Applications of Continuous Spatial Models in Multiple Antenna Signal Processing

This thesis covers the investigation and application of continuous spatial models for multiple antenna signal processing. The use of antenna arrays for advanced sensing and communications systems has been facilitated by the rapid increase in the capabilities of digital signal processing systems. The wireless communications channel will vary across space as different signal paths from the same source combine and interfere. This creates a level of spatial diversity that can be exploited to improve the robustness and overall capacity of the wireless channel. Conventional approaches to using spatial diversity have centered on smart, adaptive antennas and spatial beam forming. Recently, the more general theory of multiple input, multiple output (MIMO) systems has been developed to utilise the independent spatial communication modes offered in a scattering environment. ¶ ..

Soundfield representation, reconstruction and perception

This thesis covers the area of soundfield representation, reconstruction and perception. The complexity and information content of a soundfield presents many mathematical and engineering challenges for accurate reconstruction. After an in-depth review of the field of mathematical soundfield representation, an analysis of the numerical and practical constraints for soundfield reconstruction is presented. A review of work in experimental psycho-acoustics higlights the variability of spatial sound perception. It is shown that the error and uncertainty in perception is of a comparable magnitude to the accuracy achievable by present soundfield systems. Therefore, the effects of hearing adaption, sensory bias, sensory conflict, and contextual memory cannot be ignored. If the listening environment is inappropriate or in conflict with the desired perceptual experience, little is gained from more complex soundfield representation or reconstruction. The imp! lications of this result to the delivery of spatial audio is discussed and some open problems for further exploration and experimentation are detailed

Spatial sound intensity vectors in spherical harmonic domain

Sound intensity is a fundamental quantity describing acoustic wave fields and it contains both energy and directivity information. It is used in a variety of applications such as source localization, reproduction, and power measurement. Until now, intensity is defined at a point in space, however given sound propagates over space, knowing its spatial distribution could be more powerful. This paper formulates spatial sound intensity vectors in spherical harmonic domain such that the vectors contain energy and directivity information over continuous spatial regions. These representations are derived with finite sets of closed form coefficients enabling ease of implementation

Spherical harmonics based generalized image source method for simulating room acoustics

Allen and Berkley's image source method (ISM) is proven to be a very useful and popular technique for simulating the acoustic room transfer function (RTF) in reverberant rooms. It is based on the assumption that the source and receiver of interest are both omnidirectional. With the inherent directional nature of practical loudspeakers and the increasing use of directional microphones, the above assumption is often invalid. The main objective of this paper is to generalize the frequency domain ISM in the spherical harmonics domain such that it could simulate the RTF between practical transducers with higher-order directivity. This is achieved by decomposing transducer directivity patterns in terms of spherical harmonics and by applying the concept of image sources in spherical harmonics based propagation patterns. Therefore, from now on, any transducer can be modeled in the spherical harmonics domain with a realistic directivity pattern and incorporated with the proposed method to simulate room acoustics more accurately. We show that the proposed generalization also has an alternate use in terms of enabling RTF simulations for moving point-transducers inside pre-defined source and receiver regions.Thanks to Australian Research Council Linkage Grant funding scheme (Project No. LP160100379)

A Novel Method for Obtaining Diffuse Field Measurements for Microphone Calibration

We propose a straightforward and cost-effective method to perform diffuse soundfield measurements for calibrating the magnitude response of a microphone array. Typically, such calibration is performed in a diffuse soundfield created in reverberation chambers, an expensive and time-consuming process. A method is proposed for obtaining diffuse field measurements in untreated environments. First, a closed-form expression for the spatial correlation of a wideband signal in a diffuse field is derived. Next, we describe a practical procedure for obtaining the diffuse field response of a microphone array in the presence of a non-diffuse soundfield by the introduction of random perturbations in the microphone location. Experimental spatial correlation data obtained is compared with the theoretical model, confirming that it is possible to obtain diffuse field measurements in untreated environments with relatively few loudspeakers. A 30 second test signal played from 4-8 loudspeakers is shown to be sufficient in obtaining a diffuse field measurement using the proposed method. An Eigenmike is then successfully calibrated at two different geographical locations.Comment: Accepted to appear in IEEE ICASSP 202

Modeling Characteristics of Real Loudspeakers Using Various Acoustic Models: Modal-domain Approaches

The accuracy and perception of soundfields produced by loudspeaker arrays are strongly influenced by the inherent characteristics of the commercial loudspeakers. This paper analyzes such characteristics of loudspeakers by deriving equivalent theoretical models, and by studying their impact on soundfield reproduction. A number of acoustic models are investigated, including plane waves decomposition, point source decomposition and mixed source decomposition. Each proposed model employs three effective sparse decomposition algorithms for optimized solutions, including iteratively reweighted least squares (IRLS), matching pursuit (MP) and least absolute shrinkage and selection operator (LASSO). A successful model shall enable the prediction of the soundfield outside the original recording region. Therefore, we validate the effectiveness of the models by comparing the simulated soundfield with secondary measurements obtained beyond the original area. Experimental results have confirmed that both the plane wave and mixed source model achieve promising performance with respect to the proposed metrics.Australian Research Council Linkage Grant Projects funding scheme (project no. LP160100379). Yonggang Hu is sponsored by CSC agency for funding

Reducing Modal Error Propagation through Correcting Mismatched Microphone Gains Using Rapid

Microphone array calibration is required to accurately capture the information in an audio source recording. Existing calibration methods require expensive hardware and setup procedures to compute filters for correcting microphone responses. Typically, such methods struggle to extend measurement accuracy to low frequencies. As a result, the error due to microphone gain mismatch propagates to all the modes in the spherical harmonic domain representation of a signal. Several existing algorithms use modal representation of sound and error propagation in modes fundamentally limits the performance of such algorithms. A method for reducing the error propagation in modes by correcting the mismatched microphone gains is proposed, where RAndom PerturbatIons for Diffuse-field (RAPID) is used to design filters for correcting the mismatch. Experimental results show that the directivity pattern of a calibrated spherical microphone array using RAPID provides up to 6dB improvement in the front back factorThis research work is funded by Australian Research Council Linkage Project LP160100379

Factors associated with the presence and prevalence of contagious ovine digital dermatitis: a 2013 study of 1136 random English sheep flocks

In 2013, a questionnaire was used to gather data on risks for introduction, and factors associated with prevalence, of contagious ovine digital dermatitis (CODD). There were 1136 (28.4%) usable responses from 4000 randomly selected sheep farmers in England. CODD was present in 58% (662) of flocks, with a reported prevalence of CODD lesions of 2.3%. The geometric mean period prevalence of all lameness was 4.2% and 2.8% in CODD positive and negative flocks respectively. Factors associated with a greater risk of presence of CODD were purchasing replacement ewes, not always checking the feet of sheep before purchase, not isolating purchased sheep, foot bathing returning ewes, foot trimming the flock more than twice in the year all compared with not doing these activities and increasing log10 flock size. Farmers who vaccinated sheep with Footvax™ were less likely to report presence of CODD. Factors associated with increasing prevalence of CODD lesions were not always checking the feet of purchased sheep, flocks that mixed with other flocks and sheep that left the farm for summer grazing and later returned. In addition, flocks where farmers followed the current recommended managements for control of footrot, had a lower prevalence of CODD whilst those who used foot bathing and where feet bled during routine foot trimming had a higher prevalence of CODD. The prevalence of CODD decreased with each log10 increase in flock size. We conclude that CODD is an infectious cause of lameness in sheep of increasing importance in GB. Introduction is linked to poor biosecurity with one likely source of the pathogen being introduction of or mixing with infected sheep. As with footrot, prevalence of CODD was lower in flocks where farmers focused on individual treatment to manage lameness and avoided foot bathing and trimming feet. We conclude that most of the currently recommended biosecurity and treatment approaches to control footrot in GB are also effective for control of CODD