Has Ambisonics come of age?

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

The generation of panning laws for irregular speaker arrays using heuristic methods

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.EPSR

Hearing Without Ears

We report on on-going work investigating the feasibility of using tissue conduction to evince auditory spatial perception. Early results indicate that it is possible to coherently control externalization, range, directionality (including elevation), movement and some sense of spaciousness without presenting acoustic signals to the outer ear. Signal control techniques so far have utilised discrete signal feeds, stereo and 1st order ambisonic hierarchies. Some deficiencies in frontal externalization have been observed. We conclude that, whilst the putative components of the head related transfer function are absent, empirical tests indicate that coherent equivalents are perceptually utilisable. Some implications for perceptual theory and technological implementations are discussed along with potential practical applications and future lines of enquiry

Tissue-conducted spatial sound fields

We describe experiments using multiple cranial transducers to achieve auditory spatial perceptual impressions via bone (BC) and tissue conduction (TC), bypassing the peripheral hearing apparatus. This could be useful in cases of peripheral hearing damage or where ear-occlusion is undesirable. Previous work (e.g. Stanley and Walker 2006, MacDonald and Letowski 2006)1,2 indicated robust lateralization is feasible via tissue conduction. We have utilized discrete signals, stereo and first order ambisonics to investigate control of externalization, range, direction in azimuth and elevation, movement and spaciousness. Early results indicate robust and coherent effects. Current technological implementations are presented and potential development paths discussed

Inside-outside: 3-D music through tissue conduction

Eliciting an auditory perception by means of mechanical transduction bypassing the peripheral hearing apparatus has been recorded as early as the 16th 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 lateralization is feasible via mechanical transduction. We have extended this, adding the front-back and up-down axes

Analysis of binaural cue matching using ambisonics to binaural decoding techniques

Last year Google enabled spatial audio in head-tracked 360 videos using Ambisonics to binaural decoding on Android mobile devices. There was some early criticism of the 1st order to binaural conversion employed by Google, in terms of the quality of localisation and noticeable frequency response colouration. In this paper, the algorithm used by Google is discussed and the Ambisonics to Binaural conversion using virtual speakers analysed with respect to the resulting inter-aural time, level, and spectrum differences compared to an example HRTF data set. 1st to 35th order Ambisonics using multiple virtual speaker arrays are implemented and analysed with inverse filtering techniques for smoothing the frequency spectrum also discussed demonstrating 8th order decoding correctly reproducing binaural cues up to 4 kHz.N/

An investigation into the real-time manipulation and control of three-dimensional sound fields

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.EPRS

AmbiFreeVerb 2—Development of a 3D ambisonic reverb with spatial warping and variable scattering

In this paper the development of a three dimensional Ambisonic reverb based on the open source FreeVerb algorithm will be presented and discussed. This model is then extended to include processing in over-specified A-format, rather than B-format, variable scattering between channels along with controls for warping the distribution of the reflections to implement a reverb that is able to react to the source position in a spatially coherent way with an acoustical analysis of its performance

GASP: Guitars with ambisonic spatial performance

‘Guitars with Ambisonic Spatial Performance’ (GASP) is an ongoing project where our expertise in surround sound algorithmic research is combined with off-the-shelf hardware and bespoke software to create a spatial multichannel surround guitar performance system. This poster was funded through the ‘Undergraduate Research Scholarship Scheme’ (URSS) and presented at the University of Derby Buxton Campus 10th Annual Learning & Teaching conference on Wednesday 1st July 2015. The theme being ‘Students as Partners: Linking Teaching, Research and Enterprise’. The poster was also utilised as a contribution to the Creative Technologies Research Group (CTRG) ‘Sounds in Space’ symposium held at the University of Derby in June 2015, at which three pieces of multichannel guitar recordings were demonstrated.‘Undergraduate Research Scholarship Scheme’ (URSS) University of Derb