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Chapter 12. SNHC Audio and Audio Composi... > Synthetic-Natural Hybrid Coding of A...

12.1. Synthetic-Natural Hybrid Coding of Audio

The SNHC of audio is enabled by MPEG-4 Systems through the AudioBIFS architecture and can be used to combine sound streams that are decoded with different (synthetic or natural) audio decoders, as depicted in Figure 12.2. In this figure, it can be seen that at the BIFS level the decoded sounds are processed in a similar way, regardless of the decoder used. The hierarchical AudioBIFS subtree mixes the decoder outputs, and these mixed sounds are associated with the rest of the scene with the aid of the topmost nodes of the AudioBIFS graphs. In Figure 12.2, the topmost node is Sound, which can be given a 3D spatial location and a simple, directive sound radiation pattern. Thus, the AudioBIFS graph describes the audio signal flow from the output of the audio decoders through the processing subtree defined in AudioBIFS, and finally to the playing of the sound. The final output at the terminal is the sum of the outputs of the different AudioBIFS subtrees in the overall BIFS scene graph. If spatial properties are given to the topmost sound nodes of the audio subtrees, they can be thought of as representing real-world sound sources in a virtual scene [ScVH99, ScLY00]. This effect can be further enhanced if these nodes are associated with visual objects.

Figure 12.2. Hybrid coding of synthetic and natural sounds: AudioBIFS nodes define how the different streaming and decoded sound objects are composed together.



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