Author(s)Peter Noll, Tilman Liebchen
TitleLossless and Perceptual Coding of Digital Audio
Book titleBeiträge zur Geschichte und neueren Entwicklung der Sprachakustik und Informationsverarbeitung - Werner Endres zum 90. Geburtstag
Publisherw.e.b. Universitätsverlag
AbstractWe 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.1 Introduction Wideband (high fidelity) audio representations including multichannel audio need bandwidths of at least 20 kHz. The conventional digital format of digital audio is PCM, with sampling rates of 32, 44.1, or 48 kHz and an amplitude resolution (PCM bits per sample) of 16 bit. Typical application areas for digital audio are in the fields of audio production,program distribution and exchange, digital sound broadcasting, digital storage, and various multimedia applications. For archiving and processing of audio signals, highest quality
formats with up to 192 kHz sampling and 24 to 32-bit Amplitude resolution are already used.Audio coding is employed in order to reduce bit rate compared to the PCM epresentation.In some applications coding will have to be lossless, with compression factors around two as will be shown shortly. For other applications, perceptually transparent coding will be sufficient, which allows to compress the audio data to less than a tenth of its original size.The Compact Disc (CD) is today's de facto standard for disc-base delivery of digital audio.The CD uses the PCM format with 16-bit amplitude resolution and 44.1 kHz sampling rate,