236 lines
9.0 KiB
HTML
236 lines
9.0 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<head>
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<meta http-equiv="content-type" content="text/html; charset=UTF-8">
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<title>Linear reading strategy</title>
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<link rel="stylesheet" type="text/css" href="../include/dvdisaster.css">
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</head>
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<body>
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<!-- Title header -->
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<table width="100%" cellpadding="0" border="0">
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<tr>
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<td align="left">
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<span class="fxxl"><b>dvdisaster</b></span>
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<i>Version 0.72.4 / 0.72 (pl4)</i>
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</td>
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<td align="right">
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<span class="fxxl"> </span><a href="http://dvdisaster.net/en/">To the Internet version</a>
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<a href="../cs/qa34.html"></a>
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<a href="../de/qa34.html"></a>
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<table width="100%" cellpadding="10">
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<td>
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<span class="fs"><b>Contents</b></span>
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<table width="100%" cellpadding="0" cellspacing="0">
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<tr>
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<td class="hsep"></td>
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</table><p>
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<table width="100%">
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<tr><td colspan=3><span class="fs"><a href="news.html">News</a></span></td></tr>
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<tr><td></td><td></td><td style="height:10px;"></td></tr>
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<tr><td colspan=3><span class="fs"><a href="index.html">Overview</a></span></td></tr>
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<tr><td colspan=3><span class="fs"><a href="howtos.html">Howtos</a></span></td></tr>
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<tr><td colspan=3><span class="fs"><a href="download.html">Download</a></span></td></tr>
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<tr><td></td><td></td><td style="height:10px;"></td></tr>
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<tr><td colspan=3><span class="fs"><a href="qa.html">Questions and Answers</a></span></td></tr>
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<tr><td style="width:1%; vertical-align:top;">·</td><td colspan=2><span class="fs"><a href="qa10.html">Technical Questions</a></span></td></tr>
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<tr><td style="width:1%; vertical-align:top;">·</td><td colspan=2><span class="fs"><a href="qa20.html">Error messages</a></span></td></tr>
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<tr><td style="width:1%; vertical-align:top;">·</td><td colspan=2><span class="fs"><a href="qa30.html">Background information</a></span></td></tr>
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<tr><td></td><td style="vertical-align:top; font-size:small;">-</td><td><span class="fs"><a href="qa31.html">Error correction properties</a></span></td></tr>
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<tr><td></td><td style="vertical-align:top; font-size:small;">-</td><td><span class="fs"><a href="qa32.html">Image level data recovery</a></span></td></tr>
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<tr><td></td><td style="vertical-align:top; font-size:small;">-</td><td><span class="fs"><a href="qa33.html">RS01, RS02 and RS03</a></span></td></tr>
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<tr><td></td><td style="vertical-align:top; font-size:small;">-</td><td><span class="fs">Linear reading strategy</span></td></tr>
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<tr><td></td><td style="vertical-align:top; font-size:small;">-</td><td><span class="fs"><a href="qa35.html">Adaptive reading strategy</a></span></td></tr>
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<tr><td></td><td style="vertical-align:top; font-size:small;">-</td><td><span class="fs"><a href="qa36.html">Remarks on read errors</a></span></td></tr>
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<tr><td></td><td style="vertical-align:top; font-size:small;">-</td><td><span class="fs"><a href="qa37.html">Hints for storing the error correction files</a></span></td></tr>
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<tr><td></td><td></td><td style="height:10px;"></td></tr>
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<tr><td colspan=3><span class="fs"><a href="feedback.html">Bug reporting</a></span></td></tr>
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</table>
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<td></td>
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<td style="vertical-align:top; width:77%;" rowspan="2">
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<!-- Insert actual page content below -->
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<h3 class="top">The linear reading strategy</h3>
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dvdisaster contains two different reading strategies.<p>
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<b>The linear reading strategy is recommended for:</b><p>
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<ul>
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<li><a href="howtos23.html">creating images</a> from undamaged media, e.g. to generate the error correction file</li>
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<li><a href="howtos12.html">scanning the medium</a> for reading speed and read errors</li>
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</ul>
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<b>The <a href="qa35.html">adaptive reading strategy</a> is recommended for:</b><p>
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<ul>
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<li> <a href="howtos42.html">extracting data</a> from damaged media
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</li>
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</ul>
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<pre> </pre>
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<b>Properties of the linear reading strategy.</b><p>
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<!-- begin of screen shot -->
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<table class="screenshot">
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<tr><td>
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<a name="snap1"></a>
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Optical media are organized into sectors which are
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continously numbered beginning with zero. Each sector contains 2048 bytes of data.<p>
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The linear reading strategy reads the medium from the start (sector 0)
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until the end (last sector). The reading speed is shown graphically to provide
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information about the <a href="qa34_expand=1.html#quality">medium quality</a>:
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</td></tr>
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<tr><td align="center">
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<a href="qa34.html#snap1">
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<img src="images/weak-cd.png" alt="" class="noborder">
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</a>
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</td></tr>
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<tr><td align="center">Screen shot: Weak CD</td></tr>
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</table><pre> </pre>
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<!-- end of screen shot -->
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<p>
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<pre> </pre>
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<a name="configure"></a>
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<b>Configuration.</b><p>
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<b>Number of sectors to skip after a read error.</b>
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Reading attempts for defective sectors are slow and may wear out the drive mechanics
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under unfavourable conditions. A series of read errors, spread over a continous sector
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range, is more common than single spot defects. Therefore a
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<a href="howtos11.html#read_attempts"> configuration option</a> exists so that a certain number
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of sectors will be skipped after the occurance of a read error. The skipped sectors are
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assumed to be defective without further reading attempts.
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Some guide lines for selecting the number of skipped sectors are:<p>
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<ul>
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<li>Skipping a large number of sectors (e.g. <b>1024</b>) gives a quick overview of the
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degree of damage, but will usually not collect enough data for repairing the medium image.
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<p></li>
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<li>Smaller values like <b>16, 32 or 64</b> are a good trade-off:
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The processing time will be considerably shortened, but still enough data for repairing
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the image will be collected.<p></li>
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</ul>
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On DVD media read errors do usually extend over at least 16 sectors for technical
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reasons. Therefore a sector skip less than 16 is not recommended for DVD media.
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<p>
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<a name="range"></a>
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<b>Limiting the reading range.</b>
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Reading can be
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<a href="howtos11.html#image">limited to a given range of sectors</a> in the
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"Image" preferences tab. This comes in handy
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during multiple read attempts of damaged media.
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<pre> </pre>
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<a name="quality"></a>
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<b>Estimating media quality.</b><p>
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<a name="error"></a>
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<b>The speed curve.</b>
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Most drives will slow down while reading medium areas which are in bad condition:
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<ul>
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<li>
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Drops in the reading speed can be a warning for an imminent medium failure.
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</li>
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<li>
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However some drives will read with full speed until the bitter end.
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With such drives media deterioration may not show up in the reading curve
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until actual read errors occur.
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</li>
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</ul><p>
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The reading curve is most accurate when using the
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<a href="howtos10.html"> "Scan"</a> function. During the
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<a href="howtos23.html">"Read"</a> operation the read data will be written to
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the hard drive at the same time, which may cause irregularities in the reading
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curve depending on the operating system and hardware used.<p>
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<b>Read errors.</b>
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Read errors cause <a href="howtos13.html#defective">red markings in the spiral</a> or respective
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messages at the command line.
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This means that the medium could not be
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read at these places during the current reading pass:
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<ul>
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<li>The medium is most likely defective.</li>
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<li>The image should be
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<a href="howtos40.html">repaired</a> as soon as possible and then be transferred
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to a new medium.</li>
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</ul>
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<td align="center">
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<span class="fs">
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<i> Copyright 2004-2012 Carsten Gnörlich.<br>
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Verbatim copying and distribution of this entire article is permitted in any medium, provided this notice is preserved.
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</i>
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</span>
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</html>
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