192 lines
8.7 KiB
HTML
192 lines
8.7 KiB
HTML
<!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>Error correction properties</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>Версия 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/ru/">К интернет-версии</a>
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<a href="../cs/qa31.html"></a>
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<a href="../de/qa31.html"></a>
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<a href="../en/qa31.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>Содержание</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">Новости</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">Обзор</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">Howto's</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">Скачать</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">Вопросы и ответы</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">Технические вопросы</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">Сообщения об ошибках</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">Error correction properties</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"><a href="qa35.html">Adaptive 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"><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">Сообщения об ошибках</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">Technical properties of the error correction</h3>
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This page outlines the basic ideas behind dvdisaster
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so that you can see for yourself whether
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it meets your demands on data safety.
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If in doubt, you should not use dvdisaster
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or deploy additional data backup strategies.<p>
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<b>Method of error correction.</b> dvdisaster uses a
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<a href="http://en.wikipedia.org/wiki/Reed-Solomon_error_correction">Reed-Solomon</a> code
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together with an error correction algorithm optimized for the treatment of erasures.
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The implementation draws a lot of inspiration and program code from the excellent
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<a href="http://www.ka9q.net/code/fec/">Reed-Solomon library</a> written by
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<a href="http://www.ka9q.net/">Phil Karn</a>.
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<p>
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Using the <a href="howtos22.html#ecc">standard settings</a> of an
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<a href="howtos20.html">error correction file</a>
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223 medium sectors are combined into one error correction code ("ECC") block.
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Medium read errors are regarded as "erasures"; therefore a maximum
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of 32 bad medium sectors<sup><a href="qa31.html#footnote1">*)</a></sup>
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are correctable per ECC block. <p>
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The 223 sectors are selected so that they are evenly distributed over the medium surface.
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Therefore large contigous areas of defective sectors can be corrected before the threshold
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of 32 defects per ECC block<sup><a href="qa31.html#footnote1">*)</a></sup>
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is reached. This kind of error pattern is especially
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common for an aging medium where the outer area is starting to degenerate,
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and for scratches along the data spiral. <p>
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On the other hand, radial or diagonal scratches are expected to be correctable in the
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CD/DVD drive itself. If not, the employed error correction strategy performs rather neutral
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in these cases (neither especially good nor extraordinary bad). <p>
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<b>Limits of error correction.</b> In the wost case,
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33 defective sectors<sup><a href="qa31.html#footnote1">*)</a></sup> are sufficient
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to prevent a full data restoration. However to achieve this effect, the errors have to
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be distributed over the medium in such a way that they occur in the same ECC block - such a pattern
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is very unlikely.<br>
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Empirical tests have shown that on aging media about 10% of the overall sector count
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may become defective before
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the threshold of 33 defects per ECC block<sup><a href="qa31.html#footnote1">*)</a></sup>
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is reached. <br>
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Scratches will cause the threshold to be reached earlier,
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therefore it is recommended to visually check the media in regular intervals.
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Media with read errors caused by scratches should be replaced immediately.<p>
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<b>Hardware limits.</b> Most drives will not recognize media when the lead-in area
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before the first sector
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(near the center hole) is damaged. In such cases, dvdisaster will not be able to recover
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any content from the media. <p>
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It is <i>not feasible</i> to enhance the reliability of poor quality media by using
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dvdisaster. Cheap media can decay within a few days to an extent which will exceed
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the capabilities of the error correction code. <p>
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<pre> </pre>
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<table width="50%"><tr><td><hr></td></tr></table>
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<span class="fs">
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<a name="footnote1"><sup>*)</sup></a>
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The given threshold of 32 correctable errors per ECC block is the standard setting.
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It is possible to <a href="howtos22.html#ecc">select other values</a>
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for higher or lower error correction capabilities. When
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<a href="howtos30.html">error correction data is put directly on the medium</a>, the number of correctable
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errors depends on the free space on the medium.
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</span>
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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 2007-2009 Igor Gorbounov.<br>
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Дословное копирование и распространение всей этой статьи разрешается на любом носителе, при условии, что это уведомление сохраняется.
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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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