Compact Preservation of Scrambled CD-ROM Data

When preserving CD-ROM discs, data sectors are often read in a so-called “scrambled mode” in order to preserve as much data as possible. This scrambled data is later unscrambled and further processed into a standard CD-ROM disc image. The process of converting the scrambled data into a standard CD-ROM disc image is potentially lossy, but standard CD-ROM disc images exhibit much higher software compatibility and have greater usability compared to the scrambled data from which they are derived. Consequently, for preservation purposes, it is often necessary to store both the scrambled data and the corresponding standard disc image, resulting in greatly increased storage demands compared to storing just one or the other. Here, a method that enables compact storage of scrambled data alongside the corresponding (unscrambled) standard CD-ROM disc image is introduced. The method produces a compact representation of the scrambled data that is derived from the unscrambled disc image. The method allows for (1) storage of the standard unscrambled disc image in unmodified form, (2) easy reconstruction of the scrambled data as needed, and (3) a substantial space savings (in the typical case) compared to storing the scrambled data using standard data compression techniques.