An Investigation Into Improving Speech Intelligibility Using Binaural Signal Processing
Total Page：16
File Type：pdf, Size：1020Kb
An investigation into improving speech intelligibility using binaural signal processing Gary Anthony Spittle PhD Electronics July 2009 1 Abstract This thesis sets out to improve the intelligibility of a target speech sound source when presented with simultaneous masking sounds. A summary of the human hearing system and methods for spatialising sounds is provided as background to the problem. A detailed review of relevant research in auditory masking, auditory continuity and speech intelligibility is discussed. Angular separation and sound object differentiation through amplitude modification are used to enhance a target speech sound. A novel method is developed for achieving this using only the binaural signals received at the ears of a listener. This new approach is termed an auditory lens. Psychoacoustic evaluation of the auditory lens processing has shown comparable intelligibility scores to direct spatialisation techniques which require prior knowledge of sound source spectral content and direction. The success of the auditory lens has led to a number of potential further research projects that will take the processing system closer to a real wearable product. 2 Table of contents Abstract ..................................................................................................................2 Table of contents ....................................................................................................3 List of Figures ........................................................................................................6 List of Tables ........................................................................................................16 Acknowledgements ..............................................................................................18 Declaration ...........................................................................................................19 Chapter 1 Introduction ......................................................................................20 1.1 Background of the research ..................................................................20 1.2 Objective of the research ......................................................................22 1.3 Statement of hypothesis ........................................................................23 1.4 Thesis structure .....................................................................................24 Chapter 2 Human hearing .................................................................................26 2.1 Directions of sound ..............................................................................26 2.2 Ear Physiology .....................................................................................27 2.2.1 The peripheral auditory system ........................................................27 2.2.2 The internal auditory system ............................................................28 2.3 Masking ................................................................................................29 2.4 Binaural hearing ...................................................................................32 2.4.1 Sound localisation ............................................................................32 2.4.2 Directional cues ................................................................................32 2.4.2.1 Interaural time difference .........................................................33 2.4.2.2 Interaural envelope difference ..................................................38 2.4.2.3 Interaural intensity difference...................................................39 2.4.2.4 Spectral cues .............................................................................42 2.4.3 Distance cues ....................................................................................45 2.4.3.1 Overall signal level ...................................................................45 2.4.3.2 Changes in interaural intensity differences ..............................46 2.4.3.3 Spectral attenuation ..................................................................46 2.4.3.4 Direct-to-reverberant ratio ........................................................47 2.4.4 Binaural advantage ...........................................................................47 2.5 Summary ..............................................................................................48 Chapter 3 Sound spatialisation ..........................................................................50 3.1 Sound field reconstruction methods .....................................................50 3.1.1 Stereo ................................................................................................50 3.1.2 Surround sound systems ...................................................................52 3.1.3 Ambisonics .......................................................................................53 3.1.4 Wave-field synthesis ........................................................................53 3.2 Binaural synthesis .................................................................................54 3.2.1 Head-related transfer functions ........................................................55 3.2.2 HRTF measurement .........................................................................56 3.2.3 Personalised and generic HRTFs .....................................................62 3.2.4 Sound spatialisation using HRTFS ...................................................64 3.2.4.1 Convolution ..............................................................................64 3.2.4.2 Binaural synthesis of a single sound source .............................66 3.2.4.3 Binaural synthesis of multiple sound sources ..........................68 3 3.3 Summary ..............................................................................................69 Chapter 4 Auditory masking and the factors that influence it ..........................71 4.1 Auditory grouping ................................................................................71 4.1.1 Auditory Scene Analysis ..................................................................72 4.2 Masking and critical bands ...................................................................78 4.3 The influence of spatial separation between sound sources .................79 4.4 The influence of sound duration ...........................................................84 4.5 The influence of spectral content of sounds .........................................86 4.6 Sound source signal content .................................................................87 4.6.1 Phoneme recognition with a noise masker .......................................87 4.6.2 Speech recognition with a noise masker ..........................................87 4.6.3 The articulation rate of speech .........................................................88 4.6.4 Similarity in target and masker speech content ................................90 4.7 Summary ..............................................................................................94 Chapter 5 The cause and effects of auditory continuity ....................................97 5.1 Homophonic auditory continuity ..........................................................97 5.1.1 The influence of signal level differences..........................................98 5.1.2 The influence of signal duration .....................................................100 5.2 Heterophonic auditory continuity .......................................................102 5.2.1 The influence of spectral content and frequency separation ..........104 5.2.2 The influence of spatial separation .................................................109 5.2.3 Contralateral induction ...................................................................114 5.3 Phonemic restoration ..........................................................................115 5.3.1 The spectral content of inducer signals ..........................................116 5.3.2 The relevance of target speech content ..........................................120 5.3.3 The influence of spatial separation of auditory objects ..................121 5.3.4 Methods for improving phonemic restoration ................................123 5.4 Summary ............................................................................................125 Chapter 6 Speech intelligibility with multiple sound sources .........................128 6.1 Blind Source Separation .....................................................................129 6.1.1 Independent Component Analysis ..................................................131 6.1.2 Beamforming ..................................................................................132 6.1.3 Spectral processing .........................................................................134 6.1.4 Summary ........................................................................................134 6.2 The influence of spectral content .......................................................135 6.2.1 Auditory glimpsing – the peek theory in practice ..........................143 6.3 The influence of spatial separation .....................................................146 6.3.1 The number, type and location of interferers .................................156 6.4 Summary ............................................................................................161 Chapter 7 Binaural