Imported Upstream version 0.72.4
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TANIGUCHI Takaki
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documentation/en/qa35.html
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<head>
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<meta http-equiv="content-type" content="text/html; charset=UTF-8">
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<title>Adaptive reading strategy</title>
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<link rel="stylesheet" type="text/css" href="../include/dvdisaster.css">
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</head>
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<body>
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<!-- Title header -->
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<table width="100%" cellpadding="0" border="0">
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<tr>
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<td align="left">
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<span class="fxxl"><b>dvdisaster</b></span>
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<i>Version 0.72.4 / 0.72 (pl4)</i>
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</td>
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<td align="right">
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<span class="fxxl"> </span><a href="http://dvdisaster.net/en/">To the Internet version</a>
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<a href="../cs/qa35.html"></a>
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<a href="../de/qa35.html"></a>
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<a href="../ru/qa35.html"></a>
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</td>
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</tr>
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</table>
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<!-- Language chooser or separator -->
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<table width="100%" cellpadding="0" border="0">
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<tr>
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<td colspan="2" class="hsep"></td>
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</tr>
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<tr><td colspan="2" style="height: 10px">
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</td></tr>
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</table>
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<!-- Main body (Navigation, actual page content, optional news column) -->
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<table width="100%" cellspacing=0>
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<tr>
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<!-- Navigation -->
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<td style="background-color:#f0f0f0; vertical-align:top; width:20%;">
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<table width="100%" cellpadding="10">
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<tr>
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<td>
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<span class="fs"><b>Contents</b></span>
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<table width="100%" cellpadding="0" cellspacing="0">
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<tr>
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<td class="hsep"></td>
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</tr>
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</table><p>
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<table width="100%">
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<tr><td colspan=3><span class="fs"><a href="news.html">News</a></span></td></tr>
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<tr><td></td><td></td><td style="height:10px;"></td></tr>
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<tr><td colspan=3><span class="fs"><a href="index.html">Overview</a></span></td></tr>
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<tr><td></td><td></td><td style="height:10px;"></td></tr>
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<tr><td colspan=3><span class="fs"><a href="howtos.html">Howtos</a></span></td></tr>
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<tr><td></td><td></td><td style="height:10px;"></td></tr>
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<tr><td colspan=3><span class="fs"><a href="download.html">Download</a></span></td></tr>
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<tr><td></td><td></td><td style="height:10px;"></td></tr>
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<tr><td colspan=3><span class="fs"><a href="qa.html">Questions and Answers</a></span></td></tr>
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<tr><td style="width:1%; vertical-align:top;">·</td><td colspan=2><span class="fs"><a href="qa10.html">Technical Questions</a></span></td></tr>
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<tr><td style="width:1%; vertical-align:top;">·</td><td colspan=2><span class="fs"><a href="qa20.html">Error messages</a></span></td></tr>
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<tr><td style="width:1%; vertical-align:top;">·</td><td colspan=2><span class="fs"><a href="qa30.html">Background information</a></span></td></tr>
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<tr><td></td><td style="vertical-align:top; font-size:small;">-</td><td><span class="fs"><a href="qa31.html">Error correction properties</a></span></td></tr>
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<tr><td></td><td style="vertical-align:top; font-size:small;">-</td><td><span class="fs"><a href="qa32.html">Image level data recovery</a></span></td></tr>
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<tr><td></td><td style="vertical-align:top; font-size:small;">-</td><td><span class="fs"><a href="qa33.html">RS01, RS02 and RS03</a></span></td></tr>
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<tr><td></td><td style="vertical-align:top; font-size:small;">-</td><td><span class="fs"><a href="qa34.html">Linear reading strategy</a></span></td></tr>
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<tr><td></td><td style="vertical-align:top; font-size:small;">-</td><td><span class="fs">Adaptive reading strategy</span></td></tr>
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<tr><td></td><td style="vertical-align:top; font-size:small;">-</td><td><span class="fs"><a href="qa36.html">Remarks on read errors</a></span></td></tr>
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<tr><td></td><td style="vertical-align:top; font-size:small;">-</td><td><span class="fs"><a href="qa37.html">Hints for storing the error correction files</a></span></td></tr>
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<tr><td></td><td></td><td style="height:10px;"></td></tr>
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<tr><td colspan=3><span class="fs"><a href="feedback.html">Bug reporting</a></span></td></tr>
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</table>
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</td>
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</tr>
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</table>
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</td>
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<td></td>
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<!-- Actual page contents -->
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<td style="vertical-align:top; width:77%;" rowspan="2">
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<!-- Insert actual page content below -->
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<h3 class="top">The adaptive reading strategy</h3>
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dvdisaster contains two different reading strategies.<p>
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<b>The adaptive reading strategy is recommended for:</b><p>
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<ul>
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<li><a href="howtos42.html">extracting data</a> from damaged media
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</li>
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</ul>
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<b>The <a href="qa34.html">linear reading strategy</a> is recommended for:</b><p>
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<ul>
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<li><a href="howtos23.html">creating images</a> from undamaged media, e.g. to generate the error correction file</li>
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<li><a href="howtos12.html">scanning the medium</a> for reading speed and read errors</li>
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</ul>
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<pre> </pre>
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<b>Properties of the adaptive reading strategy.</b><p>
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The adaptive reading strategy uses a "divide and conquer" approach for locating
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still readable portions of a damaged medium. The strategy is based upon an article
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published by Harald Bögeholz in c't-Magazin 16/2005 where it was published together
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with the program <i>h2cdimage</i>:
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<ol>
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<li>
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At the beginning the medium is considered as a single unread interval. Reading begins
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with sector zero.<p>
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</li>
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<li>
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The reading process continues sequentially unless either the end of the current interval
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or a read error is encountered.<p>
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</li>
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<li>
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The reading process is terminated if either (3a) sufficient sectors for a successful
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error correction have been read or (3b) no unreadable intervals exceeding a given size
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remain.
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<p>
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</li>
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<li>
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Otherwise the largest remaining unread interval will be determined. Reading continues in the middle
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(e.g. second half) of this interval;
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the first half of this interval is kept for a later reading pass.
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</li>
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</ol>
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<!-- begin of screen shot -->
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<table>
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<tr>
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<td class="valignt">
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The termination criterium (3a) is especially efficient: Reading will stop as soon
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as enough sectors have been collected for a successful image recovery using the
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error correction file. This can reduce the reading time by as much as 90 percent
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compared with a full read attempt, but does of course only work when
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an error correction file is available.<p>
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</td>
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<td>
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<a name="snap1"></a>
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<table class="thumbnail" width="160">
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<tr><td><span class="fs">Adaptive reading in progress</span></td></tr>
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<tr><td><a href="qa35_expand=1.html#snap1"><img src="thumbnails/adaptive-progress.png" alt="" class="noborder"></a></td></tr>
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</table>
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</td>
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</tr>
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</table><p>
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<!-- end of screen shot -->
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<p>
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<pre> </pre>
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<a name="configure"></a>
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<b>Configuration</b><p>
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<b>Error correction file.</b>
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Adaptive reading works best when error correction data is available.
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Obviously the error correction data must have been
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<a href="howtos21.html">created</a>
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at a time where the medium was still fully readable. To use a error correction file
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during adaptive reading,
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<a href="howtos42.html#select_eccfile">enter its name</a> before starting the reading process.<p>
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<b>Limiting the adaptive reading range.</b> Reading can be
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<a href="howtos11.html#image">limited</a> to a part of the medium.
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This is not recommended when error correction data is used since the limit
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might prevent sectors from being read which are required
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for a succesful error correction.
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If no error correction data is available,
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limiting the reading range might be helpful
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during multiple reading attempts.<p>
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<b>Early reading termination.</b>
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If no error correction data is available, adaptive reading will stop when no unread
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intervals
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<a href="howtos41.html#reading_attempts">larger than a selectable size</a> remain.<p>
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The termination value should not be smaller than 128.
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Otherwise the laser head will have to carry out lots of repositioning moves during the
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final phase of the reading process. This will negatively affect both the life expectancy
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of the drive and its reading capability. A better approach is to stop adaptive
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reading earlier and then try reading the remaining sectors with an additional
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<a href="qa34.html">linear reading</a> pass.
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<!-- do not change below -->
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</td> <!-- end of page contents -->
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</tr>
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<!--
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<tr valign="bottom">
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<td style="background-color:#f0f0f0;">
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<table cellpadding="10"><tr><td><a href="http://dvdisaster.net/legacy/en/index.html"><span class="fs"></span></a></td></tr></table>
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</td>
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<td></td>
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<td></td>
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</tr>
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-->
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</table> <!-- end of main body table -->
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<!-- Page footer -->
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<table width="100%" cellpadding="0" border="0">
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<tr><td colspan="2" style="width:100%; height:10px;"></td>
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</tr>
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<tr><td colspan="2" class="hsep"></td>
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</tr>
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<tr>
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<td align="center">
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<span class="fs">
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<i> Copyright 2004-2012 Carsten Gnörlich.<br>
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Verbatim copying and distribution of this entire article is permitted in any medium, provided this notice is preserved.
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</i>
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</span>
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</td>
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</tr>
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<tr><td colspan="2" class="hsep"></td>
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</tr>
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</table>
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</body>
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</html>
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