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12.4. Audio Composition

This section addresses the audio composition and presentation capabilities specified in the BIFS part of the MPEG-4 Systems standard [MPEG4-1]. The tools providing these functionalities are a subset of the BIFS tools called AudioBIFS (together with its extension, Advanced AudioBIFS, which was specified in the first amendment to the MPEG-4 Systems standard [AMD1-1]). The nodes included in AudioBIFS and Advanced AudioBIFS act as an interface between the systems and the audio components of MPEG-4 content. Audio composition is a process by which decoded audio streams (corresponding to different audio objects, possibly encoded with different coding methods) are combined at the terminal to form one or several audio tracks. Audio presentation, on the other hand, includes the processing related to playing back the composed soundtrack and the actual reproduction of the sound via headphones or loudspeakers. The presentation stage also can include the spatial processing of sound that is carried out according to the relative positions of a sound source and a listening point.

In this section, the VRML sound model [VRML97] is reviewed briefly as a background to the object-based audio scene composition developed for MPEG-4. Furthermore, it is explained why the 3D sound model and the sound composition capabilities provided by the VRML sound model are not sufficient for most MPEG-4 applications. After that, the functionality of each AudioBIFS node with relevance regarding the composition and presentation of audio in MPEG-4 will be presented. In the following section, the details on how the spatial properties of audio scenes can be enhanced with the support of Advanced AudioBIFS nodes to produce immersive audiovisual scenes in 3D applications will be presented. Also, the production and control of advanced spatial sound processing effects in the MPEG-4 terminal will be covered.


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