Synthesis of volumetric virtual sources is a useful technique for auditory displays and virtual worlds. This task can be simplified into synthesis of perceived spatial extent. Previous research in virtual-world Directional Audio Coding has shown that spatial extent can be synthesized with monophonic sources by applying a time-frequency-space decomposition, i.e., randomly distributing time-frequency bins of the source signal. However, although this technique often achieved perception of spatial extent, it was not guaranteed and the timbre could degrade. In this article this technique is revisited in detail and the effect of different parameters is examined to ultimately achieve optimal quality and perception in all situations. The results of a series of informal and formal experiments are presented here, and they suggest that the revised method is viable in many cases. There is some dependency on the signal content that requires proper tuning of parameters. Furthermore, it is shown that different distribution widths can be produced with the method as well. From a psychoacoustical perspective, it is interesting that distributed narrow frequency bands form a spatially extended auditory event with no apparent directional focus.
Directional audio coding (DirAC) is a parametric time-frequency domain method for processing spatial audio based on psychophysical assumptions and on energetic analysis of the sound field. Methods to use DirAC in spatial sound synthesis for virtual worlds are presented in this article. Formal listening tests are used to show that DirAC can be used to position and to control the spatial extent of virtual sound sources with good audio quality. It is also shown that DirAC can be used to generate reverberation for N-channel horizontal listening with only two monophonic reverberators without a prominent loss in quality when compared with quality obtained with N-channel reverberators.
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