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Chapter 11. General Audio Coding > MPEG-4 Scalable Audio Coding

11.4. MPEG-4 Scalable Audio Coding

According to the traditional approach to perceptual audio coding (see Section 11.1), the bit rate used for the compressed representation of the audio signal is specified at the time of encoding. This may, however, not be appropriate if the bitstream is to be distributed subsequently via transmission channels not known beforehand or channels with varying transmission capacity (e.g., Internet, wireless transmission). This problem is addressed by the concept of MPEG-4 scalable audio coding, which enables the transmission and decoding of the bitstream with a bit rate that can be adapted to dynamically varying requirements, like the instantaneous transmission channel capacity (or, in some cases, the decoder computational resources). Although certain types of scalability were already available for video coding in previous MPEG standards, MPEG-4 is the first standard to provide this core functionality in the context of MPEG Audio standards.

11.4.1. Large-Step Scalable Audio Coding

Scalability within the MPEG-4 Audio standard is mostly achieved by using the concept of hierarchical embedded coding—that is, scalable audio bitstreams consisting of several partial bitstreams that can be decoded on their own and are combined to form a meaningful decoding result. Thus, the decoding of a hierarchical subset of the full bitstream will lead to a valid decoded signal, although at a lower quality. Combining the concepts of scalability and tool-based structure, MPEG-4 Audio has defined a flexible framework in which the existing MPEG-4 audio coding schemes have been integrated into a combined scalable coding structure.


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