A Critical Evaluation of the Dolby Digital Compression Algorithm
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UNLV Retrospective Theses & Dissertations 1-1-1996 A critical evaluation of the Dolby Digital compression algorithm David Andrew Ingbretsen University of Nevada, Las Vegas Follow this and additional works at: https://digitalscholarship.unlv.edu/rtds Repository Citation Ingbretsen, David Andrew, "A critical evaluation of the Dolby Digital compression algorithm" (1996). UNLV Retrospective Theses & Dissertations. 3279. http://dx.doi.org/10.25669/dw8q-xx1b processed, response: 201 This Thesis is protected by copyright and/or related rights. It has been brought to you by Digital Scholarship@UNLV with permission from the rights-holder(s). You are free to use this Thesis in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s) directly, unless additional rights are indicated by a Creative Commons license in the record and/ or on the work itself. 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Ingbretsen A thesis submitted in partial fulfillment of the requirements of the degree of Master of Science in Computer Science Department of Computer Science University of Nevada, Las Vegas May 1997 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UMI Number: 1385177 UMI Microform 1385177 Copyright 1997, by UMI Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. UMI 300 North Zeeb Road Ann Arbor, MI 48103 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The Thesis of David A. Ingbretsen for the degree of Master of Science in Computer Science is approved. Chairperson, E. A. Yfantis, Ph D. Examining Committee Member, A. Datta, Ph.D. y Examining Committee Member, L. Gewali, Ph.D. G raduat^aculty Representative, E. McGaugh, Ph.D. Dean of the Graduate College, R. Smith, Ph.D. University of Nevada, Las Vegas May 1997 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT Dolby Digital, or AC-3, provides for the compression of high-quality audio signals at a bit rate of 384 kbps, for 5.1 audio channels. The purpose of this thesis is to conduct an analysis and evaluation of the AC-3 compression algorithm. The main conclusion is that AC-3 allocates too few bits to the upper frequency ranges. This conclusion is supported by two lines of argument. First, a comparison of the AC-3 perceptual coding model with known facts concerning the masking properties of sound reveals omissions in the AC-3 perceptual model that lead to a loss of high frequency resolution. Second, a test program comparing the bit allocation generated by AC-3 with the bit allocation generated by MPEG corroborates the conclusion. A remedy for the high frequency loss problem is put forward. in Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS ABSTRACT...............................................................................................................................ni LIST OF TABLES....................................................................................................................vi ACKNOWLEDGEMENTS.....................................................................................................vii CHAPTER I INTRODUCTION TO AUDIO COMPRESSION......................................... 1 I. Overview..............................................................................................................................1 II. The Physics of Sound ........................................................................................................2 A. The nature of waves ......................................................................................................2 B. Mathematical properties of sound waves .....................................................................3 C. The measurement of sound ...........................................................................................5 m . Digitized Sound ............................................................................................................... 5 A. Digital sampling ............................................................................................................ 5 B. The Nyquist sampling rate ............................................................................................6 rv. Quantization .....................................................................................................................7 V. Perceptual Coding ............................................................................................................. 9 A. Basic concepts ............................................................................................................... 9 B. Threshold in quiet........................................................................................................ 10 C. Critical bands ................................................................................................................11 VI. Criteria for Evaluating a Perceptual Coder ..................................................................13 CHAPTER 2 THE MODIFIED DISCRETE COSINE TRANSFORM.............................15 I. Introduction ...................................................................................................................... 15 n. The Forward and Inverse Fourier Transforms ...............................................................17 in. The Forward and Inverse Discrete Cosine Transforms ............................................... 19 rv. The Windowing Function ..............................................................................................22 V. A Generalized Windowing Function ............................................................................25 CHAPTER 3 THE AC-3 CODEC.........................................................................................32 I. Introduction .................................................................................................................... 32 n. Overview of AC-3 ........................................................................................................... 33 in. Representation of Exponents .........................................................................................35 IV Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 4 PERCEPTUAL CODING............................................................................ 39 I. Introduction ....................................................................................................................39 n. Types of Masking Effects ...............................................................................................40 A. Masking effects of noise .............................................................................................41 B. Masking effects of tones .............................................................................................43 C. The spreading function .............................................................................................44 D. Summary of masking effects ....................................................................................45 m . Summary of the AC-3 Masking Algorithm ................................................................46 A. Power spectral density function ...............................................................................47