Hierarchical bandwidth limitation of surround sound.

This thesis describes the development of a psychoacoustically optimised bandwidth limitation method for multichannel audio. The proposed method takes advantages from the hierarchical representation of multichannel audio signals. Several hierarchical transform techniques were reviewed and their applicability to be used in the proposed hierarchical bandwidth limitation method were verified using psychoacoustical studies. The Karhunen-Loeve Transform (KLT) outperformed other hierarchical transform techniques due to its ability of rearranging the information in a perceptually hierarchical manner. Another advantage of KLT is that the transform matrices are signal-content-dependent and are adaptive to the actual spatial characteristics of audio material. The perceptual effects of the adaptive KLT-based hierarchical bandwidth limitation were studied. The results showed that applying KLT to short-term multichannel audio signals and updating the KLT transform matrices adaptively over time will result in better audio quality compared with non-adaptive KLT for those multichannel audio programmes that had significantly varying statistical characteristics over time. In addition, the bandwidth allocation strategy in KLT-based hierarchical bandwidth limitation was optimised for 3/2 stereo format for two levels of overall bandwidth: 40 and 60 kHz. An evaluation has been made of the optimised KLT-based hierarchical bandwidth limitation algorithm by comparing it with other bandwidth limitation algorithms. The results showed that the KLT-based hierarchical bandwidth limitation with the optimal bandwidth allocation strategy provided higher audio quality than the other traditional bandwidth limitation algorithms used for the limitation of bandwidth of multichannel sound.