Arts, Design and Technology

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.

Capturing and replaying distance cues for multi-channel audio is currently an under-explored and under-exploited area.  Panners that successfully give control of distance do not, currently exist.  However, recordings made with 1st order ambisonic, Soundfield microphones replayed over an ambisonic rig can give realistic results with respect to distance perception (particularly when bringing sound sources inside the speaker array). 

Near-field effect, resulting from the wave front curvature of near-field sources, is one cue recorded by the microphone, but not reproduced by software or hardware panners.  Papers by Daniel (2003, 2004) discuss the encoding and decoding of ambisonic material with particular reference to higher-order ambisonics, and describe ‘near-field coding’ filters which encode near-field effect while pre-compensating for finite loudspeaker reproduction distance.  While existing research concentrates on its simulation, this report documents an investigation into near-field effect in Soundfield ST350 and MK V tetrahedral microphones.  It is found that, as a result of calibration for a flat frequency response at a practical source distance, the Soundfield microphone responses bear strong similarity to various near-field coding filters, suggesting the existence of an optimum loudspeaker array radius for positional localisation.  On determination of this distance, recordings may be adapted for proper reproduction at any chosen reference distance using the WigWare ambisonic plug-ins created at the University of Derby.

Ambisonics was developed in the 1970’s as a flexible, psycho-acoustically aware system1.  Developed at the same time as Quadraphonics2, Ambisonics is an often mis-understood system that was far ahead of it’s time.  Due to the ubiquity of surround sound equipment in modern computers and interest in live surround events becoming more widespread, is the time, finally, right for Ambisonics to come into its’ own?

In this paper, the definition of what makes a system Ambisonic is clarified with reference made to the traditional energy and velocity vector theory, higher order systems and use in both the live and domestic environment.  More recent developments by the author are discussed with respect to irregular Ambisonic decoder design (such as for the ITU 5.1 speaker array) and analysis using Head Related Transfer Function data showing the extra insight this can give into the performance of one, seemingly similar, decoder design over another.  The freely available suite of VST plug-ins (comprising of decoders, panners and an Ambisonic reverb) created using this technology are also presented, with case studies of their use in student projects at the University of Derby.

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.

The reduction of spatial variation in the quality of reproduced sound within a defined space using varied loudspeaker placements is a significant challenge for sound engineers. Dr Bruce Wiggins has conducted research into encoding, decoding and processing algorithms using Ambisonics, a system based around full-sphere sound reproduction. The outcomes of the research have been made accessible to the wider community by the creation of a suite of software plug-ins (WigWare), a production workflow, and associated teaching materials which can enable commercial audio workstations to benefit from Ambisonics. There are numerous recorded instances of successful use.

This presentation covers the basic design of the WigWare AmbiFreeverb 2 3D reverberation VST plug-in for Mac and PC. This was followed by a demonstration on a 20 speaker with-height rig at the University of Derby's Sounds in Space research symposium event. To download the plug-in, visit http://www.brucewiggins.co.uk/

Eliciting an auditory perception by means of mechanical transduction bypassing the peripheral hearing apparatus has been recorded as early as the 16 th century. Excluding its audiometric use to assess ear pathology, bone and soft tissue conduction has received very little interest until the last two decades. Previous work during this time (Stanley and Walker 2006, MacDonald and Letowski 2006) has indicated robust lateralisation is feasible via mechanical transduction. This paper reports on an extension to this work, adding the front-back and up-down axes. Background Eliciting an auditory perception by means of mechanical transduction has been in use for hundreds of years. Several transmission pathways may be employed to deliver an audible experience, these pathways are often collectively referred to as bone conduction; as soft tissues and cerebrospinal fluid may feature significantly in these we refer to them as tissue conduction (TC). Four pathways out of several have been identifi...

The reduction of spatial variation in the quality of reproduced sound within a defined space using varied loudspeaker placements is a significant challenge for sound engineers. Dr Bruce Wiggins has conducted research into encoding, decoding and processing algorithms using Ambisonics, a system based around full-sphere sound reproduction. The outcomes of the research have been made accessible to the wider community by the creation of a suite of software plug-ins (WigWare), a production workflow, and associated teaching materials which can enable commercial audio workstations to benefit from Ambisonics. There are numerous recorded instances of successful use. The significance and reach of this work are articulated and evidenced in this article produced for the Research Excellence Framework (REF) 2014 (http://www.ref.ac.uk/).