James A. S. Angus

Analyzing head-related transfer function measurements using surface spherical harmonics

A continuous, functional representation of a large set of head-related transfer function measurements (HRTFs) is developed. The HRTFs are represented as a weighted sum of surface spherical harmonics (SSHs) up to degree 17. A Gaussian quadrature method is used to pick out a set of experimentally efficient measurement directions. Anechoic impulse responses are measured for these directions between a source loudspeaker and the entrance to the ear canal of a head-and-torso simulator (HATS). Three separate SSH analyses are carried out: The first forms a SSH representation from the time responses, with the variable onset delay caused by interaural differences intact, by applying the analysis to each time sample in turn. The second SSH model is formed in exactly the same way, except using impulse responses in which the variable onset delays have been equalized. The final SSH analysis is carried out in the frequency domain by applying the technique on a frequency bin by frequency bin basis to the magnitude and un...

A comparison between singing pitching strategies of 8 to 11 year olds and trained adult singers

A microcomputer-based system known as SINGAD (SINGing Assessment and Development) is described which enables note pitching ability to be assessed and developed. Versions of the SINGAD system currently exist for the BBC and Atari range of computers and work is currently in progress on a version for PC compatible machines. This paper describes results from a SINGAD experiment in which the pitching abilities were assessed of 175 children aged between 8 and 11 years inclusive attending a York primary school. The assessment was made against computer generated musically meaningful three note patterns from the major tonic triad of Bb 3. Each child responded to 15 such three note stimuli. A marked contrast between the pitching abilities of boys and girls is observed and discussed in relation to SINGAD assessment data gathered from a group of trained adult singers.

Using grating modulation to achieve wideband large area diffusers

Abstract Concatenating individual diffusers to form larger ones narrows their diffusion pattern into specific directions. Solutions are proposed which use modulation of existing diffusion structures with other sequences to achieve the improved diffusion patterns. These techniques provide large area diffusion structures that do not suffer from having narrowed diffusion patterns. It can also be used to develop diffusion structures that have specific diffusion patterns or have an extended frequency range. They also have the advantage that they can be implemented easily using existing diffusers. The theory, design, advantages and limitations of these structures will be discussed and simulation results of their performance will be presented.

Diffuser assessment using surface spherical harmonics

The problem of deriving a diffusion figure of merit may be alleviated by forming a continuous, functional representation of its polar pattern, expressing the polar patterns mathematically, as a continuous function of direction. This paper develops a form of continuous, orthogonal representation by expressing a diffusers polar pattern as a weighted sum of surface spherical harmonics (a hierarchical set of basis functions which are orthogonal upon the surface of a sphere). The surface spherical harmonic weights can be calculated from a limited set of experimental measurements by means of a discrete Fourier analysis. The resulting spherical harmonic representation is continuous, yielding a modeled polar pattern for any arbitrary direction. It is also hierarchical, in that the more harmonics that are included the greater the accuracy of the model, and has a meaningful spatial structure, with particular surface spherical harmonic weights expressing particular patterns of directional variation in the polar patt...

Ternary and quadriphase sequence diffusers.

A room acoustic diffuser breaks up reflected wavefronts, and this can be achieved by presenting a spatially varying surface impedance. In hybrid surfaces, varying impedance is achieved by patches of absorption and reflection, giving reflection coefficients nominally of 0 and 1. These surfaces are hybrids, absorbing some of the incident sound while diffusing any reflected energy. A problem with planar hybrid surfaces is that specular energy is only removed by absorption. By exploiting interference, by reflecting waves out-of-phase with the specular energy, it is possible to diminish the specular energy further. This can be achieved by using a diffuser based on a ternary sequence that nominally has reflection coefficients of 0, -1, and +1. Ternary sequences are therefore a way of forming hybrid absorber-diffusers that achieve better scattering performance without additional absorption. This paper discusses methods for making ternary sequence diffusers, including giving sequence generation methods. It presents prediction results based on Fourier and boundary element method models to examine the performance. While ternary diffusers have better performance than unipolar binary diffusers at most frequencies, there are frequencies at which the performances are the same. This can be overcome by forming diffusers from four-level, quadriphase sequences.

Diffusive benefits of cylinders in front of a Schroeder diffuser

A numerical investigation is performed into the diffusive effects of cylinders positioned in front of a Schroeder diffuser. A regular line of cylinders is shown to offer notable improvements to diffusion from a periodic Schroeder device, provided lateral cylinder spacing is incommensurable with the Schroeder period width. Further investigation considers angular dependence and low frequency results in greater detail, as well as the effects on narrowband and modulated Schroeder devices. An optimization procedure is subsequently performed to investigate the effects of an irregular cylinder arrangement, which provides further diffusive benefits.

Multiple scattering between cylinders and a schroeder diffuser

A multiple scattering method is developed to model an array of N cylinders positioned in front of a Schroeder diffuser. Results are compared against the more computationally intensive boundary element method, as well as existing multiple scattering methods for cylinders alone. An investigation of scattering performance from a cylinder arrangement in front of a Schroeder diffuser is also performed. The addition of cylinders is shown to offer potential benefits such as reducing the effects of periodicity in the Schroeder diffuser.

Low-cost speech recognizer

Abstract A speech recognition system has been developed for application in a voice-operated telephone dialling system. The requirement for such a system are that it is low cost, consumes little power and contains few components. Based on a Hitachi 6303X single-chip microcomputer, the system uses a zero-crossing pattern extraction technique and a pattern matching approach using a path constraint dynamic time warp. The paper explains the operation of the extraction and matching methods. Hardware of a prototype system is described. The demonstrator-trained system has been tested in public and was shown to work successfully independent of the speaker, although recognition is limited to isolated words. Further work is underway to enable continuous word recognition.

Phase reflection diffusers design using Huffman sequences

Phase reflection diffusers based on integer sequences are, unfortunately, limited in their frequency response. The purpose of this paper is to present an alternative form of diffusion structure using noninteger sequence based phase reflection gratings. This paper will present a new method of generating such diffusers based on Huffman sequences. The theory, design, advantages, and limitations of these structures will be discussed and simulation results of their performance will be presented. In particular, their independence from the frequency response effects typical of integer based sequences. Methods of extending and improving their performance will also be suggested. Methods of construction will also be discussed. The paper will show that it is possible to develop diffusing structures which have a better frequency performance than conventional phase reflecting structures.

Perfect audio signal processing using finite field transforms

The design and implementation of lossless audio signal processing using Finite Field Transforms is discussed. Finite field signal processing techniques are described. The effects of filter length and coefficient accuracy are also discussed. Finite field transform algorithms which would be suitable for lossless signal processing are presented.