Digital Sound

Everybody, including myself, was astonished to find that it was impossible to distinguish between my own voice, and Mr. Edison's re-creation of it.—Anna Case, Metropolitan Opera Soprano, 1915

In the July 1990 "As We See It" (Vol.13 No.7, p.5), I examined the conflict between those who believe existing measurements can reveal and quantify every audible aspect of a component's behavior, and those who consider the listening experience a far better indicator of a component's performance than the numbers generated by "objective" measurements. Implicit in the objectivist position is the assumption that phenomena affecting a device's audible characteristics are well understood: any mysteries have long since been crushed by the juggernaut of scientific method. If people then hear differences that "science" cannot measure or quantify, those differences exist only in people's minds and have no basis in reality. Consequently, observers' listening impressions are virtually excluded from consideration as merely "subjective," unworthy of acceptance by audio science. This belief structure is at the very core of the audio engineering establishment, and is the guiding force behind their research efforts (footnote 1).

The subjectivists believe that the ear is far superior to test instruments in resolving differences. It is also axiomatic that vast areas of audio reproduction, far from being fully researched and understood, are instead considerably more complex than the simple scientific models used to describe them.

A good example of this is the well-publicized subject of CD treatments. In the October issue (Vol.13 No.10, p.5) I described my experiences with cryogenically frozen CDs, as well as CDs pressed from the same stamper but made from different molding materials. I am particularly fascinated by CD treatments not for what they do, but for what they represent. The fact that cryogenically freezing a CD results in an easily audible change in sound points to the uncomfortable (for the engineering establishment) conclusion that everything is not as simple and well-understood as is thought.

A result of this dichotomy is that academic researchers—the very people in whose hands lie the tools and knowledge to discover the physical causes of these phenomena—are the least likely to listen critically and the most likely to dismiss the audiophile's claims as nothing more than voodoo. Consequently, their research direction is dictated by improving measured performance rather than increasing subjective performance; the latter is far more meaningful when the goal of research is to better communicate the musical experience.

Those who make their livings from digital audio (like mastering engineers) have long complained about sonic anomalies and perceptual differences where no differences should theoretically exist. The academic audio community, as well as manufacturers of professional digital audio equipment, maintain that these differences—unsupported by theory and unmeasurable—are products of the listeners' imaginations.

This thinking was exemplified by events during a two-day Sony seminar on digital mastering technology I attended a few years ago. The designers of the Sony PCM-1630, DAE-1100 digital editor, and other digital mastering equipment were present. The seminar was attended by mastering engineers who work with, and listen to, this equipment daily.

One of the mastering engineers expressed his concern over the audible degradation that occurs when making digital-to-digital tape copies, and the sonic differences introduced by the digital editor, especially when using the editor's level adjustment (footnote 2). These comments set off an outspoken flurry of concurrence among the assembled mastering engineers. The Sony designers argued vehemently that no differences were possible, and regarded the collective perception with some amusement. This exchange is a microcosm of the conflict between those who listen and those who measure.

Such is the background of this essay's subject, a paper by Dr. Roger Lagadec entitled "New Frontiers in Digital Audio" presented at the most recent Audio Engineering Society Convention in Los Angeles. I believe this paper will one day be considered a turning point in digital audio's evolution. Copernican in scope, it is likely to radically change the direction of and thinking in audio engineering. Lagadec's thesis is of utmost importance to the audiophile, both because of its promise of greatly improved digital audio, and for its validation of a fundamental audiophile philosophy: the importance of critical listening in evaluating audio technology over the belief that existing measurements can reveal all differences. Furthermore, and perhaps most significant, the paper was written by a man considered by many to be the world's foremost thinker in digital audio, whose ideas carry enormous influence in the audio engineering community.

As a pioneer in digital audio since 1973, Dr. Lagadec has conducted fundamental research into digital signal processing, was one of the developers of the Digital Audio Stationary Head (DASH) format while at Studer, and has offered broad, conceptual insights into the nature of digital audio. He holds a Ph.D. in Technical Sciences in the field of Digital Signal Processing, and has been actively involved in setting digital audio standards within the AES, of which he was named a Fellow in 1983. He is now responsible for all professional digital products at Sony. It is difficult to overstate Dr. Lagadec's credentials or his ability to influence digital-audio thinking.

"New Frontiers in Digital Audio" is bold in concept, brilliant in its simplicity, and technically incontrovertible. The paper identifies two areas of digital audio considered fully understood—digital-domain gain adjustment and dither—and reveals fundamental concepts about these areas that had not previously been considered (footnote 3). Moreover, the paper correlates these new discoveries with the perceptions of trained listeners whose comments were once considered heresy. Significantly, Dr. Lagadec's thesis extends beyond digital gain adjustment and dither: these two relatively simple issues are paradigms for the broader and more complex conflict between measurement and human musical perception. In this analysis, I will avoid most of the paper's technical details and focus instead on the broader issues raised (footnote 4).

Dr. Lagadec challenges the conventional wisdom that requantizing a digital audio signal with a digital fader produces only a change in level accompanied by a slight noise increase. "The imprecise, but by no means uncertain, answer of experienced users has sometimes been that—with critical signals—the texture of the new signal, its fine structure, possibly its precise spatial definition, will be affected: the signal will (sometimes) have changed in a way uncorrelated to level change and noise level, in spite of the extreme simplicity of the digital signal processing it underwent....The rest of this chapter cannot have the ambition of proving that such vague (but genuine) comments are true in an absolute sense. Rather, it will try to make the point that, based on a straightforward analysis, it is implausible that well-trained personnel would not detect differences beyond noise and signal level." (emphasis in original)

This, in itself, is a remarkably bold position for Dr. Lagadec to adopt. To acknowledge that previously unidentified phenomena affect the subjective perception of digitally processed music is indeed a milestone on the road to improving digital audio. Furthermore, the thesis doesn't summarily reject the listening experience as an important contributor to understanding these phenomena. The audio engineering establishment typically rejects listening because one's perceptions cannot be proven in a scientifically acceptable method and are therefore meaningless. It is also unusual for a man of science to use a lexicon associated more with audiophiles than scientists ("the texture of the new signal, its fine structure, possibly its precise spatial definition"). The audiophile, however, would have described these perceptions in more blunt terms; textures hard rather than liquid, loss of inner instrumental detail, and a collapsed soundstage.

Footnote 1: "To doubt everything or to believe everything are two equally convenient solutions; both dispense with the necessity of reflection." Jules Henri Poincaré, as quoted by Bertrand Russell in the preface to Science and Method.—Robert Harley

Footnote 2: It is common practice among many record companies, when transferring the analog master tape to digital, to "clean up" the tape by following the fades on the analog master with the digital fader. This reduces the analog tape hiss during the fade and inserts "digital silence" (all encoded data words are zero) between tracks. However, this standard practice causes low-level signals—already a challenge to digital—to be requantized in an extremely coarse way. The end of a fade often includes reverberation decay that contains spatial information—the worst place for loss of low-level resolution. And now since the analog master has been converted to digital and is "perfect," "permanent," and "impervious to tape degradation," the analog master is sometimes destroyed to clear vault space. Recently, however, there has been a trend away from subjecting the program to such "improvement."—Robert Harley

Footnote 3: When a signal is attenuated in the digital domain, each sample is multiplied by a number less than 1 (the coefficient), with the coefficient determined by the amount of attenuation. Requantization noise is added to the signal since the new number generated (the product of the sample and the coefficient) must be rounded off to the nearest quantization step, creating an amplitude error that is manifested as quantization noise. Dither is typically analog white noise added to the signal before it is digitized. The addition of dither allows resolution below the Least Significant Bit (LSB), and renders quantization noise less audible by making it more random and less correlated. I refer readers interested in learning more about digital audio to Ken Pohlmann's excellent book, Principles of Digital Audio, Second Edition, published by Howard W. Sams & Company.—Robert Harley

Footnote 4: A Preprint of "New Frontiers in Digital Audio" (preprint #3002, Session Paper #K-2) is available from the Audio Engineering Society, 60 E. 42nd Street, New York, NY 10165. I must stress that this essay is merely my interpretation of Dr. Lagadec's ideas, which I offer with great trepidation. Readers are encouraged to order the preprint and form their own conclusions. In addition, I should distance my opinions and speculations from those put forth in the paper. I have taken a very broad interpretation and am, admittedly, far from unbiased and dispassionate about the subject.—Robert Harley

Share | |

Enter your username.
Enter the password that accompanies your username.