# Background information

dvdisaster Version 0.70 | To the Internet version

Contents

Overview
Examples and screen shots
Download
Questions and Answers
Background information
· | Error correction properties
· | Image level data recovery
· | RS01 and RS02
· | Linear reading strategy
· | Adaptive reading strategy
· | Remarks on read errors
· | Hints for storing the error correction files

### The RS01 and RS02 methods

dvdisaster contains two error correction methods named RS01 and RS02. RS01 is the existing and proven method while RS02 is still under development. RS02 is currently only available at the command line and will be fully integrated into the graphical user interface in Version 0.70.

Comparison of both methods. RS01 and RS02 build on the same Reed-Solomon error correction. They calculate error correction information for CD/DVD images which is used to recover unreadable sectors if the disc becomes damaged afterwards.

The methods differ in the way the error correction information is stored:

- RS01 creates error correction files which are stored separately from the image they belong to. Since data protection at the file level is difficult, error correction files must be stored on media which are protected against data loss by dvdisaster, too.

- To apply the RS02 method an image is first created on hard disc using a CD/DVD writing software. Before the image is written on the medium, dvdisaster is used to augment the image with error correction data. Therefore the data to be protected and the error correction information are located at the same medium. Damaged sectors in the error correction information reduce the data recovery capacity, but do not make recovery impossible - a second medium for keeping or protecting the error correction information is not required.

Comparison of error correction storage.

Error correction files | Image augmented with error correction data
any possible redundancy can be chosen | redundancy is limited by free space on medium (= medium capacity - size of data image)
already effective at 15% redundancy since error correction files are required to be free of damage | requires more redundancy (recommended: 20-30%) to compensate defects in the error correction data
medium can be completely filled with data | usable medium capacity is reduced by amount of error correction data
can be created for already existing media | only applicable before writing the new medium since the image must be augmented with error correction information in advance
separately storing the error correction file from user data strengthens data protection | common storage of user data and error correction data may reduce error correction capacity
Mapping between error correction files and media must be kept. Error correction files must be protected against damage. | Easy one-medium solution; error correction information needs not to be cataloged or explicitly protected.
no compatibilty issues with play-back units | media with augmented images may not play correctly on all units

Copyright 2004-2008 Carsten Gnörlich. Verbatim copying and distribution of this entire article is permitted in any medium, provided this notice is preserved.