132 lines
5.6 KiB
PHP
132 lines
5.6 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()../images/
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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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require("../include/screenshot.php");
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begin_page();
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howto_headline("Scanning media for errors", "Interpreting results", "images/scan-icon.png");
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?>
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<!-- Insert actual page content below -->
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<?php begin_screen_shot("Overview","defective-cd.png"); ?>
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<b>Overview.</b> dvdisaster provides several information about the scanning results:
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<ul>
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<li>The spiral under "<b>Medium State</b>" (to the right).<p>
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The spiral provides information about the medium readability. The medium is
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fully readable when all segments of the spiral are colored green. Yellow or red
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blocks mark places where data could not be correctly read from the medium.
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The total number of unreadable sectors is printed
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in the <i>"Scanning finished:"</i> message
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at the window bottom.<p>
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</li>
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<li>"<b>Speed</b>" - The reading speed curve (upper left).<p>
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The reading speed is not an absolute gauge of the medium health,
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but it is usable as a rule of thumb:
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The more regular the curve, the better the medium.
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You will find examples of good and bad reading speed curves further down this
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page.<p></li>
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<li>"<b>C2 errors</b>" - A medium state gauge provided by the drive (down left).<p>
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This kind of analysis
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is <a href="qa.php?pipo">currently only available for CD media</a>.
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CD drives have a built-in error correction which can eliminate small data losses
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caused by minor defects on the medium. The number of C2 errors is a measurement
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of how often the drive needed to employ its internal error correction during
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the read - this value should be zero on good media.</li>
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</ul>
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<?php end_screen_shot(); ?>
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<b>Examples for good media</b><p>
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<?php begin_screen_shot("Good CD","good-cd.png"); ?>
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<b>Good CD</b>: This screen shot shows a perfect CD:
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All blocks under "Medium state" are green, no C2 errors have been reported
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and the reading curve runs smoothly. A rising reading speed is normal for most
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media (see the next screen shot for a counter example). The small spikes
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at the beginning and at the end of the curve are normal; minor glitches like
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the one shown at 250M are also harmless.
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<?php end_screen_shot(); ?>
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<?php begin_screen_shot("Good two layered DVD","good-dvd9.png"); ?>
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<b>Sometimes the reading curve won't rise steadily</b>: Multi-layered media
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might yield reading curves which are rising and dropping in a symmetric pattern.
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Not shown but also possible are flat curves without any change in reading speed
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(most typically seen with DVD-RAM).
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<?php end_screen_shot(); ?><p>
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<b>An example for a weak medium</b><p>
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<?php begin_screen_shot("Weak CD","weak-cd.png"); ?>
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This medium is still readable as indicated by the green spiral shown under
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"Medium state". However there are clear signs of serious trouble ahead:
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The drive must slow down significantly towards the end of the medium in order
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to read from it. Note the steep fall of reading speed after the 600M mark.
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This comes along with C2 error rates rising to the 100 mark; this is another
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warning that the medium is decaying in the outer region.
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If you have not created <a href="howtos20.php">error correction</a> data
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this is probably the last opportunity to do so as the medium will develop
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the first read errors soon.
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<?php end_screen_shot(); ?><p>
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<b>Examples of defective media</b><p>
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<?php begin_screen_shot("Defective CD","defective-cd.png"); ?>
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<b>Defective CD.</b>
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The red sectors in the spiral visualize large unreadable sections
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in the outer region of the medium. At the bottom of the window you will find
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the information that the medium contains 28752 unreadable sectors.
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This sums up to about 8.2% defective sectors (of 352486 sectors total) and
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is well within the
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<a href="howtos40.php">recovery</a> bounds by
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<a href="howtos20.php">error correction (ecc) data</a> made
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with default settings - if you have made the ecc data in time!
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Otherwise the contents of the red
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sectors are lost since ecc data cannot be created from already defective media.
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<?php end_screen_shot(); ?><p>
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<a name="crc"></a>
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<?php begin_screen_shot("Checksum errors","crc-cd.png"); ?>
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<b>Checksum errors.</b> Yellow spots in the spiral depict places
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where the medium was fully readable, but the data read did not match
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checksums in the error correction data. There are two main causes:
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<p>
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<ul><li>
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<b>The image has been manipulated</b> after the creation
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of error correction data and before writing it to the medium.
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This can happen on Unix systems when the image is mounted with write access
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after ecc data has been created. Typical signs are CRC errors in sector 64
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and in sectors 200 to 400 as the system updates the file access times there.
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Performing a data recovery using dvdisaster is typically harmless in this
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situation.<p>
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However if you have modified files in the image after creating the ecc data,
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the error correction data will be both worthless and dangerous.
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Applying a recovery to the medium will restore the image state
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at the time the ecc data has been created, and this will obviously not
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represent the most recent contents of the medium.<p></li>
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<li><b>There are technical problems with the computer system,</b>
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especially in mass storage communication. Perform the scan again
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and observe the CRC error locations.
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If CRC errors disappear or surface at different locations your system
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might have defective RAM, bad drive cabling/controllers or incorrect clock
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speeds.</li></ul>
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<?php end_screen_shot(); ?><p>
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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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