133 lines
4.7 KiB
PHP
133 lines
4.7 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">The RS01, RS02 and RS03 methods</h3>
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dvdisaster contains three error correction methods.
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RS01 and RS02 are the existing and proven methods while RS03 is still
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under development.<p>
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<b>Comparison of the methods.</b>
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All three methods build on the same
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<a href="qa31.php">Reed-Solomon</a> error correction.
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They calculate error correction information for ISO images which
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is used to recover unreadable sectors if the disc becomes damaged afterwards.<p>
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The methods differ in the way the error correction information is stored:<p>
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<ul>
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<li>
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<a name="file"> </a>
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RS01 creates <b>error correction files</b> which are stored separately from
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the image they belong to. Since data protection at the
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<a href="qa32.php#file">file level</a> is difficult,
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error correction files must be stored on media which are protected
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against data loss by dvdisaster, too.<p></li>
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<li>
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<a name="image"> </a>
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To apply the RS02 method an image is first created on hard disc using a
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CD/DVD writing software. Before the image is written on the medium,
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dvdisaster is used to <b>augment the image</b> with error correction data.
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Therefore the data to be protected and the error correction information
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are located at the same medium. Damaged sectors in the error correction
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information reduce the data recovery capacity, but do not make recovery
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impossible - a second medium for keeping or protecting the error correction
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information is not required.<p></li>
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</ul>
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RS03 is a further development of RS01 and RS02. It can create both
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error correction files and augmented images:
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<ul>
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<li>RS03 can distribute work over multiple processor cores and is therefore
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much faster than RS01/RS02 on modern hardware.</li>
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<li>RS03 <b>error correction files</b> are - contrary to RS01 - robust against
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damage. This should not delude you into careless handling of your error
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correction files though - the disadvantages of <a href="qa32.php#file">reading
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at the filesystem level</a> are still valid.</li>
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<li>RS03 <b>augmented images</b> do not require so-called master blocks
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holding important information. This makes RS03 a bit more robust, but also
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more restrictive: The augmented image must completely fill the medium now
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while the size of augmented image can be freely chosen in RS02.
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</li>
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</ul>
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The changes for parallel computation and higher robustness make RS03 a bit less
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space efficient, e.g. RS03 error correction data has slighly less error
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correction capacity than its RS01/RS02 counterpart with equal size.<p>
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<a name="table"> </a>
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<b>Comparison of error correction storage.</b><p>
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The following table summarizes the differences between
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error correction files (RS01, RS03) and augmented images (RS02, RS03):<p>
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<table width="100%" border="1" cellspacing="0" cellpadding="5">
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<tr>
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<td class="w50p"><i>Error correction files</i></td>
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<td class="w50p"><i>Image augmented with error correction data</i></td>
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</tr>
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<tr valign="top">
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<td>
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any possible redundancy can be chosen</td>
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<td> redundancy is limited by free space on medium<br>
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(= medium capacity - size of data image)</td>
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</tr>
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<tr valign="top">
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<td>already effective at 15% redundancy
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since error correction files are required to be free of damage</td>
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<td>requires more redundancy (recommended: 20-30%)
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to compensate defects in the error correction data</td>
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</tr>
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<tr valign="top">
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<td>medium can be completely filled with data</td>
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<td>usable medium capacity is reduced by amount of error correction data</td>
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</tr>
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<tr valign="top">
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<td> can be created for already existing media</td>
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<td> only applicable before writing the new medium since
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the image must be augmented with error correction information in advance</td>
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</tr>
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<tr valign="top">
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<td> separately storing the error correction file from user data
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strengthens data protection</td>
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<td> common storage of user data and error correction data may reduce error
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correction capacity</td>
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</tr>
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<tr valign="top">
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<td>Mapping between error correction files and media must be kept.
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Error correction files must be protected against damage.</td>
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<td>Easy one-medium solution; error correction information
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needs not to be cataloged or explicitly protected.</td></tr>
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<tr valign="top">
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<td> no compatibilty issues with play-back units</td>
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<td> media with augmented images may not play correctly on all units</td>
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</tr>
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</table><p>
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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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