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+<?php
+# dvdisaster: English homepage translation
+# Copyright (C) 2004-2012 Carsten Gnörlich
+#
+# UTF-8 trigger: äöüß 
+#
+# Include our PHP sub routines, then call begin_page()
+# to start the HTML page, insert the header, 
+# navigation and news if appropriate.
+
+require("../include/dvdisaster.php");
+begin_page();
+?>
+
+<!-- Insert actual page content below -->
+
+<h3 class="top">Technical properties of the error correction</h3>
+
+This page outlines the basic ideas behind dvdisaster
+so that you can see for yourself whether
+it meets your demands on data safety.
+If in doubt, you should not use dvdisaster
+or deploy additional data backup strategies.<p>
+
+<b>Method of error correction.</b> &nbsp; dvdisaster uses a
+<a href="http://en.wikipedia.org/wiki/Reed-Solomon_error_correction">Reed-Solomon</a> code
+together with an error correction algorithm optimized for the treatment of erasures.
+The implementation draws a lot of inspiration and program code from the excellent
+<a href="http://www.ka9q.net/code/fec/">Reed-Solomon library</a> written by
+<a href="http://www.ka9q.net/">Phil Karn</a>.
+
+<p>
+
+Using the <a href="howtos22.php#ecc">standard settings</a> of an 
+<a href="howtos20.php">error correction file</a> 
+223 medium sectors are combined into one error correction code ("ECC") block.
+Medium read errors are regarded as "erasures"; therefore a maximum 
+of 32 bad medium sectors<sup><a href="#footnote1">*)</a></sup>
+are correctable per ECC block. <p>
+
+The 223 sectors are selected so that they are evenly distributed over the medium surface.
+Therefore large contigous areas of defective sectors can be corrected before the threshold
+of 32 defects per ECC block<sup><a href="#footnote1">*)</a></sup>
+is reached. This kind of error pattern is especially
+common for an aging medium where the outer area is starting to degenerate, 
+and for scratches along the data spiral. <p>
+
+On the other hand, radial or diagonal scratches are expected to be correctable in the
+CD/DVD drive itself. If not, the employed error correction strategy performs rather neutral
+in these cases (neither especially good nor extraordinary bad). <p>
+
+<b>Limits of error correction.</b> &nbsp; In the wost case, 
+33 defective sectors<sup><a href="#footnote1">*)</a></sup> are sufficient
+to prevent a full data restoration. However to achieve this effect, the errors have to
+be distributed over the medium in such a way that they occur in the same ECC block - such a pattern
+is very unlikely.<br>
+Empirical tests have shown that on aging media about 10% of the overall sector count
+may become defective before
+the threshold of 33 defects per ECC block<sup><a href="#footnote1">*)</a></sup>
+ is reached. <br>
+Scratches will cause the threshold to be reached earlier, 
+therefore it is recommended to visually check the media in regular intervals.
+Media with read errors caused by scratches should be replaced immediately.<p>
+
+<b>Hardware limits.</b> &nbsp; Most drives will not recognize media when the lead-in area
+before the first sector
+(near the center hole) is damaged. In such cases, dvdisaster will not be able to recover
+any content from the media. <p>
+
+It is <i>not feasible</i> to enhance the reliability of poor quality media by using
+dvdisaster. Cheap media can decay within a few days to an extent which will exceed
+the capabilities of the error correction code. <p>
+
+<pre> </pre>
+<table width="50%"><tr><td><hr></td></tr></table>
+
+<span class="fs">
+<a name="footnote1"><sup>*)</sup></a> 
+The given threshold of 32 correctable errors per ECC block is the standard setting.
+It is possible to <a href="howtos22.php#ecc">select other values</a>
+for higher or lower error correction capabilities. When 
+<a href="howtos30.php">error correction data is put directly on the medium</a>, the number of correctable
+errors depends on the free space on the medium.
+</span>
+
+<!-- do not change below -->
+
+<?php
+# end_page() adds the footer line and closes the HTML properly.
+
+end_page();
+?>