The development of a design tool for 5-speaker surround sound decoders

This thesis presents the development of a software-based decoder design tool (DDT) for producing Ambisonic decoders optimised for playback over 5-speaker layouts. The research specifically focuses on developing decoders for irregular layouts with loudspeakers at a constant radial distance from the central listening position. It was motivated by the desire to provide better surround sound over the standard ITU 5-speaker layout for listeners in the sweet spot and off-centre positions. A wide-ranging literature review is presented revealing the need for such work.
The DDT employs the Tabu Search algorithm to seek improved decoder parameters according to a multi-objective fitness function. The fitness function encapsulates criteria from psychoacoustic models as a set of objectives. In order to ensure the objectives were treated equally a method known as „range-removal‟ was used for the first time in Ambisonic decoder design. A companion technique termed „importance‟ allows the systematic prioritisation of range-removed objectives giving a designer control over desired decoder criteria.
Additional elements exist in the DDT that can be turned on or off in different combinations. They include: a novel component for producing decoders with even performance by angle, a novel component for producing performance that correlates with the pattern of human spatial resolution estimated in previous Minimum Audible Angle experiments, and the ability to produce frequency dependent or independent decoders of different orders. Moreover, the user of the DDT can optimise performance for a single listener or multiple distributed listeners. To make the DDT as interactive as possible searches can optionally run on a High Performance Computer.
This thesis also details the extensive testing of Ambisonic decoders for the ITU layout. Decoders have been assessed subjectively in listening tests and objectively using binaural measurements which has verified the methods developed in this research and the DDT‟s concept. Furthermore, decoders derived by the DDT have been compared to existing decoders and the results show they give equal or better performance.
The development of a fully-functioning DDT which incorporates techniques for range-removal, importance, even performance by angle, minimum audible angle, off-centre listeners and their use in any combination represent the key outcomes of this work.