations </p>
<p>
<dl>
<dt><b>horizontal sync frequency (HSF)</b><dd><p>Horizontal scans per second (see above). </p>

<dt><b>vertical sync frequency (VSF) </b><dd><p>Vertical scans per second (see above).  Mainly important as the upper
limit on your refresh rate. </p>

<dt><b>dot clock (DCF)</b><dd><p>More formally, `driving clock frequency'; The frequency of the
crystal or VCO on your adaptor --- the maximum dots-per-second it can
emit. </p>

<dt><b>video bandwidth (VB)</b><dd><p>The highest frequency you can feed into your monitor's video
input and still expect to see anything discernible. If your adaptor
produces an alternating on/off pattern, its lowest frequency is half
the DCF, so in theory bandwidth starts making sense at DCF/2. For
tolerably crisp display of fine details in the video image, however,
you don't want it much below your highest DCF, and preferably higher.</p>

<dt><b>frame length (HFL, VFL)</b><dd><p>Horizontal frame length (HFL) is the number of dot-clock ticks
needed for your monitor's electron gun to scan one horizontal line,
<em>including the inactive left and right borders</em>.  Vertical
frame length (VFL) is the number of scan lines in the
<em>entire</em> image, including the inactive top and bottom
borders.</p>

<dt><b>screen refresh rate (RR)</b><dd><p>The number of times per second your screen is repainted (this is
also called "frame rate").  Higher frequencies are better, as they
reduce flicker.  60Hz is good, VESA-standard 72Hz is better.
Compute it as
<blockquote><code>
<pre>
	RR = DCF / (HFL * VFL)
</pre>
</code></blockquote>
</p>
<p>Note that the product in the denominator is <em>not</em> the same
as the monitor's visible resolution, but typically somewhat larger.
We'll get to the details of this below.</p>
<p>The rates for which interlaced modes are usually specified (like 87Hz
interlaced) are actually the half-frame rates: an entire screen seems
to have about that flicker frequency for typical displays, but every
single line is refreshed only half as often.</p>
<p>For calculation purposes we reckon an interlaced display at its
full-frame (refresh) rate, i.e. 43.5Hz. The quality of an interlaced
mode is better than that of a non-interlaced mode with the same
full-frame rate, but definitely worse then the non-interlaced one
corresponding to the half-frame rate.</p>
</dl>
</p>

<H3><A NAME="8"></A>5.1. About Bandwidth:</H3>

<p>Monitor makers like to advertise high bandwidth because it constrains the
sharpness of intensity and color changes on the screen.  A high bandwidth
means smaller visible details. </p>
<p>Your monitor uses electronic signals to present an image to
your eyes.  Such signals always come in in wave form once they are converted
into analog form from digitized form.  They can be considered as combinations
of many simpler wave forms each one of which has a fixed frequency, many of
them are in the Mhz range, eg, 20Mhz, 40Mhz, or even 70Mhz.  Your monitor
video bandwidth is, effectively, the highest-frequency analog signal it can
handle without distortion. </p>
<p>For our purposes, bandwidth is mainly important as an approximate cutoff point
for the highest dot clock you can use. </p>

<H3><A NAME="9"></A>5.2. Sync Frequencies and the Refresh Rate:</H3>

<p>Each horizontal scan line on the display is just the visible portion of a
frame-length scan.  At any instant there is actually only one dot active on
the screen, but with a fast enough refresh rate your eye's persistence of
vision enables you to "see" the whole image.</p>
<p>Here are some pictures to help: </p>
<p>
<hr>
<pre>
     _______________________
    |                       |     The horizontal sync frequency
    |-&gt;-&gt;-&gt;-&gt;-&gt;-&gt;-&gt;-&gt;-&gt;-&gt;-&gt; |     is the number of times per
    |                      )|     second that the monitor's
    |&lt;-----&lt;-----&lt;-----&lt;--- |     electron beam can trace
    |                       |     a pattern like this
    |                       |
    |                       |
    |                       |
    |_______________________|    
     _______________________
    |        ^              |     The vertical sync frequency
    |       ^ |             |     is the number of times per
    |       | v             |     second that the monitor's
    |       ^ |             |     electron beam can trace
    |       | |             |     a pattern like this
    |       ^ |             |
    |       | v             |
    |       ^ |             |
    |_______|_v_____________|    
</pre>
<hr>
</p>
<p>Remember that the actual raster scan is a very tight zigzag pattern; that is,
the beam moves left-right and at the same time up-down. </p>
<p>Now we can see how the dot clock and frame size relates to refresh rate.  By
definition, one hertz (hz) is one cycle per second.  So, if your horizontal
frame length is HFL and your vertical frame length is VFL, then to cover the
entire screen takes (HFL * VFL) ticks.  Since your card emits DCF ticks per
second by definition, then obviously your monitor's electron gun(s) can sweep
the screen from left to right and back and from bottom to top and back DCF /
(HFL * VFL) times/sec.  This is your screen's refresh rate, because it's how
many times your screen can be updated (thus <em>refreshed</em>) per second! </p>
<p>You need to understand this concept to design a configuration which trades off
resolution against flicker in whatever way suits your needs. </p>
<p>For those of you who handle visuals better than text, here is one: </p>
<p>
<hr>
<pre>
        RR                                      VB
         |   min HSF                     max HSF |
         |    |             R1        R2  |      |
max VSF -+----|------------/----------/---|------+----- max VSF
         |    |:::::::::::/::::::::::/:::::\     |
         |    \::::::::::/::::::::::/:::::::\    |
         |     |::::::::/::::::::::/:::::::::|   |
         |     |:::::::/::::::::::/::::::::::\   |
         |     \::::::/::::::::::/::::::::::::\  |
         |      \::::/::::::::::/::::::::::::::| |
         |       |::/::::::::::/:::::::::::::::| |
         |        \/::::::::::/:::::::::::::::::\| 
         |        /\:::::::::/:::::::::::::::::::|
         |       /  \:::::::/::::::::::::::::::::|\
         |      /    |:::::/:::::::::::::::::::::| |
         |     /     \::::/::::::::::::::::::::::| \
min VSF -+----/-------\--/-----------------------|--\--- min VSF
         |   /         \/                        |   \
         +--/----------/\------------------------+----\- DCF
           R1        R2  \                       |     \
                          min HSF                |    max HSF
                                                 VB
</pre>
<hr>
</p>
<p>This is a generic monitor mode diagram.  The x axis of the diagram
shows the clock rate (DCF), the y axis represents the refresh rate
(RR). The filled region of the diagram describes the monitor's
capabilities: every point within this region is a possible video
mode. </p>
<p>The lines labeled `R1' and `R2' represent a fixed resolutions (such as
640x480); they are meant to illustrate how one resolution can be realized
by many different combinations of dot clock and refresh rate. The R2
line would represent a higher resolution than R1. </p>
<p>The top and bottom boundaries of the permitted region are simply
horizontal lines representing the limiting values for the vertical sync
frequency. The video bandwidth is an upper limit to the clock rate and
hence is represented by a vertical line bounding the capability region on
the right.  </p>
<p>Under 
<A HREF="VideoModes15.html#cplot">Plotting Monitor Capabilities</A>) you'll
find a program that will help you plot a diagram like
this (but much nicer, with X graphics) for your individual monitor.
That section also discusses the interesting part; the derivation of
the boundaries resulting from the limits on the horizontal sync
frequency. </p>

 <HR NOSHADE>
<A HREF="VideoModes.html"><EM>XFree86 Video Timings HOWTO</EM></A>
 <b>:</b> <EM>Interpreting the Basic Specifications</EM><BR>
<b>Previous:</b> <A HREF="VideoModes4.html"><EM>Basic Things to Know about your Display and Adapter</EM></A><BR>
<b>Next:</b> <A HREF="VideoModes6.html"><EM>Tradeoffs in Configuring your System</EM></A>



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