A PROPOSED FRAMEWORK FOR FORENSIC AUDIO ENHANCEMENT by JAMES ZJALIC BSc, Birmingham City University, 2015 A thesis submitted to the Faculty of the Graduate School of the University of Colorado in partial fulfillment of the requirements for the degree of Master of Science Recording Arts Program 2017 © 2017 JAMES ZJALIC ALL RIGHTS RESERVED ii This thesis for the Master of Science degree by James Zjalic has been approved for the Recording Arts Program by Catalin Grigoras, Chair Jeffrey Smith Lorne Bregitzer December 1, 2017 iii Zjalic, James (MS, Recording Arts Program) A Proposed Framework For Forensic Audio Enhancement Thesis directed by Associate Professor Catalin Grigoras ABSTRACT Although there are many processes available to the forensic audio analyst seeking to enhance a recording, the order in which they are executed is vital in achieving the optimal enhancement, as each tool is used in isolation and modifies the signal in a unique way based on algorithms applied. The output of a processor in a sequence is determined by the output of the previous, and the order of this chain has a cumulative effect that provides differing quality enhancement results. There are currently various papers available detailing the individual methods of audio enhancement processes, but no document yet exists which provides a context for the interaction of these processes and how the sequence of methods can be ordered in a myriad of ways to produce differing results. Using both scientific reasoning and experimental procedures, this research will propose a framework to produce optimal results when performing audio enhancement for forensic purposes. The form and content of this abstract are approved. I recommend its publication. Approved: Dr. Catalin Grigoras iv ACKNOWLEDGEMENTS I would like to thank everybody at the National Centre of Media Forensics for their support, knowledge, and patience during my studies. Specifically, thank you to my professors Dr. Catalin Grigoras and Jeff Smith for the guidance, support, and knowledge. More explicitly, Catalin’s late night seminars provided me with an informal platform to discuss ideas freely, however rudimentary or implausible they may have been. Jeff’s enthusiasm for the subject matter is infectious and he was always highly responsive and helpful with any questions I had, however mundane. Thank you to Leah & Emma for keeping me on track with the clerical side of my studies, allowing me to focus on getting as much out of my Masters as possible. I would also like to thank my cohorts for the laughs, conversations, and support during my time at the NCMF. v TABLE OF CONTENTS CHAPTER I. INTRODUCTION ........................................................................................................................ 1 Scope ............................................................................................................................................ 4 Audio Enhancement Characteristics ............................................................................................. 6 Enhancement or manipulation? .................................................................................................... 7 Legalities and Best Practices ........................................................................................................ 8 II. SOUND FUNDAMENTALS .................................................................................................... 10 Auditory Systems ....................................................................................................................... 10 DSP Basics ................................................................................................................................. 14 Recording Limitations ................................................................................................................ 18 III. SETTING THE STAGE ........................................................................................................... 21 Evidence Preparation .................................................................................................................. 21 Listening Environment Optimization ......................................................................................... 22 Initial Analysis ........................................................................................................................... 23 System Calibration ..................................................................................................................... 23 IV. HYPOTHESIS ......................................................................................................................... 25 V. PRE-PROCESSING .................................................................................................................. 27 Critical Listening ........................................................................................................................ 27 Noise ....................................................................................................................................... 27 Distortion ................................................................................................................................ 27 Fast Fourier Transform ............................................................................................................... 28 VI. REVIEW OF AUDIO ENHANCEMENTS ............................................................................. 31 vi De-Click ..................................................................................................................................... 31 De-Clip ....................................................................................................................................... 32 Spectral Repair ........................................................................................................................... 35 Stereo Source Separation ............................................................................................................ 35 Reference cancellation................................................................................................................ 36 Equalization ................................................................................................................................ 38 Static Filters ............................................................................................................................ 39 Shelving Filters ....................................................................................................................... 41 Dynamic Equalization ............................................................................................................ 42 Adaptive Filters ...................................................................................................................... 42 De-Hum ...................................................................................................................................... 43 Broadband Noise Reduction ....................................................................................................... 44 Time Domain .......................................................................................................................... 45 Spectral Domain ..................................................................................................................... 45 Gain ............................................................................................................................................ 46 Compression ........................................................................................................................... 46 Limiters .................................................................................................................................. 49 De-reverberation ......................................................................................................................... 49 Normalization ............................................................................................................................. 51 Time-stretching .......................................................................................................................... 53 VII. PROPOSED FRAMEWORK ................................................................................................. 54 Framework Rationale ..................................................................................................................... 56 Evidence Preparation .................................................................................................................. 56 vii Analysis ...................................................................................................................................... 58 Processing ................................................................................................................................... 60 Output ......................................................................................................................................... 64 VIII. CASE STUDIES ................................................................................................................... 66 Case One – Clicks, noise and reverb .......................................................................................... 70 Case Two – Clipping, hum, background speech ........................................................................ 77 Case Three
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