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