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documentation/en/howtos13.php

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+<?php
+# dvdisaster: English homepage translation
+# Copyright (C) 2004-2009 Carsten Gnörlich
+#
+# UTF-8 trigger: äöüß 
+#
+# Include our PHP sub routines, then call begin_page()../images/
+# to start the HTML page, insert the header, 
+# navigation and news if appropriate.
+
+require("../include/dvdisaster.php");
+require("../include/screenshot.php");
+
+begin_page();
+
+howto_headline("Scanning media for errors", "Interpreting results", "images/scan-icon.png");
+?>
+
+<!--- Insert actual page content below --->
+
+<?php begin_screen_shot("Overview","defective-cd.png"); ?>
+<b>Overview.</b> dvdisaster provides several information about the scanning results:
+<ul>
+<li>The spiral under "<b>Medium State</b>" (to the right).<p>
+The spiral provides information about the medium readability. The medium is
+fully readable when all segments of the spiral are colored green. Yellow or red
+blocks mark places where data could not be correctly read from the medium.
+The total number of unreadable sectors is printed
+in the <i>"Scanning finished:"</i> message 
+at the window bottom.<p>
+</li>
+<li>"<b>Speed</b>" - The reading speed curve (upper left).<p>
+The reading speed is not an absolute gauge of the medium health,
+but it is usable as a rule of thumb: 
+The more regular the curve, the better the medium.
+You will find examples of good and bad reading speed curves further down this
+page.<p></li>
+<li>"<b>C2 errors</b>" - A medium state gauge provided by the drive (down left).<p>
+This kind of analysis 
+is <a href="qa.php?pipo">currently only available for CD media</a>.
+CD drives have a built-in error correction which can eliminate small data losses
+caused by minor defects on the medium. The number of C2 errors is a measurement
+of how often the drive needed to employ its  internal error correction during
+the read - this value should be zero on good media.</li>
+</ul>
+<?php end_screen_shot(); ?>
+
+<b>Examples for good media</b><p>
+
+<?php begin_screen_shot("Good CD","good-cd.png"); ?>
+<b>Good CD</b>: This screen shot shows a perfect CD:
+All blocks under "Medium state" are green, no C2 errors have been reported
+and the reading curve runs smoothly. A rising reading speed is normal for most
+media (see the next screen shot for a counter example). The small spikes
+at the beginning and at the end of the curve are normal; minor glitches like
+the one shown at 250M are also harmless.
+<?php end_screen_shot(); ?>
+
+
+<?php begin_screen_shot("Good two layered DVD","good-dvd9.png"); ?>
+<b>Sometimes the reading curve won't rise steadily</b>: Multi-layered media
+might yield reading curves which are rising and dropping in a symmetric pattern.
+Not shown but also possible are flat curves without any change in reading speed
+(most typically seen with DVD-RAM).
+<?php end_screen_shot(); ?><p>
+
+<b>An example for a weak medium</b><p>
+
+<?php begin_screen_shot("Weak CD","weak-cd.png"); ?>
+This medium is still readable as indicated by the green spiral shown under
+"Medium state". However there are clear signs of serious trouble ahead:
+The drive must slow down significantly towards the end of the medium in order
+to read from it. Note the steep fall of reading speed after the 600M mark.
+This comes along with C2 error rates rising to the 100 mark; this is another
+warning that the medium is decaying in the outer region.
+If you have not created <a href="howtos20.php">error correction</a>  data
+this is probably the last opportunity to do so as the medium will develop
+the first read errors soon.
+<?php end_screen_shot(); ?><p>
+
+<b>Examples of defective media</b><p>
+
+<?php begin_screen_shot("Defective CD","defective-cd.png"); ?>
+<b>Defective CD.</b> 
+The red sectors in the spiral visualize large unreadable sections
+in the outer region of the medium. At the bottom of the window you will find
+the information that the medium contains 28752 unreadable sectors.
+This sums up to about 8.2% defective sectors (of 352486 sectors total) and
+is well within the 
+<a href="howtos40.php">recovery</a> bounds by 
+<a href="howtos20.php">error correction (ecc) data</a> made 
+with default settings - if you have made the ecc data in time! 
+Otherwise the contents of the red
+sectors are lost since ecc data cannot be created from already defective media.
+<?php end_screen_shot(); ?><p>
+
+<a name="crc"></a>
+<?php begin_screen_shot("Checksum errors","crc-cd.png"); ?>
+<b>Checksum errors.</b> Yellow spots in the spiral depict places
+where the medium was fully readable, but the data read did not match
+checksums in the error correction data. There are two main causes:
+<p>
+
+<ul><li>
+<b>The image has been manipulated</b> after the creation 
+of error correction data and before writing it to the medium.
+This can happen on Unix systems when the image is mounted with write access
+after ecc data has been created. Typical signs are CRC errors in sector 64
+and in sectors 200 to 400 as the system updates the file access times there.
+Performing a data recovery using dvdisaster is typically harmless in this
+situation.<p>
+
+However if you have modified files in the image after creating the ecc data,
+the error correction data will be both worthless and dangerous.
+Applying a recovery to the medium will restore the image state
+at the time the ecc data has been created, and this will obviously not 
+represent the most recent contents of the medium.<p></li>
+
+<li><b>There are technical problems with the computer system,</b>
+especially in mass storage communication. Perform the scan again
+and observe the CRC error locations.
+If CRC errors disappear or surface at different locations your system
+might have defective RAM, bad drive cabling/controllers or incorrect clock
+speeds.</li></ul>
+<?php end_screen_shot(); ?><p>
+
+<?php
+# end_page() adds the footer line and closes the HTML properly.
+
+end_page();
+?>