A Likelihood-Ratio Based Forensic Voice Comparison in Standard Thai Supawan Pingjai August 2019 A thesis submitted for the degree of Doctor of Philosophy The Australian National University © Copyright by Supawan Pingjai 2019 All Rights Reserved Declaration I hereby declare that this thesis is entirely my own work, except where acknowledgement to the contrary is made in the text. I also confirm that I have fully cited and referenced all material and results that are not original. SIGNED:…………………………………………………. Supawan Pingjai DATE……………………………………………………… i Acknowledgements I want to express my deep sense of gratitude to Dr Shunichi Ishihara for his valuable guidance and encouragement during the production of this thesis. He devoted a lot of his time to addressing the many difficulties I encountered during my research. It would not have been possible to complete this thesis without the financial support of the University of Phayao, Thailand. I would similarly like to thank my parents for their financial support during my stay in Australia, and for the love they have shown throughout. I am also very thankful to Dr Philip Rose, who inspired me to conduct this Forensic Voice Comparison research. I express my deep and sincere gratitude to Dr Rose for his suggestions at various stages of my thesis. His guidance immensely contributed to the evolution of my ideas. This thesis received editorial input from Maxine McArthur and Bert Peeters. I would like to express my sincere thanks and deep appreciation for their professional support. I also acknowledge, with thanks, the moral support of the ANU Thai Association in Canberra. Finally, yet importantly, I wish to thank all the informants who contributed many hours of their time to participate in three recording sessions. Without their patience, the speech database for this PhD thesis would not have eventuated. ii Abstract This research uses a likelihood ratio (LR) framework to assess the discriminatory power of a range of acoustic parameters extracted from speech samples produced by male speakers of Standard Thai. The thesis aims to answer two main questions: 1) to what extent the tested linguistic-phonetic segments of Standard Thai perform in forensic voice comparison (FVC); and 2) how such linguistic-phonetic segments are profitably combined through logistic regression using the FoCal Toolkit (Brümmer, 2007). The segments focused on in this study are the four consonants /s, ʨh, n, m/ and the two diphthongs [ɔi, ai]. First of all, using the alveolar fricative /s/, two different sets of features were compared in terms of their performance in FVC. The first comprised the spectrum-based distributional features of four spectral moments, namely mean, variance, skew and kurtosis; the second consisted of the coefficients of the Discrete Cosine Transform (DCTs) applied to a spectrum. As DCTs were found to perform better, they were subsequently used to model the consonant spectrum of the remaining consonants. The consonant spectrum was extracted at the center point of the /s, ʨh, n, m/ consonants with a Hamming window of 31.25 msec. For the diphthongs [ɔi] - [nɔi L] and [ai] - [mai HL], the cubic polynomials fitted to the F2 and F1-F3 formants were tested separately. The quadratic polynomials fitted to the tonal F0 contours of [ɔi] - [nɔi L] and [ai] - [mai HL] were tested as well. Long-term F0 distribution (LTF0) was also trialed. The results show the promising discriminatory power of the Standard Thai acoustic features and segments tested in this thesis. The main findings are as follows. 1. The fricative /s/ performed better with the DCTs (Cllr = 0.70) than with the spectral moments (Cllr = 0.92). h 2. The nasals /n, m/ (Cllr = 0.47) performed better than the affricate /tɕ / (Cllr = 0.54) and the fricative /s/ (Cllr = 0.70) when their DCT coefficients were parameterized. 3. F1-F3 trajectories (Cllr = 0.42 and Cllr = 0.49) outperformed F2 trajectory (Cllr = 0.69 and Cllr = 0.67) for both diphthongs [ɔi] and [ai]. iii 4. F1-F3 trajectories of the diphthong [ɔi] (Cllr = 0.42) outperformed those of [ai] (Cllr = 0.49). 5. Tonal F0 (Cllr = 0.52) outperformed LTF0 (Cllr = 0.74). 6. Overall, better results were obtained when DCTs of /n/ - [na: HL] and /n/ - [nɔi L] were fused. (Cllr = 0.40 with the largest consistent-with-fact SSLog10LR = 2.53). In light of the findings, we can conclude that Standard Thai is generally amenable to FVC, especially when linguistic-phonetic segments are being combined; it is recommended that the latter procedure be followed when dealing with forensically realistic casework. iv Table of contents Declaration i Acknowledgements ii Abstract iii Table of Contents v List of Tables xi List of Figures xiii Chapter 1 Introduction .................................................................................................................. 1 1.1 Introduction ......................................................................................................................... 1 1.2 A brief history of the study of speaker recognition............................................................. 1 1.3 Types of speaker recognition: Speaker identification/verification/forensic voice comparison .......................................................................................................................... 2 1.4 What is Forensic Voice Comparison? ................................................................................. 5 1.5 Motivations ......................................................................................................................... 9 1.6 The research approach ...................................................................................................... 10 1.6.1 Traditional vs automatic parameters .............................................................................. 10 1.6.2 Statistical modelling techniques..................................................................................... 11 1.7 Linguistic-phonetic segments and acoustic parameters .................................................... 12 1.8 Research questions ............................................................................................................ 14 1.9 Thesis outline .................................................................................................................... 14 1.10 Summary ......................................................................................................................... 15 Chapter 2 Literature review ........................................................................................................ 16 2.1 Introduction ....................................................................................................................... 16 2.2 Ideal features of forensic scientific evidence .................................................................... 16 2.2.1 Lack of control over variation ........................................................................................ 17 2.2.1.1 Between-speaker variation .......................................................................................... 17 2.2.1.2 Within-speaker variation ............................................................................................. 18 2.2.2 Reduction in dimensionality .......................................................................................... 20 2.3 Speech variation and the role of probability theory .......................................................... 21 v 2.4 What is the Likelihood Ratio? .......................................................................................... 22 2.4.1 Why forensic experts should limit themselves to the calculation of Likelihood Ratio (LR) ..................................................................................................................... 29 2.5 The Thai legal system ....................................................................................................... 29 2.6 Shift to a new paradigm .................................................................................................... 31 2.7 Standard Thai and other main dialects: Phonetics and phonology.................................... 34 2.7.1 Standard Thai consonant phonemes ............................................................................... 35 2.7.2 Standard Thai clusters .................................................................................................... 35 2.7.3 Standard Thai vowels ..................................................................................................... 36 2.7.4 Standard Thai tones ........................................................................................................ 37 2.8 Northern Thai dialect ........................................................................................................ 38 2.8.1 Northern Thai consonant phonemes .............................................................................. 38 2.8.2 Northern Thai clusters .................................................................................................... 39 2.8.3 Northern Thai vowels .................................................................................................... 39 2.8.3.1 Monophthongs ............................................................................................................ 39 2.8.3.2 Diphthongs .................................................................................................................. 39 2.8.4 Northern Thai