DualStream is aimed at filling the big gap between perceptual coding and lossless coding. The goal is to offer real transparent audio coding (not only perceptually transparent) at half or less the bitrate generally used by lossless coding, and also to permit progressive consistent increase of the quality level, until lossless coding is reached. To eliminate the problem of choosing between lossless and near lossless, DualStream has an option to create a correction file, which may be eventually stored separately and used at a later time to restore the original. The advantage is that the two files (near lossless and correction) have together approximately the size of the lossless coded original file.

http://losslessaudiocompression.com/DualStream.php