Introduction
Traditional high quality audio coding schemes like MPEG-1 Layer-3 (aka .mp3) have found
their way into many applications including widespread acceptance on the Internet. MPEG-4 audio is scheduled to
be the successor of these, building and expanding on the acceptance of earlier audio coding formats. To do this,
MPEG-4 natural audio coding has been designed to fit well into the philosophy of MPEG-4. It enables new functionalities
and implements a paradigm shift from the linear storage or streaming architecture of MPEG-1 and MPEG-2 into objects
and presentation rendering. While most of these new functionalities live within the tools of MPEG-4 structured
audio and audio BIFS, the syntax of the classical audio coding algorithms within MPEG-4 natural audio has been
defined and amended to implement scalability and the notion of audio objects. This way MPEG-4 natural audio goes
well beyond classic speech and audio coding algorithms into a new world which we will see unfold in the coming years.
Overview
The tools defined by MPEG-4 natural audio coding can be combined to different
audio coding algorithms. Since no single coding paradigm was found to span
the complete range from very low bitrate coding of speech signals up to high
quality multi-channel audio coding, a set of different algorithms has been
defined to establish optimum coding efficiency for the broad range of anticipated
applications (Fig. 1, [1]). The following list
introduces the main algorithms and the reason for their inclusion into MPEG-4.
The following chapters will give more detailed descriptions for each of the
tools used to implement the coding algorithms. The following table lists the
major algorithms of MPEG-4 natural audio. Each algorithm was defined from
separate coding tools with the goal of maximizing the overlap of tools between
different algorithms and maximizing the flexibility in which tools can be
used to generate different flavors of the basic coding algorithms.
Figure 1: Assignment of codecs to bitrate ranges
- HVXC Low rate clean speech coder
- CELP Telephone speech / wideband speech coder
- GA General Audio coding for medium and high qualities
- TwinVQ Additional coding tools to increase the codingefficiency at very low bitrates
In addition to the coding tools used for the basic coding functionality,
MPEG-4 provides techniques for additional features like bit stream scalability.
Tools for these features will be explained later
in this paper.
