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2004, University of Derby Doctoral Thesis
This thesis describes a system that can be used for the decoding of a three dimensional audio recording over headphones or two, or more, speakers. A literature review of psychoacoustics and a review (both historical and current) of surround sound systems is carried out. The need for a system which is platform independent is discussed, and the proposal for a system based on an amalgamation of Ambisonics, binaural and transaural reproduction schemes is given. In order for this system to function optimally, each of the three systems rely on providing the listener with the relevant psychoacoustic cues. The conversion from a five speaker ITU array to binaural decode is well documented but pair-wise panning algorithms will not produce the correct lateralisation parameters at the ears of a centrally seated listener. Although Ambisonics has been well researched, no one has, as yet, produced a psychoacoustically optimised decoder for the standard irregular five speaker array as specified by the ITU as the original theory, as proposed by Gerzon and Barton (1992) was produced (known as a Vienna decoder), and example solutions given, before the standard had been decided on. In this work, the original work by Gerzon and Barton (1992) is analysed, and shown to be suboptimal, showing a high/low frequency decoder mismatch due to the method of solving the set of non-linear simultaneous equations. A method, based on the Tabu search algorithm, is applied to the Vienna decoder problem and is shown to provide superior results to those shown by Gerzon and Barton (1992) and is capable of producing multiple solutions to the Vienna decoder problem. During the write up of this report Craven (2003) has shown how 4th order circular harmonics (as used in Ambisonics) can be used to create a frequency independent panning law for the five speaker ITU array, and this report also shows how the Tabu search algorithm can be used to optimise these decoders further. A new method is then demonstrated using the Tabu search algorithm coupled with lateralisation parameters extracted from a binaural simulation of the Ambisonic system to be optimised (as these are the parameters that the Vienna system is approximating). This method can then be altered to take into account head rotations directly which have been shown as an important psychoacoustic parameter in the localisation of a sound source (Spikofski et al., 2001) and is also shown to be useful in differentiating between decoders optimised using the Tabu search form of the Vienna optimisations as no objective measure had been suggested. Optimisations for both Binaural and Transaural reproductions are then discussed so as to maximise the performance of generic HRTF data (i.e. not individualised) using inverse filtering methods, and a technique is shown that minimises the amount of frequency dependant regularisation needed when calculating cross-talk cancellation filters.
Currently, the ITU standard surround sound speaker arrangement is based on an irregular 5 speaker array. However, this may change to an irregular 7 speaker array (as is now the standard on computer hardware) or more in the future. The Ambisonic system, pioneered by Micheal Gerzon, among others, in the late 1960’s, is very well suited to situations where the end system speaker configuration is not fixed in terms of number or position while also offering a simple way (via energy and velocity vector analysis) of quantifying the performance of such systems. However, while the derivation of the decoders is well documented for regular speaker arrangements [1], optimising the decoders for irregular layouts is not a simple task, where optimisation requires the solution of a set of non linear simultaneous equations, complicated further by the fact that multiple solutions are possible [2]. Craven [3] extended the system to use higher order circular harmonics and presented a 4th order Ambisonic decoder (9 input channels), although the derivation method used was not presented. In this paper a semi-automated decoder optimisation system using heuristic methods will be presented that will be shown to be robust enough to generate higher order Ambisonic decoders based on the energy and velocity vector parameters. This method is then analytically compared to Craven’s decoder using both energy/velocity vector and head related transfer function based methods. [1] Gerzon, M.A. (1985) Ambisonics in Multichannel Broadcasting and Video. Journal of the Audio Engineering Society, Vol. 33, No. 11, p. 851-871. [2] Gerzon, M. A. & Barton, G. J. (1992) Ambisonic Decoders for HDTV. Proceedings of the 92nd International AES Convention, Vienna. 24 – 27 March. Preprint 3345. [3] Craven, P. (2003), Continuous Surround Panning for 5-speaker Reproduction, AES 24th International Conference, Banff, Canada.
2003 •
IEEE International Symposium on Virtual Environments, Human-Computer Interfaces and Measurement Systems, 2003. VECIMS '03. 2003
3D binaural sound reproduction using a virtual ambisonic approach2003 •
We report on the practical implementation of an Ambisonics decoder for irregular 3D speaker layouts. The developed decoder, which uses a non-linear search algorithm to look for the optimal Ambisonics coefficients for each loudspeaker, has a number of features specially tailored for reproduction in real-world 3D audio venues (for example, special 3D audio installations, concert halls, audiovisual installations in museums, etc). In particular, it performs well even for highly irregular speaker arrays, giving an acceptable listening experience over a large audience area.
Convincing sound reproduction via headphones requires filtering of virtual sound sources with head related transfer functions (HRTF). HRTFs describe signal differences at the two ear drums in level, time and frequency weighting [1,2]. Another psychoacoustic phenomenon of the ...
Journal of The Audio Engineering Society
Comparison of Techniques for Binaural Navigation of Higher-Order Ambisonic Soundfields2015 •
In previous papers, the present authors described techniques for design, implementation, and evaluation of Ambi-sonic decoders for regular loudspeaker arrays. However, irregular arrays are often required to accommodate domes-tic listening rooms. Because the figures of merit used to predict decoder performance are non-linear functions of speaker positions, non-linear optimization techniques are needed. In this paper we discuss the implementation of an open-source application, based on the NLopt non-linear optimization software library, that derives decoders for arbi-trary arrays of loudspeakers, as well as providing a prediction of their performance using psychoacoustic criteria, such as Gerzon's velocity and energy localization vectors. We describe the implementation and optimization crite-ria, and report on informal listening tests comparing the decoders produced.
2011 •
The aim of this project is to expand on the techniques and knowledge used in binaural audio. This includes main characteristics: Interaural Time Difference (ITD), Interaural Level Difference (ILD) and Head Related Transfer Function (HRTF). Recordings were made at the University's anechoic chamber with a dummy head and binaural microphones to test the effect of turning the head in front of a speaker. The recordings done included a range of pure tunes at different frequencies, white noise and sine sweeps. Programs were done in MATLAB to determine ITDs and IILs as well as HRTFs based on Fourier analysis and cross correlation and autocorrelation of the sounds recorded at the microphones and the sounds played. The outcome of the project was a set of binaural cues and data used to generate transfer functions that can be applied to dry mono sounds to perform virtual localization on them. Declaration
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