Compatible scrambling of compressed audio

Stimulated by the technological revolution in both networking technology (the Internet) and highly efficient perceptual audio coding algorithms (e.g. MPEG audio), a tremendous amount of music piracy has emerged recently. In contrast to this, a controlled distribution of music or multimedia content commonly employs so-called secure envelope techniques which "package" the audio bitstream into a secure container by means of ciphering all or part of the payload bitstream. In this way, access to the payload (i.e. decoding of the bitstream) is possible only for authorized persons who are in the possession of the proper key for decryption. While decoding of such a secure envelope bitstream requires a two-stage process (deciphering and source decoding), this paper presents a novel technique integrating both deciphering and source decoding into one combined process. This is achieved by "scrambling" the bitstream of the coded signal in a syntax-compatible way such that playback of the scrambled bitstream without access to the proper key will result in a stable playback at a degraded quality level ("soft-envelope" technique). The approach allows the content authors to select the amount of degradation, does not impose a bitrate or quality burden and can be applied to a wide range of coders. Examples of the scrambling technique are given for an MPEG-2 advanced audio coding (AAC) system.