40 NTT Technical Review 1. Background In parallel with the evolution of broadband net- works and digital audio equipment, information rates for delivery and storage keep growing rapidly in response to demands for high-quality audio signals (high sampling rates, high word resolution, and mul- tiple channels). NTT Communication Science Labo- ratories recognized the importance of lossless com- pression of audio signals and the standardization of its technology, considering interoperability, long- term maintenance, and clear status of intellectual property rights. The Laboratories took the initiative in promoting this technology and advancing it to a standard in MPEG (moving picture experts group), which is a working group within ISO/IEC (Interna- tional Standards Organization, International Elec- trotechnical Commission). NTT initiated discussion on the need for and requirements for such a standard and prepared the technical call for technologies. Through the normal standardization process, several improvements and integration efforts were applied to the initial refer- ence model. The partners for this standardization included the Technical University of Berlin (Ger- many), RealNetworks Corp. (USA), and I2R (Singa- pore). As a result of the final ballot in December 2005, the specifications of the lossless coding were officially established as “14496-4 3rd edition amendment 2 (ALS: audio lossless coding)” and published in March 2006 [1]-[5]. Note that, in parallel, MPEG-4 also established MPEG-4 SLS (scalable lossless cod- ing), which provides the special functionality of lossy-to-lossless compression and bit-rate scalable compression. In this article, we focus on the more general compression tool: MPEG-4 ALS. As shown in Fig. 1, MPEG audio standards have made significant contributions to communication systems including broadcasting, mobile services, and Internet services. Most of the audio coding standards, such as MP3 and AAC or the technology for the MiniDisc system *1 , are based on perceptual coding with a high compression ratio in exchange for minor waveform distortion at the decoder. These encoders carefully control the quantization distortion by utiliz- ing the characteristics of the human ear. The wave- form is different from the original, although it is per- ceptually very close to it. Unlike perceptual coding, lossless coding ensures perfect reconstruction of the waveform without a sin- Takehiro Moriya † , Noboru Harada, Yutaka Kamamoto, and Hiroshi Sekigawa Abstract This article explains the technologies and applications of lossless audio coding. NTT started research on lossless coding of audio signals and proposed the initiation of international standardization in 2002, aiming at high-quality services suitable for broadband networks. As a result of cooperative work with other organizations, the specifications of this technology were officially incorporated in the ISO/IEC MPEG standard published in March 2006 (ISO: International Standards Organization, IEC: Internation- al Electrotechnical Commission, MPEG: moving picture experts group). MPEG-4 ALS—International Standard for Lossless Audio Coding Letters † NTT Communication Science Laboratories Atsugi-shi, 243-0198 Japan E-mail: moriya@idea.brl.ntt.co.jp *1 MiniDisc: System introduced by Sony Corporation as a digital replacement for conventional analog compact audio cassettes.