|dvdisaster Version 0.72.5 / 0.79 (devel-3)|
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Remarks on read errorsOptical media have their own error correction code which protects the data against small manufacturing errors and inaccuracies during writing. If the writer and medium are compatible and of high quality, the error correction built into the medium will at first be mainly idle. This leaves enough reserves to compensate normal wear and aging effects during many years of the medium usage.
When the capacity of the built-in error correction is finally exhausted, read errors will start to appear on the medium. These will be reported by the "Scan"-operation of dvdisaster. Depending on the time of first occurrence, two types of read errors are of particular interest:
Read errors appearing right after writing the medium. This is a sign of:
Please withstand the temptation of trying to preserve the faulty media by means of an error correction file - this will most likely end with data loss.
Read errors after a few months/years. The built-in error correction of the medium will be increasingly loaded during its life time until it finally fails and read errors show up. This happens for mechanical reasons (scratches, warping of the plastic material) as well as for chemical causes (decaying dye and/or reflective layer).
These effects typically occur while the medium is stored away for a few months, and it may not be possible to read in all sectors afterwards.
Therefore it is crucial to create the error correction data in time. The ecc data contains information for recalculating the contents of missing sectors (within certain limits). Therefore with the help of the ecc data dvdisaster can recover images even if not all sectors could actually be read by the drive.
Since the error correction can reconstruct missing sectors up to a certain number, it is not necessary to squeeze out a defective medium for every readable sector. The adaptive reading strategy checks during reading whether enough data for error correction has been collected. As soon as this is the case, reading stops and still unread sectors will be recovered using the ecc data.
The outcome from reading damaged media depends on several factors:
Copyright 2004-2013 Carsten Gnörlich.
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