REFERENTIAL TOME

The Art of Digital Audio by John Watkinson

Focal Press ( Stoneham, Mass.), hardbound, 489 pp., $49.95.

I was pleased to have this book come to me for review. In the past several months, I have not only read it thoroughly, some sections many times, but have also used some of its explanations and information as references in teaching a course on audio system design for senior electrical engineering students. It is without doubt one of the most useful books I have come across for obtaining an overview of the art of digital audio. I recommend that everyone with either a professional, or even a more casual, interest in digital audio rush to get a copy.

A reading of this book by anyone, but particularly by persons with some electrical engineering or computer sciences background, will be refreshingly useful. Jammed with information on everything from floppy disks to Compact Discs, it seems to include discussions of every kind of digital recording ever conceived, whether for numerical data or audio signals.

Now that you are sold on The Art of Digital Audio, let me issue some warnings. Most readers will, as I have, develop a love/hate relationship with this book. Though some 480 pages long, it is either too short or contains too much. As a consequence, many topics are covered too briefly. The brevity is a special problem in the discussions of some of the more basic topics, such as digital conversion, coding, and signal processing.

John Watkinson points out in the preface that he has not tried to write a textbook. He moves from the overly simplified to the incredibly difficult with great leaps and fewer connecting links than are necessary to teach the material. There are few facts in the book, he says, because facts can be forgotten; his emphasis is on thinking and understanding. I found the book loaded with facts-very interesting facts, indeed.

Unfortunately, there is often no explanation of their significance, nor, frequently, are they tied into the thread of the concepts being discussed. I found many of the discussions of important theoretical issues not only lacking equations but written in such a convoluted manner as to make understanding these complex matters more difficult than necessary.

Many fine figures are presented, some copied from the references, many new. Some are self-explanatory and enlightening; unfortunately, many are not. Often the figures receive little or no discussion in the text and have short, infuriatingly incomplete captions. In many cases, the figures need much-improved captions or more extended discussion in the text. Examples are numerous; I will give only one.

Figure 6.16 shows the autocorrelation functions for 10 channel codes, none of which have been discussed yet (some are discussed later). The caption reads, "Comparison of codes by autocorrelation function of run length." The text explains, "Most of the parameters of a code can be read from the autocorrelation function at a glance, whereas the more common use of code spectrum makes this more difficult." Clear? Neither "autocorrelation function" nor "code spectrum" are to be found in the index.

It is simply not possible to teach or understand many of the complex concepts in this book without some mathematics and without a thorough discussion of the terminology being used.

Nevertheless, this is in many ways a wonderful book. I will try to give you a balanced view of its contents.

In general, the chapters on basics are inadequate, while those on specific topics are full of interesting facts.

Chapter 2 covers basics, such as conversion of signals from analog to digital form and vice-versa. Sampling and quantization are covered without benefit of mathematics; only the roughest idea of the process can be gained from the figures and text. The author has distilled several good references on these matters and condensed them into a series of "facts." I suggest that the original papers, or other textbooks on theory, be consulted for an understanding of the basics of digital processing of signals. A treatment of the accuracy and speed of the currently available D/A and A/D converters, as they are applied in the latest digital equipment, would be more in keeping with the level and content of the later chapters.

Chapter 3 contains about 10 pages on binary logic, adding, and the like.

Its last 20 pages are on a strange, almost random collection of digital topics, some of which seem quite out of place: Level metering, gain control, mixing, cross-fading, and companding. Only digital dither, time-base correction, and FIFO (first-in, first-out) time-base correction seem to fit well into the presentation at this point.

Overall, however, Chapters 2 and 3 seem weak.

Chapter 4 is a 55-page treatment of digital filters. This includes finite impulse response filters, infinite impulse response filters, z-transforms, filter design, and zero/pole positioning, with 14 pages on sampling rate conversion thrown in for good measure. If you understand these topics before you read this chapter, you will find it an interesting treatment. It is a bit like Alice in Wonderland, since it goes through topics fast, fast, fast. Despite its faults, the chapter will give the novice a flavor of digital filtering practice; the expert will find the treatment gives interesting insights into digital filtering. All in all, not a bad chapter.

The 21-page Chapter 5 seems too short. Very interesting and well written, it goes into detail about the various digital interconnection standards for digital equipment. Many standards are discussed, including those used in PCM tape machines, Compact Disc players, digital audio tape machines, and other equipment. The topics fit the author's style beautifully. There are facts, details, and well-done figures.

One can really understand both the problems of interfacing among various standards and how to go about solving some of these problems. The detail presented is excellent, and I would like to have seen even more of it.

Chapter 6 is where The Art of Digital Audio really comes to life. The author clearly knows a great deal about digital recording, and it shows. The dozen pages on the magnetic recording process and the qualities of the channel are enlightening and to the point. It is made clear why channel coding is needed. The discussion of simple channel codes is useful (even though the dreadful Figure 6.16, mentioned above, appears in this section ). A 10 page review of group codes and convolutional codes follows. I would like to have seen this material expanded, as it is one of the most important topics in digital audio and the material is very well written. Partial response receives five pages. This subject, which involves very advanced concepts of trellis coding and Viterbi decoding (a topic usually called delayed decision coding) unfortunately cannot be explained in five pages and probably should not be included in this book. These are topics much more applicable to modems and communications systems, since they are not effective for burst errors, which are more typical in digital storage media. This chapter would have been better had it treated fewer topics more thoroughly.

Chapter 7, 46 pages long, is nicely presented in that it starts with relatively simple examples, those of parity code checking, and proceeds to more complex topics by giving a nice example of a Hamming code. Cyclic codes are discussed in the next seven pages.

The discussion is clearly presented and at a level which can be followed easily if the reader puts in some effort on working through the examples in detail. Codes such as cyclic redundancy check codes (CRCC) are very important because they are widely used in digital audio systems. Even the brief discussion of Galois field manipulations seems in place in this chapter.

It is greatly simplified but nevertheless gives some insight into the complexities required in useful and practical coding systems. Correction of burst errors is essential in digital recording, since errors are caused by defects in the medium which wipe out "chunks" of data. The author, in 16 pages, attempts to ease the reader through the burst-correction maze by treating several coding techniques in order, from the simpler to the more complex. The Fire code, B-adjacent code, and Reed Solomon code (used for Compact Discs) are discussed. It would have been useful to improve the code structure figures, and I find the discussion of the Fire code more confusing than clarifying. When the author allows himself to write a few equations and to give examples, his exposition becomes very clear. I find his treatment of the Reed-Solomon code a very nice starting point for understanding the purpose of all the complexity required to make the Compact Disc work so well. The final eight pages of Chapter 7, on interleaving, are nicely written, readable, and understandable.

The 48-page Chapter 8 is on rotary head recorders: Video recorders, PCM machines, EIAJ-format recorders, and the R-DAT machine. This is where the book really starts to pay off in a big way. Chapters 8 through 11 alone are worth the price of admission: Roughly one-half of Chapter 8 is on video-type rotating-head machines (including PCM adaptors and the like) and the rest on DAT (specifically R-DAT). The mechanical problems, channel limitations, and coding techniques used are discussed with just the right amount of detail and at the right level for readers who want an overview of this important digital recording technology. The treatment of R-DAT is excellent-here is another case when the interested reader will wish for more.

A full 53 pages are devoted to stationary head recorders in Chapter 9, half on the DASH formats and half on the Mitsubishi formats (Pro-Digi). Again, the treatment is excellent. There is enough detail for the reader to understand the problems involved and the elegant solutions that have been devised to solve them. This chapter will be of interest to the studio engineer who simply wants to know more about the technology, as well as to those merely interested in digital techniques and how they are applied to audio recording. There are even a few pages devoted to the S-DAT formal.

Chapter 10 is a short, nine-page section on digital technology applied to 8-mm video. Chapter 11, 25 pages, discusses professional VTR formats such as EBU/SMPTE, C-format, and other PCM formats. These chapters are adequate discussions of the technology for the specialist; they do not, however, add much to the book in terms of extending the reader's understanding of digital technology. These chapters are more lists of facts than anything else. Though not as thorough as Chapters 8 and 9, they nevertheless will be of interest to studio and broadcast personnel.

Chapter 12 is rather long, 52 pages, on various disk drives that are or might be used for audio recording. It is a sort of mishmash of interesting information about all kinds of disks, from floppy to hard and including optical. I liked the chapter and learned a lot of "facts" about these drives, but I am not sure the information learned will be very useful. The casual reader will find this chapter interesting; the expert will find it too elementary and sketchy.

Chapter 13, 46 pages, is on the Compact Disc. "Finally!" you might say. While this is the last chapter, it is by no means the least. A few pages are devoted to how the CD is manufactured and to a discussion of the mechanics of the tracks and how the "bumps" are read. Yes, "bumps." Why not "pits," which is standard usage in most of the literature? Pits are just bumps looked at from the bottom-or the other way around! Eight additional pages are spent on the reading, tracking, and optics of the CD system.

There are a few pages on the pickup structure as well. This is as good and clear a treatment of the mechanics of the CD as I have seen, and the coverage includes most of what the casual reader would like to know. A nice six page section on the channel coding of the CD includes an explanation of the use of eight-to-fourteen modulation (EFM). The following 12 pages give great detail about the data and control formats on the disc. The serial frame structure is described clearly. The CD subcode blocks used for control and other information (i.e., the P, O, R, S, T, U, V, and W word blocks) are discussed in depth; the treatment is detailed and well written. The final nine pages describe specific circuits and control circuit designs of CD players.

The chapter on the CD is adequate but disappointing in some ways. It passes off CD error-correction techniques with the comment that they have been discussed in Chapter 7. I feel that a section on the specific application of interleaving and Reed-Solomon coding techniques to the CD should have been included. These techniques are too important to the CD to be slighted as they have been here.

A large number of potential readers will be interested mainly in the CD. The other lack in this chapter is its failure to discuss oversampling and digital filtering as they relate to modern digital CD machines. These are two of the most important recent technical developments. They have added greatly to the quality of CD playback processes and deserve extensive treatment. The author mentions over sampling in Chapter 2 but unfortunately fails to discuss the latest CD technology using these techniques.

As indicated at the start of this review, I have a love/hate relationship with The Art of Digital Audio. It is very good in many of its parts, and that makes it worth having. In fact, I now keep a copy at home and have put one in my electroacoustics laboratory library as a reference for my graduate students. It is not a textbook, but it is a very good reference for many topics.

(It has a good list of references at the end of each chapter and a good index.) I consider this probably the best book available at its technical level at this time, and I would urge anyone with an interest in digital audio technology to buy it. It takes concentration and hard work to read and understand, but with this investment, reading it will be very worthwhile.

-R. A. Greiner

The New Trouser Press Record Guide, Revised Edition

edited by Ira A. Robbins. Collier Books, 657 pp., softcover, $16.95.

So you want to know about alternative rock bands like !Action Pact!, Men They Couldn't Hang, or Xdreamysts.

Where do you turn? About the only place is The New Trouser Press Record Guide, edited by Ira A. Robbins, now out in an expanded third edition.

The book chronicles the various alternative rock movements that began reacting to the highly formularized pop/rock of the late 1970s. Beginning originally with New Wave and punk, the Guide has expanded its scope to include styles from reggae, rap, and hardcore to a small amount of independent heavy metal and new folk music. Entries are alphabetical.

What you'll find here is a wealth of information on both obscure and better known groups that are iconoclastically inclined, either in terms of music (Einstürzende Neubauten) or attitude (Flux of Pink Indians). The majority will never show up in your Harmony Encyclopedia of Rock and Roll, and that alone makes this Guide valuable.

When it comes to substance, however, there's good news/bad news.

The good news is that you get a tremendous range of entries to dip into, and good basic discographies (title, label, year, country, special format info, and CD availability; no ID numbers). An index would have been a helpful addition, but maybe next time.

The bad news is that many entries consist more of highly subjective record reviews than of helpful biographical information (e.g., names of band members) or other more objective, historical perspectives. Implicit in this is a tone of somewhat egocentric, elitist critical snobbery which can be annoying if you don't buy the notion that only music which is not commercially successful is intellectually acceptable. In the worst cases, all you learn is that the reviewer did or didn't like a record.

Yet despite a subjectivist critical ethos more appropriate to a fanzine, Ira Robbins' The New Trouser Press Record Guide provides interesting information on a major chunk of rock music that you won't find out about anywhere else. Now, let's see, what do they have to say about Mofungo?

-Michael Wright

(adapted from Audio magazine, Jan. 1990)

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