Readers familiar with earlier editions of Principles of Digital Audio may be surprised at the extent of the revisions in this third edition. Clearly, the essential nature of theoretical topics such as sampling and quantizing have not changed, but our understanding of them has. Similarly, the relative importance of topics continually undergoes change; advances in technology diminish the magnitude of some issues, while simultaneously magnifying others. In the same way that history continually rewrites itself, our understanding of scientific principles evolves. Moreover, and even more significantly, in the interim following the last edition, numerous entirely new digital audio applications have been developed. In short, this third edition is both needed, and is long overdue.
Examination of this new edition will show that over one-half of the text has been extensively revised. A number of topics have been eliminated to conserve the page count (and cost), a number of discussions have been significantly expanded, and a variety of new topics, both theoretical and practical in nature, have been introduced. Specifically, readers will find a considerable amount of entirely new material on channel codes, jitter, optics, optical storage, compact disc formats, digital audio interfaces, networks, psychoacoustics, perceptual coding, the MiniDisc, digital audio broadcasting, multimedia applications, digital filters, digital signal processing, low-bit conversion, noise shaping, and other subjects.
An introductory approach is retained, but the greater depth given to some material has somewhat increased the reading sophistication required; of course, readers may pick and choose according to their level, and need. Also, the wider scope of topics should serve to better satisfy a broader range of students and professional practitioners. The latter will certainly notice an increased emphasis on applications-oriented material.
One thing has not changed. This book is neither a compendium of every possible fact, nor an advanced treatise. It is an introductory text that attempts to provide the most lucid explanations possible, and to strike that all-important balance between mere information, and understanding. In other words, this is a learning tool, brought to you by someone who enjoys spending time on both sides of the lectern. With this in mind, the chapter contents can be summarized:
Chapter 1 introduces the most elemental concepts of sound pressure functions, as well as binary arithmetic and Boolean algebra. This nomenclature lays the foundation for everything to follow.
Chapter 2 presents the fundamentals of digital audio theory. Principles such as discrete time sampling, amplitude quantization, aliasing, and dither are alien to the analog world, but essential to the digital realm. This chapter carefully introduces each of these topics in an understandable way.
Chapter 3 describes the recording side of a audio digitization system, using a PCM architecture as its model. The hardware design of this encoder includes the anti-aliasing low-pass filter, sample and hold circuit, and A/D converter. In addition, this chapter includes a discussion of channel coding, the ultimate objective of an encoding system.
Chapter 4 explains the output processing required to transform binary data into an analog waveform. The hardware blocks of the decoder, D/A converter, output sample and hold circuit, and anti-imaging filter are described. Because all practical digital audio players now use digital filters, these devices are described. In addition to the PCM architecture, many specialized coding systems have been devised. In addition, this chapter includes a section on timebase correction, describing the need to control jitter tolerances throughout a digitization chain.
Chapter 5 surveys the field of error correction, a highly mathematical endeavor that is critical to the successful storage of high density digital audio data. Basic error detection techniques such as parity are described, along with simple detection codes. Error correction schemes such as block and convolutional codes, interleaving, and Reed Solomon codes are discussed in theoretical terms, as are practical implementations such as the Cross Interleave Reed Solomon Code. In addition, error concealment is described.
Chapter 6 examines magnetic tape, a venerable and still vital form a digital storage. A discussion of magnetic recording theory precedes descriptions of the most prominently used magnetic storage systems, including stationary and rotary head systems, with attention to the constraints underlying any particular format.
Chapter 7 presents the digital audio tape (DAT) format, a rotary-head system used to record two audio channels, along with extensive subcode. DAT design is examined, including azimuth recording, track format, modulation, and error correction. Professional DAT applications of timecode recording and editing are also presented.
Chapter 8 discusses optical storage and transmission. The chapter begins with a review of optical phenomena such as diffraction, resolution, and polarization. Both nonerasable and erasable optical disk systems are discussed. Each system may be implemented with a variety of technologies. The chapter also considers fiber optics. The advantages of fiber optics, the operation of such a system, and limitations of fiber optic interconnection are discussed.
Chapter 9 is devoted to the audio compact disc, and its many variations. The physical characteristics of discs and the nature of the data encoded on discs are presented. The theory of operation of the player is described; the laser pickup, EFM, CIRC, subcode, manufacturing and other topics are discussed. Alternative CD formats are described including CD-ROM, CD-I, CD-WO (CD-R), Photo CD, Video CD and DVD.
Chapter 10 describes the numerous professional and consumer digital interfaces that are used to interconnect digital audio devices. These include the SDIF-2, AES3 (AES/EBU), and AES10 (MADI), and S/DIF protocols. In addition, SCMS and AES11, as well as sample rate conversion, are described. The chapter further discusses communications topics such as ISDN and file transfer, as well as computer networks, including the Internet.
Chapter 11 explores the increasingly important topic of perceptual coding, also as known as low bit-rate coding. Data compression techniques such as Huffman coding are described. Psychoacoustics forms the basis for perceptual coding, whether the architecture employs subband or transform coding. The ISO/MPEG standard is described in detail, along with AC-3 and other coders. Key to the development of perceptual coders is the means used to evaluate their transparency. Finally, the MPEG-1 video standard is considered.
Chapter 12 presents the MiniDisc format, as used in both consumer and professional applications. The disc physical design, optical pickup, magneto-optical recording, ATRAC perceptual coder, disc mastering and manufacturing and other topics are all explained.
Chapter 13 describes the rapidly evolving topic of digital audio broadcasting. Satellite transmission, a particularly powerful distribution method, is explained, including direct broadcast satellite applications. Technical considerations such as multipath performance and bandwidth are important to any digital broadcasting system. Both the Eureka 147 and IBOC systems are explained.
Chapter 14 examines the widespread use of digital audio workstations, following the introduction of digital mixing consoles. Workstations have changed post-production methods in many professional audio and multimedia applications; however, the multiplicity of file formats can complicate this environment. Specific examples of workstation operation are presented.
Chapter 15 tackles the topic of digital signal processing. Subjects such as linearity, time-invariance, impulse response, convolution, and transforms are explained in a largely nonmathematical fashion. Digital filter theory is discussed, with a look at both FIR and IIR filters. Parameters for filter design are presented, as are DSP hardware chips and DSP programming, and their respective roles in a digital system. The chapter concludes with a look at applications such as delay, reverberation and noise removal.
Chapter 16 describes low-bit converters, sometimes known as sigma-delta converters. With the help of noise shaping, low-bit D/A converters can provide an extremely low in-band noise floor and distortion. Similarly, low-bit A/D converters use a high sampling rate and decimation to achieve excellent performance. Several practical chip examples are considered. When converting a long wordlength file to a shorter length, it is important to preserve information with the use of psychoacoustically optimized noise shaping techniques.
Much of the material in this book stems from the work of the many pioneers and leaders in the field of digital audio technology. We owe a tremendous debt to them for their efforts in developing and fulfilling the potential of this young science. Clearly, their vision has profoundly changed both our industry and society.
Principles of Digital Audio
Ken Pohlmann
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