Lossless and Perceptual Coding of Digital Audio

We have seen rapid progress in high-quality compression of wideband audio signals. Today’s coding algorithms can achieve substantially better compression than was thought possible only a few years ago. In the case of audio coding with its bandwidth of 20 kHz and more, the concept of perceptual coding has paved the way for significant bit rate reductions. However, multiple coding can reveal originally masked distortions. In addition, reproduction of critical music items shows that even the best systems can not be considered as truly transparent. Therefore lossless audio coding has become a topic of high interest both for professional and customer applications. This paper will explain approaches to lossless and lossy compression, both with emphasis on MPEG standards which have found a wide range of communications-based and storagebased applications. As an example for state-of the-art lossless coding, an overview of the forthcoming MPEG-4 Audio Lossless Coding (ALS) standard will be presented. On the other hand, it will be shown that the recent MPEG-4 Advanced Audio Coding (AAC) standard outperforms many other perceptual coding algorithms (including MP3 coders). Finally, we will address the current MPEG-4 speech and audio coding standardization work which merges the whole range of audio from high fidelity audio coding and speech coding down to synthetic audio, synthetic speech and text-to-speech conversion.