Abstract
Spatial Audio Coding (SAC) is a technique used to encode multichannel audio signals by extracting the spatial parameters and downmixing the audio signals to a mono or stereo audio signal. Recently, various SAC techniques have been proposed to efficiently encode multichannel audio signals. However, all of them operate in open-loop, where the encoder and decoder operate sequentially and independently, and, thus, lack a mechanism for minimising the decoded audio reconstruction error. This thesis proposes a novel SAC technique that utilises the closed-loop system configuration, termed Analysis by Synthesis (AbS), in order to optimise the down-mix signal and the spatial parameters, so as to minimise the decoded signal error. In order to show the effect of the AbS optimisations, the Reverse One-To-Two (R-OTT) module, used in the MPEG Surround (MPS), must first be applied in the frequency domain to recalculate the downmix and residual signals based on the quantised spatial parameters. These parameters show that the AbS scheme can minimise the quantisation errors of the spatial parameters. As the full AbS is far too complicated to be applied in practice, a simplified AbS algorithm for finding sub-optimal parameters, based on the adapted R-OTT module, is also proposed. Subjective tests show that the proposed Analysis by Synthesis Spatial Audio Coding (AbS-SAC), encoding 5-channel audio signals at a bitrate of 51. 2 kb/s per audio channel, achieves higher Subjective Difference Grade (SDG) scores than the tested Advanced Audio Coding (AAC) technique. Furthermore, the objective test also shows that the proposed AbS-SAC method, operating at bitrates of 40 to 96 kb/s per audio channel, significantly outperforms (in terms of Objective Difference Grade (ODG) scores) the tested AAC multichannel technique.