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<A HREF="VideoModes.html"><EM>XFree86 Video Timings HOWTO</EM></A>
 <b>:</b> <EM>Overdriving Your Monitor</EM><BR>
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  <H2><A NAME="17"></A>11. Overdriving Your Monitor<A NAME="overd"></A></H2>

<p>You should absolutely <em>not</em> try exceeding your monitor's scan
rates if it's a fixed-frequency type.  You can smoke your hardware
doing this!  There are potentially subtler problems with overdriving a
multisync monitor which you should be aware of.  </p>
<p>Having a pixel clock higher than the monitor's maximum bandwidth is
rather harmless, in contrast.  (Note: the theoretical limit of
discernible features is reached when the pixel clock reaches double
the monitor's bandwidth.  This is a straightforward application of
Nyquist's Theorem: consider the pixels as a spatially distributed
series of samples of the drive signals and you'll see why.) </p>
<p>It's exceeding the rated maximum sync frequencies that's problematic.
Some modern monitors might have protection circuitry that shuts the
monitor down at dangerous scan rates, but don't rely on it.  In
particular there are older multisync monitors (like the Multisync II)
which use just one horizontal transformer. These monitors will not
have much protection against overdriving them.  While you necessarily
have high voltage regulation circuitry (which can be absent in fixed
frequency monitors), it will not necessarily cover every conceivable
frequency range, especially in cheaper models. This not only implies
more wear on the circuitry, it can also cause the screen phosphors to
age faster, and cause more than the specified radiation (including X-rays)
to be emitted from the monitor. </p>
<p>Another importance of the bandwidth is that the monitor's input
impedance is specified only for that range, and using higher
frequencies can cause reflections probably causing minor screen
interferences, and radio disturbance. </p>
<p>However, the basic problematic magnitude in question here is the slew
rate (the steepness of the video signals) of the video output drivers,
and that is usually independent of the actual pixel frequency, but
(if your board manufacturer cares about such problems) related
to the maximum pixel frequency of the board. </p>
<p>So be careful out there... </p>

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<A HREF="VideoModes.html"><EM>XFree86 Video Timings HOWTO</EM></A>
 <b>:</b> <EM>Overdriving Your Monitor</EM><BR>
<b>Previous:</b> <A HREF="VideoModes10.html"><EM>Putting it All Together</EM></A><BR>
<b>Next:</b> <A HREF="VideoModes12.html"><EM>Using Interlaced Modes</EM></A>



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