<title>MPEG-4 low-delay general audio coding</title>

Abstract

Traditionally, speech coding for communication purposes and perceptual audio coding have been separate worlds. On one hand, speech coders provide acceptable speech quality at very low data rates and low delays which are suitable for two-way communication applications, such as Voice over IP (VoIP) or teleconferencing. Due to the underlying coding paradigm, however, such coders do not perform well for non-speech signals (e.g.~music and environmental noise). Furthermore, the sound quality and naturalness is severely limited by the fact that most coders are working in narrow-band mode, i.e. with a bandwidth below 4 kHz. On the other hand, perceptual audio codecs provide excellent subjective audio quality for a broad range of signals including speech at bit rates down to 16 kbit/s. The delay of such a coder/decoder chain, however, usually exceeds 200 ms at very low data rates and in this way is not acceptable for interactive two-way communication. This paper describes a coding scheme which is designed to combine the advantages of perceptual audio coding with the low delay necessary for two-way communication. The codec was standardized within MPEG-4 Version 2 Audio under the work item ``Low Delay Audio Coding'' and is derived from the ISO/MPEG-2/4 Advanced Audio Coding (AAC) algorithm. The algorithm provides modes operating at algorithmic delay as low as 20 ms and is equipped to handle all full-bandwidth high-quality audio signals, both in monophonic, stereophonic and even multi-channel format. Despite of the low algorithmic delay, the codec delivers better audio quality than MPEG-1 Layer-3 (MP3) at the same bit rate. The paper also addresses issues pertaining to the integration of the coder into H.32x and SDP applications.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.