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documentation/en/qa36.php

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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">Remarks on read errors</h3>
+
+Optical media have their own error correction code which protects the data
+against small manufacturing errors and inaccuracies during writing.
+If the writer and medium are compatible and of high quality, the
+error correction built into the medium will at first be mainly idle.
+This leaves enough reserves to compensate normal wear and aging effects 
+during many years of the medium usage.
+<p>
+
+When the capacity of the built-in error correction is finally exhausted,
+read errors will start to appear on the medium. These will be reported by
+the <a href="howtos10.php">"Scan"</a>-operation of dvdisaster.
+Depending on the time of first occurrence, 
+two types of read errors are of particular interest:<p>
+
+<b>Read errors appearing right after writing the medium.</b>
+This is a sign of:
+
+<ul>
+<li>media from a faulty production run, or</li>
+<li>media which are not compatible with the writer.</li>
+</ul>
+
+A prudential choice is to dispose of the faulty media and
+to write the data on error-free media, possibly switching to a 
+different producer.<p>
+
+Please withstand the temptation of trying to preserve 
+the faulty media by means of an error correction file - this
+will most likely end with data loss.<p>
+
+<b>Read errors after a few months/years.</b> 
+The built-in error correction of the medium will be increasingly loaded
+during its life time until it finally fails and read errors show up.
+This happens for mechanical reasons (scratches, warping of the plastic material)
+as well as for chemical causes (decaying dye and/or reflective layer).<p>
+
+These effects typically occur while the medium is stored away for a few months,
+and it may not be possible to read in all sectors afterwards.<p>
+
+Therefore it is crucial to create the 
+<a href="howtos21.php">error correction data</a> in time. 
+The ecc data contains information for recalculating the contents of
+missing sectors
+<a href="qa31.php">(within certain limits)</a>.
+Therefore with the help of the ecc data
+dvdisaster can recover images even if not all sectors
+could actually be read by the drive.<p>
+
+Since the error correction can reconstruct missing sectors up to a certain number,
+it is not necessary to squeeze out a defective medium for every readable sector.
+The <a href="qa35.php">adaptive reading strategy</a> checks during
+reading whether enough data for error correction has been collected.
+As soon as this is the case, reading stops and still unread sectors
+will be recovered using the ecc data.<p>
+
+<a name="reading-tips"><b>
+Some hints for effectively reading damaged media</b></a><p>
+
+The outcome from reading damaged media depends on several factors:
+
+<ul>
+<li><b>Not all drives are built the same.</b><br>
+Different drives have different reading capabilities.
+Take advantage of dvdisaster's function for completing an image 
+with several reading passes and use different drives for each pass.
+Transfer the image file between computers using a network or rewritable media
+in order to use drives installed in different machines.
+<p></li>
+<li><b>Eject and insert the medium again.</b><br>
+Sometimes it makes a difference to eject the medium, turn it about a quarter,
+and then load it again for another reading pass.
+<p></li>
+<li><b>Some drives read better while being cold.</b><br>
+Turn off the computer over night and perform another reading attempt in the next
+morning.<p>
+But note: "Cold" refers to normal living room conditions - putting hardware
+or media into the fridge can be very unhealthy for them.<p></li>
+</ul>
+
+<!-- do not change below -->
+
+<?php
+# end_page() adds the footer line and closes the HTML properly.
+
+end_page();
+?>