93 lines
3.9 KiB
PHP
93 lines
3.9 KiB
PHP
<?php
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# dvdisaster: English homepage translation
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# Copyright (C) 2004-2012 Carsten Gnörlich
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#
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# UTF-8 trigger: äöüß
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#
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# Include our PHP sub routines, then call begin_page()
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# to start the HTML page, insert the header,
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# navigation and news if appropriate.
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require("../include/dvdisaster.php");
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begin_page();
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?>
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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.php#ecc">standard settings</a> of an
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<a href="howtos20.php">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="#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="#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="#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="#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.php#ecc">select other values</a>
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for higher or lower error correction capabilities. When
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<a href="howtos30.php">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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<?php
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# end_page() adds the footer line and closes the HTML properly.
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end_page();
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?>
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