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<A HREF="VideoModes.html"><EM>XFree86 Video Timings HOWTO</EM></A>
 <b>:</b> <EM>Tradeoffs in Configuring your System</EM><BR>
<b>Previous:</b> <A HREF="VideoModes5.html"><EM>Interpreting the Basic Specifications</EM></A><BR>
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  <H2><A NAME="10"></A>6. Tradeoffs in Configuring your System<A NAME="trade"></A></H2>

<p>Another way to look at the formula we derived above is </p>
<p>
<blockquote><code>
<pre>
	DCF = RR * HFL * VFL
</pre>
</code></blockquote>

That is, your dot clock is fixed.  You can use those dots per second to buy 
either refresh rate, horizontal resolution, or vertical resolution.  If one
of those increases, one or both of the others must decrease. </p>
<p>Note, though, that your refresh rate cannot be greater than the maximum
vertical sync frequency of your monitor.  Thus, for any given monitor at a
given dot clock, there is a minimum product of frame lengths below which you
can't force it. </p>
<p>In choosing your settings, remember: if you set RR too low, you will get
mugged by screen flicker. </p>
<p>You probably do not want to pull your refresh rate below 60Hz.  This is the
flicker rate of fluorescent lights; if you're sensitive to those, you need
to hang with 72Hz, the VESA ergonomic standard. </p>
<p>Flicker is very eye-fatiguing, though human eyes are adaptable and peoples'
tolerance for it varies widely.  If you face your monitor at a 90% viewing
angle, are using a dark background and a good contrasting color for
foreground, and stick with low to medium intensity, you *may* be comfortable
at as little as 45Hz. </p>
<p>The acid test is this: open a xterm with pure white back-ground and black
foreground using <code>xterm -bg white -fg black</code> and make it so large as
to cover the entire viewable area.  Now turn your monitor's intensity to 3/4 of
its maximum setting, and turn your face away from the monitor.  Try peeking at
your monitor sideways (bringing the more sensitive peripheral-vision cells into
play).  If you don't sense any flicker or if you feel the flickering is
tolerable, then that refresh rate is fine with you.  Otherwise you better
configure a higher refresh rate, because that semi-invisible flicker is going
to fatigue your eyes like crazy and give you headaches, even if the screen
looks OK to normal vision. </p>
<p>For interlaced modes, the amount of flicker depends on more factors
such as the current vertical resolution and the actual screen
contents.  So just experiment.  You won't want to go much below about
85Hz half frame rate, though. </p>
<p>So let's say you've picked a minimum acceptable refresh rate.  In choosing
your HFL and VFL, you'll have some room for maneuver. </p>

 <HR NOSHADE>
<A HREF="VideoModes.html"><EM>XFree86 Video Timings HOWTO</EM></A>
 <b>:</b> <EM>Tradeoffs in Configuring your System</EM><BR>
<b>Previous:</b> <A HREF="VideoModes5.html"><EM>Interpreting the Basic Specifications</EM></A><BR>
<b>Next:</b> <A HREF="VideoModes7.html"><EM>Memory Requirements</EM></A>



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