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.
Dr. Lagadec supports his thesis with a very simple analysis of what happens when changing level in the digital domain and the nature of the attendant requantization error. This function is considered perhaps the simplest and best-understood type of digital signal processing. However, he has discovered a previously unknown form of error created by this simple processing: the digital gain control's transfer function (difference between input and output signals) varies according to the amount of gain or attenuation. The nature of the transfer function's non-linearity (imprecision), introduced by changing gain in the digital domain, is determined by the gain pair ratios; ie, the relative beginning signal level and the signal level after gain reduction. I won't go into the details here: the phenomenon is explained and documented fully in the paper.
This discovery is extraordinary for two reasons. First, it vindicates those who have long maintained, after critical listening, that digital faders affect sound quality (footnote 5). Second and more important, it reveals that even the simplest aspects of digital audio that are thought to be well understood are, in fact, not well understood. Dr. Lagadec has worked for the past 7 of his 17 years in digital audio in the area of digital level control, yet recognized and began researching this phenomenon during only the past year. Again, I quote Dr. Lagadec's paper: "It is remarkable that such a simple system, eminently amenable to the methods used in non-linear dynamics, has not—to the author's limited knowledge—been widely publicized yet. If an element of surprise can come from analyzing such simple systems, more instructive surprises may be in store when more complex ones are scrutinized."
With this last sentence, Dr. Lagadec implies that a digital Pandora's Box may be opened by closer analysis of other aspects of digital audio. Like the Pandora's Box of Greek mythology that contained the world's troubles, this digital Pandora's Box may reveal other problems in digital audio that no one knew existed. An understanding of these cracks in conventional digital audio theory will go a long way toward correlating listeners' subjective impressions with objective fact.
The paper then examines dither, another area which, like digital gain adjustment, is considered a closed subject because it is well understood. Dr. Lagadec presents a hypothesis which states that dither should be optimized based on the ear's short-term perception of quantization noise, rather than the current mathematically based long-term analysis: "Needless to say, the 'optimal' dither types in the long-term statistical sense which have been proposed by Vanderkoy [sic] and Lipshitz (footnote 6) are a very valid first approach. As they are, however, independent of any practical detector model, it is not unfair to expect further improvements in perceived performance from dither models optimized in a different, less mathematically rigorous, and more perceptually oriented way."
With that analysis, Dr. Lagadec again proposes that an existing precept, thought to be immutable, is in fact far from a settled question. Moreover, one can infer that future research should be based on improving perceptual qualities rather than conforming to mathematical theories. This is reflected in the phrases "independent of any practical detector model" (my interpretation: "without regard for human hearing"), and "dither models optimized in a different, less mathematically rigorous, and more perceptually oriented way." (emphasis added)
Significantly, this suggests that the criterion for what is considered optimum dither should be based on human hearing rather than on purely mathematical ideas or measurements that have little relation to auditory perception (footnote 7). This represents a remarkable shift in thinking away from the scientific dictum that measurements and theory are more reliable and important than human perception in determining "what is good" in music reproduction. The human perceptual element in audio engineering has long been disregarded because it cannot be quantified. The ability to measure and quantify an entity are the criteria by which science judges that entity's reality. The scientific mind tends to mistrust anything than cannot be represented or communicated by linear symbols. These symbols that describe reality, obtained by measurement and calculation, assume a greater importance than, or are even mistaken for, the actual reality they try to represent. It is thus momentous that one of the world's foremost audio scientists has called for accepting musical perception directly rather than in the abstract, linear terms of representational thinking.
Dr. Lagadec points to some future directions in digital audio, including "a much greater word length" than the current 16-bit system, and bandwidth much wider than 20kHz. This additional bandwidth would be "kept open for, say, low-level harmonics, harmonics due to non-linear processing, and out-of-band noise shaping." An advantage he notes of having bandwidth beyond 20kHz "would be the freedom to disregard the arguments as to whether there is perceptible sound beyond 20kHz." However, he notes that "For economic reasons, it is evident that hardware capable of such parameters at an acceptable cost is not for this decade."
This concept of today's digital audio being in its infancy and subject to radical changes in fundamental precepts is in sharp contrast to the prevailing view among most academics that the 20kHz bandwidth is adequate, and that properly dithered 16-bit representation provides sufficient resolution and dynamic range. Indeed, the idea that today's digital audio parameters are perfectly satisfactory for music were expressed by Dr. Stanley Lipshitz parenthetically in his "Tutorial on Phase," given at the convention. He somewhat derisively scolded critics of digital audio for speciously (in his view) blaming today's digital audio's fundamental parameters (he specifically mentioned sampling rate and word length) as the cause of its inferiority (in the critics' view) to analog.
Dr. Lagadec then says that since the previously unknown aspects of digital audio he has discovered correlate to what critical listeners have been saying for years, perhaps other claims of audible differences should not be dismissed so cavalierly by the audio engineering establishment, despite the lack of scientific proof of such differences. I was astonished by the paper's last paragraphs (quoted below), in which Dr. Lagadec expands his thesis by bringing up the subject of audible differences between cables. Claims of differences between cables have long been a bugaboo of audio engineers. Further, he contends that if no measurable differences exist between cables, yet critical listeners report such differences, perhaps our understanding of human hearing acuity is suspect, rather than the rationality of those who hear differences.
The paper concludes by calling for the world's audio scientists and researchers to vigorously pursue these new challenges and to make room for, rather than exclude, the role of listening in advancing audio science. Dr. Lagadec writes:
"The industry is full of lore as to the superior sound quality of some cables, connectors, electronic devices, and the like. Assuming, as scientists presumably should, that things can only sound different if they cause signals to become different, and using the technology available today to ascertain whether differences do exist, and if so what they consist of, we may hope to achieve reproducible improvements; to deepen our understanding of sound quality; and to separate unfounded legends from justifiable improvements.
"Conversely, if we were to discover that, when, say, different cables are used, the signals look the same beyond the resolution of today's best converters, but still sound reproducibly different, then we would indeed still have much to learn about human audibility.
"The advanced tools available today—the recorders, computer software, workstations, DSP chips and boards, monitor systems, A/D and D/A conversion systems, instrumentation—and which are within reach of any university might be put to use, scientifically, aggressively, to find out how we hear, and how we might improve what we hear. Every generation since Edison's days has said that its sound recordings were almost better than the original, and at the very least indistinguishable from reality. Ours will hardly be an exception, neither in hubris and hype, nor in the disappointment. Yet we have tools for generating and manipulating signals, moving them in space and time, which few of our predecessors dreamed of. The tools deserve to be used, and our engineers deserve to be guided, by scientists who will advance the state of the art ahead of the state of the industry."
Although the tenets put forth in "New Frontiers in Digital Audio" are hardly new to audiophiles, the paper is revolutionary to the mindset of the audio engineering community. What makes this paper such a significant and extraordinary event is the credibility and influence of its author. You may be certain that Dr. Lagadec does not speak lightly or from a shaky platform. Consequently, the ideas expressed in the paper will be given serious consideration by those accustomed to attacking the very same ideas when espoused by those of us without Ph.Ds.
As I considered the paper's ramifications, I couldn't help thinking about the people who for years have reported sonic anomalies in digital audio, only to be met with skepticism and ridicule. This is especially true of Doug Sax, who was one of the first and most outspoken critics of digital. During the past eight years, he has reported his listening experiences to an indifferent world. Despite his pre-eminence in the field of record mastering, he is regarded as a pariah by the audio engineering community for his views, views which I believe have been taken a step toward scientific fact by "New Frontiers in Digital Audio."
If "genius" is defined as the arriving at conclusions ten years before the rest of the world reaches those same conclusions, then both Roger Lagadec and Doug Sax, disparate as their approaches are, certainly qualify. The fact that Dr. Lagadec's paper may cause the audio engineering establishment to take seriously the listening impressions of people like Doug Sax is small consolation to music lovers who must listen to inferior CDs made during the period between when the problems were first reported (1982) and when the problems' existence were proved in a scientifically acceptable method (1990).
"New Frontiers in Digital Audio" holds out the hope that one day digital audio may exceed analog's performance in all respects. It is my sincerest hope that our successors regard today's pronouncements of digital audio's quality with the same combination of humor and incredulity with which we view Anna Case's assessment of Mr. Edison's machine.
http://www.stereophile.com/asweseeit/1290awsi/
Wednesday, May 30, 2007
Frozen CDs?
Just when you thought it was safe to put green paint around the edges of your CDs without ridicule, there's yet another CD tweak that's sure to bring howls of laughter from the skeptics: cryogenically freezing CDs. They won't be laughing for long, however, when they hear for themselves the sonic results of this process.
Ed Meitner, designer of the Museatex line of electronics, has discovered that cryogenically freezing a CD changes the physical structure of polycarbonate, the plastic material from which CDs are made. The result is reportedly an audible improvement in sound quality. In this process, CDs are placed in a cryogenic freezing chamber and the temperature is slowly reduced over eight hours to 75 Kelvins, or about -300 degrees Fahrenheit. This is approximately the temperature of liquid nitrogen, the chamber's cooling agent. The temperature is then slowly brought back to room temperature over another eight hours.
This technique reportedly relaxes the lattice structure of a material (polycarbonate in the case of CD) that has been previously distorted by heat or pressure, both of which are present during CD injection molding. By reducing the molecular bonds holding the material together, the internal stress in the material is reduced, thus changing its resonant characteristics. Indeed, a treated disc feels slightly more flexible than an untreated disc.
But how could freezing a CD possibly affect its sound quality? So what if the polycarbonate has a different structure? The data are all ones and zeros. Furthermore, uncorrected data errors are almost nonexistent in most discsci without treatment, ruling out improved data integrity as an answer. I posed these questions to Ed Meitner and got the following explanation (footnote 1).
Mechanical vibration of the disc causes the HF signal to become noisy and have excessive jitter. The HF signal is the raw signal output from the CD player's photodetector (footnote 2). By freezing a CD, the disc's mechanical resonance is lowered, improving the quality of the HF signal retrieved from the disc. Although theory states that noise and jitter in the HF signal will have no effect on sound quality—the HF signal is squared, buffered, decoded, filtered, and clocked out of another buffer with quartz-crystal accuracy—many digital designers maintain that HF signal quality does affect the sound.
Ed Meitner claims that the HF signal improvement from a cryogenically treated disc is easily measurable. I looked at the HF signal on an oscilloscope from the Esoteric P2 transport with treated and untreated discs. I could see no difference in the signal quality. However, it is very difficult to make comparisons without seeing the two HF signals side by side.
Meitner is talking to some audiophile labels about mass-treating their releases. Apparently, the process is efficient and economical, with the ability to treat thousands of discs at once. Liquid nitrogen, which doesn't come in direct contact with the CDs, is inexpensive and readily available. Interestingly, this process is said to yield similar sonic improvements with a vinyl phonograph record. In addition to CDs and LPs, the process has been used on LaserVision-format video discs, speaker cable, interconnects, integrated circuits, and musical instrument strings.
Cryogenic freezing is also used to treat machine tools like drill bits, copper welding tools, and saw blades. The process reportedly improves their wear characteristics, thus extending the tool's useful life. The treatment doesn't always work, however, and there is no consensus among metallurgists that the process is always beneficial. In fact, the effects of cryogenically freezing materials is not well understood; little scientific research has been done to explain the phenomenon (footnote 3).
Another tweak developed by Ed Meitner is painting a CD's top surface black. This reportedly improves sound quality by improving the signal at the CD player's photodetector. Before describing how this works, let's look at the playback laser beam's path through the disc.
The playback beam enters the disc through the surface without the label. It travels through the 1.2mm disc thickness where it encounters pits impressed in the polycarbonate. To reflect the beam back through the disc and to the photodetector, a thin layer of aluminum is deposited on the disc surface, which conforms to the pit structure. A protective coating of varnish seals in the aluminum and prevents it from oxidizing. The label is then silk-screened on top of the protective coating.
Ed Meitner contends that several mechanisms are at work that degrade the HF signal picked up by the photodetector. One phenomenon is distortion of the aluminum layer by the laser beam's heat. Even though the beam is very low-power—about half a milliwatt—it is focused on such a small area (1.5µm) that the aluminum molecules bend, causing the aluminum layer to flex. This introduces jitter in the HF signal as well as noise in the focus signal.
This phenomenon has reportedly been measured by painting black bars on a CD's top surface (the label side) and looking at various signals. The bar pattern is readily apparent in both the focus servo and HF signals. Painting the CD black reportedly improves the thermal conditions by reducing the contrast in the aluminum molecules caused by laser-induced heat. Another mechanism that is also affected by black paint is the secondary reflection from the disc label. Some laser light passes through the aluminum layer and is reflected to the photodetector by the label. This reportedly causes noise in the HF signal which is manifested as uncertainty in the digital code transitions. Note that the above descriptions are those of Ed Meitner, and have not been independently verified.
How plausible are these explanations? I find some of them hard to believe, especially this last phenomenon. However, there is so much going on in digital audio that we don't know about—especially the optical considerations in data retrieval from CD—that I hesitate to rule out anything (footnote 4).
What really matters is if these treatments work. Since I believe that the ear is the highest-resolution instrument available to explore these phenomena, I gave Ed Meitner three copies of the Stereophile Test CD for treatment. One disc was cryogenically frozen, another was painted black, and the third was both painted and frozen. The Stereophile Test CD is ideal for this purpose: it has a wide variety of music, all recorded by Stereophile contributors. In addition, I know with absolute certainty that the three treated discs as well as my untreated control disc were made by JVC from the same master tape and CD stamper.
I began by listening to my guitar and bass recording from the untreated disc. After switching to the frozen and painted disc, the difference was immediate and obvious. First, the guitar appeared to become louder, with more clarity and detail. Subtle sounds like finger noises and minute instrumental detail jumped forward. The sonic picture became more vivid and immediate. The acoustic bass took on a more rounded character and its musical contribution seemed enhanced. There was a greater degree of air and life around the instruments; they suddenly became more palpable.
The degree to which these characteristics were apparent varied considerably with the type of music. During our annual Stereophile writers' conference in early August, I had an opportunity to play treated and untreated discs for some of the visiting writers. Arnis Balgalvis correctly identified the treated disc in a blind A/B/A comparison when he visited my listening room. He immediately knew that presentation B was different, and his description of the difference was remarkably similar to my impressions.
I repeated the blind test for Peter Mitchell in JA's listening room; Peter also immediately identified the treated disc. In fact, within seconds of hearing the treated disc with the guitar and bass recording, he let out a loud exclamation of surprise. His impressions were consistent with the differences I had heard, which I related to him after the test and his description to me of the differences.
A good point Peter raised was that although there was clearly a difference, he had doubts about which was "better" or more true to the original recording. The treated disc had a brighter, more detailed character that would exacerbate many of CD's problems.
The above listening comparisons were made between an untreated disc and one that was both cryogenically frozen and painted black. Further listening of frozen-only discs and painted-only discs revealed that most of the sonic difference was the result of freezing. The black paint, however, did add to the effect. A second frozen and painted disc sent to me by Museatex had similar differences. However, a look at the disc's inside ring, where the production number is written, revealed that it was a different pressing from my untreated control disc. I would therefore refrain from reaching any conclusions based on this disc. From my experience with the Stereophile Test CD, however, I am convinced that some unexplained phenomena are occurring in frozen and painted CDs.
http://www.stereophile.com/asweseeit/822/index1.html
Ed Meitner, designer of the Museatex line of electronics, has discovered that cryogenically freezing a CD changes the physical structure of polycarbonate, the plastic material from which CDs are made. The result is reportedly an audible improvement in sound quality. In this process, CDs are placed in a cryogenic freezing chamber and the temperature is slowly reduced over eight hours to 75 Kelvins, or about -300 degrees Fahrenheit. This is approximately the temperature of liquid nitrogen, the chamber's cooling agent. The temperature is then slowly brought back to room temperature over another eight hours.
This technique reportedly relaxes the lattice structure of a material (polycarbonate in the case of CD) that has been previously distorted by heat or pressure, both of which are present during CD injection molding. By reducing the molecular bonds holding the material together, the internal stress in the material is reduced, thus changing its resonant characteristics. Indeed, a treated disc feels slightly more flexible than an untreated disc.
But how could freezing a CD possibly affect its sound quality? So what if the polycarbonate has a different structure? The data are all ones and zeros. Furthermore, uncorrected data errors are almost nonexistent in most discsci without treatment, ruling out improved data integrity as an answer. I posed these questions to Ed Meitner and got the following explanation (footnote 1).
Mechanical vibration of the disc causes the HF signal to become noisy and have excessive jitter. The HF signal is the raw signal output from the CD player's photodetector (footnote 2). By freezing a CD, the disc's mechanical resonance is lowered, improving the quality of the HF signal retrieved from the disc. Although theory states that noise and jitter in the HF signal will have no effect on sound quality—the HF signal is squared, buffered, decoded, filtered, and clocked out of another buffer with quartz-crystal accuracy—many digital designers maintain that HF signal quality does affect the sound.
Ed Meitner claims that the HF signal improvement from a cryogenically treated disc is easily measurable. I looked at the HF signal on an oscilloscope from the Esoteric P2 transport with treated and untreated discs. I could see no difference in the signal quality. However, it is very difficult to make comparisons without seeing the two HF signals side by side.
Meitner is talking to some audiophile labels about mass-treating their releases. Apparently, the process is efficient and economical, with the ability to treat thousands of discs at once. Liquid nitrogen, which doesn't come in direct contact with the CDs, is inexpensive and readily available. Interestingly, this process is said to yield similar sonic improvements with a vinyl phonograph record. In addition to CDs and LPs, the process has been used on LaserVision-format video discs, speaker cable, interconnects, integrated circuits, and musical instrument strings.
Cryogenic freezing is also used to treat machine tools like drill bits, copper welding tools, and saw blades. The process reportedly improves their wear characteristics, thus extending the tool's useful life. The treatment doesn't always work, however, and there is no consensus among metallurgists that the process is always beneficial. In fact, the effects of cryogenically freezing materials is not well understood; little scientific research has been done to explain the phenomenon (footnote 3).
Another tweak developed by Ed Meitner is painting a CD's top surface black. This reportedly improves sound quality by improving the signal at the CD player's photodetector. Before describing how this works, let's look at the playback laser beam's path through the disc.
The playback beam enters the disc through the surface without the label. It travels through the 1.2mm disc thickness where it encounters pits impressed in the polycarbonate. To reflect the beam back through the disc and to the photodetector, a thin layer of aluminum is deposited on the disc surface, which conforms to the pit structure. A protective coating of varnish seals in the aluminum and prevents it from oxidizing. The label is then silk-screened on top of the protective coating.
Ed Meitner contends that several mechanisms are at work that degrade the HF signal picked up by the photodetector. One phenomenon is distortion of the aluminum layer by the laser beam's heat. Even though the beam is very low-power—about half a milliwatt—it is focused on such a small area (1.5µm) that the aluminum molecules bend, causing the aluminum layer to flex. This introduces jitter in the HF signal as well as noise in the focus signal.
This phenomenon has reportedly been measured by painting black bars on a CD's top surface (the label side) and looking at various signals. The bar pattern is readily apparent in both the focus servo and HF signals. Painting the CD black reportedly improves the thermal conditions by reducing the contrast in the aluminum molecules caused by laser-induced heat. Another mechanism that is also affected by black paint is the secondary reflection from the disc label. Some laser light passes through the aluminum layer and is reflected to the photodetector by the label. This reportedly causes noise in the HF signal which is manifested as uncertainty in the digital code transitions. Note that the above descriptions are those of Ed Meitner, and have not been independently verified.
How plausible are these explanations? I find some of them hard to believe, especially this last phenomenon. However, there is so much going on in digital audio that we don't know about—especially the optical considerations in data retrieval from CD—that I hesitate to rule out anything (footnote 4).
What really matters is if these treatments work. Since I believe that the ear is the highest-resolution instrument available to explore these phenomena, I gave Ed Meitner three copies of the Stereophile Test CD for treatment. One disc was cryogenically frozen, another was painted black, and the third was both painted and frozen. The Stereophile Test CD is ideal for this purpose: it has a wide variety of music, all recorded by Stereophile contributors. In addition, I know with absolute certainty that the three treated discs as well as my untreated control disc were made by JVC from the same master tape and CD stamper.
I began by listening to my guitar and bass recording from the untreated disc. After switching to the frozen and painted disc, the difference was immediate and obvious. First, the guitar appeared to become louder, with more clarity and detail. Subtle sounds like finger noises and minute instrumental detail jumped forward. The sonic picture became more vivid and immediate. The acoustic bass took on a more rounded character and its musical contribution seemed enhanced. There was a greater degree of air and life around the instruments; they suddenly became more palpable.
The degree to which these characteristics were apparent varied considerably with the type of music. During our annual Stereophile writers' conference in early August, I had an opportunity to play treated and untreated discs for some of the visiting writers. Arnis Balgalvis correctly identified the treated disc in a blind A/B/A comparison when he visited my listening room. He immediately knew that presentation B was different, and his description of the difference was remarkably similar to my impressions.
I repeated the blind test for Peter Mitchell in JA's listening room; Peter also immediately identified the treated disc. In fact, within seconds of hearing the treated disc with the guitar and bass recording, he let out a loud exclamation of surprise. His impressions were consistent with the differences I had heard, which I related to him after the test and his description to me of the differences.
A good point Peter raised was that although there was clearly a difference, he had doubts about which was "better" or more true to the original recording. The treated disc had a brighter, more detailed character that would exacerbate many of CD's problems.
The above listening comparisons were made between an untreated disc and one that was both cryogenically frozen and painted black. Further listening of frozen-only discs and painted-only discs revealed that most of the sonic difference was the result of freezing. The black paint, however, did add to the effect. A second frozen and painted disc sent to me by Museatex had similar differences. However, a look at the disc's inside ring, where the production number is written, revealed that it was a different pressing from my untreated control disc. I would therefore refrain from reaching any conclusions based on this disc. From my experience with the Stereophile Test CD, however, I am convinced that some unexplained phenomena are occurring in frozen and painted CDs.
http://www.stereophile.com/asweseeit/822/index1.html
CD: Jitter, Errors & Magic
The promise of "perfect sound forever," successfully foisted on an unwitting public by the Compact Disc's promoters, at first seemed to put an end to the audiophile's inexorable need to tweak a playback system's front end at the point of information retrieval. Several factors contributed to the demise of tweaking during the period when CD players began replacing turntables as the primary front-end signal source. First, the binary nature (ones and zeros) of digital audio would apparently preclude variations in playback sound quality due to imperfections in the recording medium. Second, if CD's sound was indeed "perfect," how could digital tweaking improve on perfection? Finally, CD players and discs presented an enigma to audiophiles accustomed to the more easily understood concept of a stylus wiggling in a phonograph groove. These conditions created a climate in which it was assumed that nothing in the optical and mechanical systems of a CD player could affect digital playback's musicality.
Recently, however, there has been a veritable explosion of interest in all manner of CD tweaks, opening a digital Pandora's box. An avalanche of CD tweak products (and the audiophile's embrace of them) has suddenly appeared in the past few months, Monster Cable's, AudioQuest's, and Euphonic Technology's CD Soundrings notwithstanding. Most of these tweaks would appear to border on voodoo, with no basis in scientific fact. Green marking pens, an automobile interior protectant, and an "optical impedance matching" fluid are just some of the products touted as producing musical nirvana. The popular media has even picked up on this phenomenon, sparked by Sam Tellig's Audio Anarchist column in Vol.13 No.2 describing the sonic benefits of applying Armor All, the automobile treatment, to a CD's surface. Print articles have appeared in the Los Angeles Times, Ice Magazine, and on television stations MTV, VH-1, and CNN, all reporting, with varying degrees of incredulity, the CD tweaking phenomenon.
The intensity of my interest in the subject was heightened by a product called "CD Stoplight," marketed by AudioPrism. CD Stoplight is a green paint applied to the outside edge of a CD (not the disc surface, but the 1.2mm disc thickness) that reportedly improves sound quality. I could not in my wildest imagination see how green paint on the disc edge could change, for better or worse, a CD's sound. However, trusting my ears as the definitive test, I compared treated to untreated discs and was flabbergasted. Soundstage depth increased, mids and highs were smoother with less grain, and the presentation became more musically involving.
Other listeners, to a person, have had similar impressions. Since I am somewhat familiar with the mechanisms by which data are retrieved from a CD (I worked in CD mastering for three years before joining Stereophile), this was perplexing: I could think of no plausible explanation for a difference in sonic quality. As we shall see, the light reflected from a CD striking the photo-detector contains all the information encoded on the disc (footnote 1). Even if CD Stoplight could somehow affect the light striking the photo-detector, how could this change make the soundstage deeper? I was simultaneously disturbed and encouraged by this experience. Disturbed because it illustrates our fundamental lack of understanding of digital audio's mysteries, and encouraged by the promise that identification of previously unexplored phenomena could improve digital audio to the point where today's digital audio era will be regarded as the stone age.
These events prompted me to conduct a scientific examination of several CD "sonic cure-all" devices and treatments. I wanted to find an objective, measurable phenomenon that explains the undeniable musical differences heard by many listeners where, at least according to established digital audio theory, no differences should exist. For this inquiry, I measured several digital-domain performance criteria on untreated CDs, and then on the same CDs treated with various CD tweaks. The parameters measured include data error rates, ability to correct (rather than conceal) data errors, and jitter.
The six CD treatments and devices chosen for this experiment include three that allegedly affect optical phenomena and three that ostensibly affect the CD player's mechanical performance. The three optical treatments tested are CD Stoplight (the green paint), Finyl (a liquid applied to a disc surface, that, according to its promoters, provides "optical impedance matching"), and Armor All. The mechanical devices include CD Soundrings, The Mod Squad's CD Damper disc, and the Arcici LaserBase, a vibration-absorbing CD-player platform. I also measured playback signal jitter in a mid-priced CD player and the $4000 Esoteric P2 transport (regarded as having superb sonics). However, this is not intended as a survey of the musical benefits of these devices and treatments. In addition, I looked at the variation in quality of discs made at various CD manufacturing facilities around the world.
Another purpose of the article is to dispel some common misconceptions about CD error correction and its effect on sonic quality. If one believes the promoters of some of these CD treatments, errors are the single biggest source of sonic degradation in digital audio. In reality, errors are the least of CD's problems. However, this has not prevented marketeers from exploiting the audiophile's errorphobia in an attempt to sell products.
For example, Digital Systems and Solutions, Inc., manufacturer of Finyl, claim in their white paper that error concealment "results in a serious degrading of playback fidelity." They also state that errors can get through undetected, leading to a litany of sonic horrors including: "poor articulation of bass and mid-bass notes, attenuation of dynamics and smearing of transients, increased noise with loss of inner detail and intertransient silence, reduced midrange presence that diminishes clarity and transparency, loss of image specificity and focus, reduction of the apparent width and depth of soundstage—virtually eliminating the possibility of holophonic [sic] imagery, decreased resolution of the low level detail that is so necessary to the recovery of hall ambience, altered instrumental and vocal timbres that lack coherence or cohesiveness, obscuring of vocal textures and expression, instrumental lines and musical themes are more difficult to sort out, complex rhythms and tempos are less easily followed, the music will not be as emotionally involving and satisfying an experience as might have otherwise been possible, subtle breath effects on brass or wind instruments are more difficult to discern as are nuances of fingering and bowing on string instruments." This list, they concede, "is not claimed to be complete."
Technical background
Encoding and data retrieval: Before getting into the measurement results, let's arm ourselves with a little technical background on how the CD works.
A CD's surface is covered by a single spiral track of alternating "pit" and "land" formations. These structures, which encode binary data, are created during the laser mastering process. The CD master disc is a glass substrate coated with a very thin layer of photosensitive material. The glass master is rotated on a turntable while exposed to a laser beam that is modulated (turned on and off) by the digital data we wish to record on the disc. This creates a spiral of exposed and unexposed areas of the disc. When the master is later put under a chemical developing solution, areas of the photosensitive material exposed to the recording laser beam are etched away, creating a pit. Unexposed areas are unaffected by the developing solution and are called lands. These formations, which are among the smallest manufactured structures, are transferred through the manufacturing process to mass-produced discs. Fig.1 is a scanning electron microscope of a CD surface. Note that a human hair is about the width of 50 tracks.
The playback laser beam in the CD player is focused on these tiny pits and held on track by a servo system as the disc rotates. This beam is reflected from the disc to a photo-detector, a device that converts light into voltage. To distinguish between pit and land areas, the pit depth is one-quarter the wavelength of the playback laser beam. When laser light strikes a pit, a portion of the beam is reflected from the surrounding land, while some light is reflected from the pit bottom. Since the portion of light reflected from the pit bottom must travel a longer distance (1/4 wavelength down plus 1/4 wavelength back up), this portion of the beam is delayed by half a wavelength in relation to the beam reflected by the land. When these two beams combine, phase cancellation occurs, resulting in decreased output from the photo-detector. This variable-intensity beam thus contains all the information encoded on the disc.
Now that we understand how the playback beam/photo-detector can distinguish between pit and land, let's look at how these distinctions represent digital audio data. One may intuitively think that it would be logical for a pit to represent binary one and a land to represent binary zero, or vice versa. This method would certainly work, but a much more sophisticated scheme has been devised that is fundamental to the CD. It is called Eight-to-Fourteen Modulation, or EFM.
This encoding system elegantly solves a variety of data-retrieval functions. In EFM encoding, pit and land do not represent binary data directly. Instead, transitions from pit-to-land or land-to-pit represent binary one, while all other surfaces (land or pit bottom) represent binary zero. EFM encoding takes symbols of 8 bits and converts them into unique 14-bit words, creating a pattern in which binary ones are separated by a minimum of two zeros and a maximum of 10 zeros. The bit stream is thus given a specific pattern of ones and zeros that result in nine discrete pit or land lengths on the disc. The shortest pit or land length encodes three bits, while the longest encodes 11 bits. The blocks of 14 bits are linked by three "merging bits," resulting in an encoding ratio of 17:8. At first glance, it may seem odd that EFM encoding, in more than doubling the number of bits to be stored, can actually increase data density. But just this occurs: Storage density is increased by 25% over unmodulated encoding.
EFM has other inherent advantages. By inserting zeros between successive ones, the bandwidth of the signal reflected from the disc is decreased. The data rate from a CD is 4.3218 million bits per second (footnote 2), but the EFM signal has a bandwidth of only 720kHz. In addition, the EFM signal serves as a clock that, among other functions, controls the player's rotational servo.
The signal reflected from the disc is comprised of nine discrete frequencies, corresponding to the nine discrete pit or land lengths (footnote 3). The highest-frequency component, called "I3," is produced by the shortest pit or land length and has a frequency of 720kHz. This represents binary data 100. The lowest-frequency component, called "I11," is produced by the longest pit or land length and has a frequency of 193kHz. This represents binary data 10000000000. The signal reflected from the disc, produced by EFM encoding, is often called the HF (high frequency) signal. The varying periods of the sinewaves correspond to the periods of time required to read the various pit lengths.
At first impression, the HF signal appears to be analog, not one that carries digital data. However, the zero crossings of the waveforms contain the digital information encoded on the disc. Fig.2 shows the relationship between binary data, pit structure, and the recovered HF signal.
Fig.2 Relationship between binary data, pit structure, and the HF signal. (Reproduced from Principles of Digital Audio, Second Edition (1989), by Kenneth C. Pohlmann, with the permission of the publisher, Howard W. Sams & Company.)
HF signal quality is a direct function of pit shape, which in turn is affected by many factors during the CD manufacturing process. There is a direct correlation between error rates and pit shape. Poorly shaped pits result in a low-amplitude HF signal with poorly defined lines. Figs.3 and 4 show an excellent HF signal and a poor HF signal respectively.
CD data errors: Any digital storage medium is prone to data errors, and the CD is no exception. An error occurs when a binary one is mistakenly read as a binary zero (or vice versa), or when the data flow is momentarily interrupted. The latter, more common in CDs, is caused by manufacturing defects, surface scratches, and dirt or other foreign particles on the disc. Fortunately, the CD format incorporates extremely powerful error detection and correction codes that can completely correct a burst error of up to 4000 successive bits. The reconstructed data are identical to what was missing. This is called error correction. If the data loss exceeds the player's ability to correctly replace missing data, the player makes a best-guess estimate of the missing data and inserts this approximation into the data stream. This is called error concealment, or interpolation.
It is important to make the distinction between correction and concealment: correction is perfect and inaudible, while concealment has the potential for a momentary sonic degradation where the interpolation occurs.
A good general indication of disc quality (and the claimed error-reduction effects of some CD tweaks) is the Block Error Rate, or BLER. BLER is the number of blocks per second that contain errant data, before error correction. The raw data stream from a CD (called "channel bits") contains 7350 blocks per second, with a maximum allowable BLER (as specified by Philips) of 220. A disc with a BLER of 100 thus has 100 blocks out of 7350 with errant or missing data. In these experiments, Block Error Rate is the primary indicator of a particular tweak's effect on error-rate performance.
In addition to measuring the effects of CD tweaks on BLER, I explored their potential to reduce interpolations. To do this, I used the Pierre Verany test CD that has intentional dropouts in the spiral track. The disc has a sequence of tracks with increasingly long periods of missing data.
First, I found the track that was just above the threshold of producing an uncorrectable error (called an "E23 error") as analyzed by the Design Science CD Analyzer (see Sidebar). The track was played repeatedly to assure consistency, thus avoiding the ascription to chance of any subsequent change. Then, the same track was played and analyzed, this time after the addition of a CD treatment or device. This twofold approach—measuring a tweak's effect on both BLER and interpolations—would seem to cover the gamut of error-reduction potential.
There are two general misconceptions about CD errors and sound quality: 1) errors are the primary source of sonic degradation; and 2) if there are no uncorrectable errors, there can be no difference in sound.
The first conclusion is largely due to the marketing programs of CD-accessory manufacturers who claim their products reduce error rates. Many of the devices tested claim to improve sound quality by reducing the amount of error concealment performed by the CD player. In fact, interpolations (error concealment) rarely occur. In the unlikely event that concealment is performed, it will be momentary and thus have no effect on the overall sound. At worst, a transient tick or glitch would be audible.
To better understand the nature of data errors, a look at CD Read-Only Memory (CD-ROM) is useful. A CD-ROM is manufactured just like an audio CD, but contains computer data (text, graphics, application software, etc.) instead of music. The data retrieved from a CD-ROM must be absolutely accurate to the bit level, after error correction. If even a single wrong bit gets past the error correction, the entire program could crash. The errant bit may be within instructions for the host computer's microprocessor, causing the whole application to come to an instant halt, making the disc useless.
To prevent this, a quality-control procedure is routinely used at the mastering and pressing facility to assure 100% error-free performance. Samples of the finished CD-ROM are compared, bit for bit, to the original source data. For high-reliability applications, each replicated disc undergoes this process. This rigorous testing reveals much about the error-correction ability of the CD's Cross Interleaved Reed-Solomon encoding (CIRC). Throughout dozens of hours of this verification procedure, I cannot remember even a single instance of one wrong bit getting through.
It could be argued that CD-ROM has additional error-correction ability not found on CD audio discs. This is true, but the additional layer of error correction is almost never invoked. Furthermore, in all the hours of error-rate measuring for this project, I never encountered an E23 error, the first and most sensitive indication of an interpolation (except on the Pierre Verany disc, which has intentional errors). In fact, I saw only one E22 error, the last stage of correction before concealment. In retesting the disc, the E22 error disappeared, indicating it was probably due to a piece of dirt on the disc. Finally, the unlikely occurrence of an uncorrectable error is exemplified by the warning system in the Design Science CD Analyzer. The system beeps and changes the computer's display color to red to alert the operator if even an E22 error (fully corrected) is detected.
http://www.stereophile.com/reference/590jitter/
Recently, however, there has been a veritable explosion of interest in all manner of CD tweaks, opening a digital Pandora's box. An avalanche of CD tweak products (and the audiophile's embrace of them) has suddenly appeared in the past few months, Monster Cable's, AudioQuest's, and Euphonic Technology's CD Soundrings notwithstanding. Most of these tweaks would appear to border on voodoo, with no basis in scientific fact. Green marking pens, an automobile interior protectant, and an "optical impedance matching" fluid are just some of the products touted as producing musical nirvana. The popular media has even picked up on this phenomenon, sparked by Sam Tellig's Audio Anarchist column in Vol.13 No.2 describing the sonic benefits of applying Armor All, the automobile treatment, to a CD's surface. Print articles have appeared in the Los Angeles Times, Ice Magazine, and on television stations MTV, VH-1, and CNN, all reporting, with varying degrees of incredulity, the CD tweaking phenomenon.
The intensity of my interest in the subject was heightened by a product called "CD Stoplight," marketed by AudioPrism. CD Stoplight is a green paint applied to the outside edge of a CD (not the disc surface, but the 1.2mm disc thickness) that reportedly improves sound quality. I could not in my wildest imagination see how green paint on the disc edge could change, for better or worse, a CD's sound. However, trusting my ears as the definitive test, I compared treated to untreated discs and was flabbergasted. Soundstage depth increased, mids and highs were smoother with less grain, and the presentation became more musically involving.
Other listeners, to a person, have had similar impressions. Since I am somewhat familiar with the mechanisms by which data are retrieved from a CD (I worked in CD mastering for three years before joining Stereophile), this was perplexing: I could think of no plausible explanation for a difference in sonic quality. As we shall see, the light reflected from a CD striking the photo-detector contains all the information encoded on the disc (footnote 1). Even if CD Stoplight could somehow affect the light striking the photo-detector, how could this change make the soundstage deeper? I was simultaneously disturbed and encouraged by this experience. Disturbed because it illustrates our fundamental lack of understanding of digital audio's mysteries, and encouraged by the promise that identification of previously unexplored phenomena could improve digital audio to the point where today's digital audio era will be regarded as the stone age.
These events prompted me to conduct a scientific examination of several CD "sonic cure-all" devices and treatments. I wanted to find an objective, measurable phenomenon that explains the undeniable musical differences heard by many listeners where, at least according to established digital audio theory, no differences should exist. For this inquiry, I measured several digital-domain performance criteria on untreated CDs, and then on the same CDs treated with various CD tweaks. The parameters measured include data error rates, ability to correct (rather than conceal) data errors, and jitter.
The six CD treatments and devices chosen for this experiment include three that allegedly affect optical phenomena and three that ostensibly affect the CD player's mechanical performance. The three optical treatments tested are CD Stoplight (the green paint), Finyl (a liquid applied to a disc surface, that, according to its promoters, provides "optical impedance matching"), and Armor All. The mechanical devices include CD Soundrings, The Mod Squad's CD Damper disc, and the Arcici LaserBase, a vibration-absorbing CD-player platform. I also measured playback signal jitter in a mid-priced CD player and the $4000 Esoteric P2 transport (regarded as having superb sonics). However, this is not intended as a survey of the musical benefits of these devices and treatments. In addition, I looked at the variation in quality of discs made at various CD manufacturing facilities around the world.
Another purpose of the article is to dispel some common misconceptions about CD error correction and its effect on sonic quality. If one believes the promoters of some of these CD treatments, errors are the single biggest source of sonic degradation in digital audio. In reality, errors are the least of CD's problems. However, this has not prevented marketeers from exploiting the audiophile's errorphobia in an attempt to sell products.
For example, Digital Systems and Solutions, Inc., manufacturer of Finyl, claim in their white paper that error concealment "results in a serious degrading of playback fidelity." They also state that errors can get through undetected, leading to a litany of sonic horrors including: "poor articulation of bass and mid-bass notes, attenuation of dynamics and smearing of transients, increased noise with loss of inner detail and intertransient silence, reduced midrange presence that diminishes clarity and transparency, loss of image specificity and focus, reduction of the apparent width and depth of soundstage—virtually eliminating the possibility of holophonic [sic] imagery, decreased resolution of the low level detail that is so necessary to the recovery of hall ambience, altered instrumental and vocal timbres that lack coherence or cohesiveness, obscuring of vocal textures and expression, instrumental lines and musical themes are more difficult to sort out, complex rhythms and tempos are less easily followed, the music will not be as emotionally involving and satisfying an experience as might have otherwise been possible, subtle breath effects on brass or wind instruments are more difficult to discern as are nuances of fingering and bowing on string instruments." This list, they concede, "is not claimed to be complete."
Technical background
Encoding and data retrieval: Before getting into the measurement results, let's arm ourselves with a little technical background on how the CD works.
A CD's surface is covered by a single spiral track of alternating "pit" and "land" formations. These structures, which encode binary data, are created during the laser mastering process. The CD master disc is a glass substrate coated with a very thin layer of photosensitive material. The glass master is rotated on a turntable while exposed to a laser beam that is modulated (turned on and off) by the digital data we wish to record on the disc. This creates a spiral of exposed and unexposed areas of the disc. When the master is later put under a chemical developing solution, areas of the photosensitive material exposed to the recording laser beam are etched away, creating a pit. Unexposed areas are unaffected by the developing solution and are called lands. These formations, which are among the smallest manufactured structures, are transferred through the manufacturing process to mass-produced discs. Fig.1 is a scanning electron microscope of a CD surface. Note that a human hair is about the width of 50 tracks.
The playback laser beam in the CD player is focused on these tiny pits and held on track by a servo system as the disc rotates. This beam is reflected from the disc to a photo-detector, a device that converts light into voltage. To distinguish between pit and land areas, the pit depth is one-quarter the wavelength of the playback laser beam. When laser light strikes a pit, a portion of the beam is reflected from the surrounding land, while some light is reflected from the pit bottom. Since the portion of light reflected from the pit bottom must travel a longer distance (1/4 wavelength down plus 1/4 wavelength back up), this portion of the beam is delayed by half a wavelength in relation to the beam reflected by the land. When these two beams combine, phase cancellation occurs, resulting in decreased output from the photo-detector. This variable-intensity beam thus contains all the information encoded on the disc.
Now that we understand how the playback beam/photo-detector can distinguish between pit and land, let's look at how these distinctions represent digital audio data. One may intuitively think that it would be logical for a pit to represent binary one and a land to represent binary zero, or vice versa. This method would certainly work, but a much more sophisticated scheme has been devised that is fundamental to the CD. It is called Eight-to-Fourteen Modulation, or EFM.
This encoding system elegantly solves a variety of data-retrieval functions. In EFM encoding, pit and land do not represent binary data directly. Instead, transitions from pit-to-land or land-to-pit represent binary one, while all other surfaces (land or pit bottom) represent binary zero. EFM encoding takes symbols of 8 bits and converts them into unique 14-bit words, creating a pattern in which binary ones are separated by a minimum of two zeros and a maximum of 10 zeros. The bit stream is thus given a specific pattern of ones and zeros that result in nine discrete pit or land lengths on the disc. The shortest pit or land length encodes three bits, while the longest encodes 11 bits. The blocks of 14 bits are linked by three "merging bits," resulting in an encoding ratio of 17:8. At first glance, it may seem odd that EFM encoding, in more than doubling the number of bits to be stored, can actually increase data density. But just this occurs: Storage density is increased by 25% over unmodulated encoding.
EFM has other inherent advantages. By inserting zeros between successive ones, the bandwidth of the signal reflected from the disc is decreased. The data rate from a CD is 4.3218 million bits per second (footnote 2), but the EFM signal has a bandwidth of only 720kHz. In addition, the EFM signal serves as a clock that, among other functions, controls the player's rotational servo.
The signal reflected from the disc is comprised of nine discrete frequencies, corresponding to the nine discrete pit or land lengths (footnote 3). The highest-frequency component, called "I3," is produced by the shortest pit or land length and has a frequency of 720kHz. This represents binary data 100. The lowest-frequency component, called "I11," is produced by the longest pit or land length and has a frequency of 193kHz. This represents binary data 10000000000. The signal reflected from the disc, produced by EFM encoding, is often called the HF (high frequency) signal. The varying periods of the sinewaves correspond to the periods of time required to read the various pit lengths.
At first impression, the HF signal appears to be analog, not one that carries digital data. However, the zero crossings of the waveforms contain the digital information encoded on the disc. Fig.2 shows the relationship between binary data, pit structure, and the recovered HF signal.
Fig.2 Relationship between binary data, pit structure, and the HF signal. (Reproduced from Principles of Digital Audio, Second Edition (1989), by Kenneth C. Pohlmann, with the permission of the publisher, Howard W. Sams & Company.)
HF signal quality is a direct function of pit shape, which in turn is affected by many factors during the CD manufacturing process. There is a direct correlation between error rates and pit shape. Poorly shaped pits result in a low-amplitude HF signal with poorly defined lines. Figs.3 and 4 show an excellent HF signal and a poor HF signal respectively.
CD data errors: Any digital storage medium is prone to data errors, and the CD is no exception. An error occurs when a binary one is mistakenly read as a binary zero (or vice versa), or when the data flow is momentarily interrupted. The latter, more common in CDs, is caused by manufacturing defects, surface scratches, and dirt or other foreign particles on the disc. Fortunately, the CD format incorporates extremely powerful error detection and correction codes that can completely correct a burst error of up to 4000 successive bits. The reconstructed data are identical to what was missing. This is called error correction. If the data loss exceeds the player's ability to correctly replace missing data, the player makes a best-guess estimate of the missing data and inserts this approximation into the data stream. This is called error concealment, or interpolation.
It is important to make the distinction between correction and concealment: correction is perfect and inaudible, while concealment has the potential for a momentary sonic degradation where the interpolation occurs.
A good general indication of disc quality (and the claimed error-reduction effects of some CD tweaks) is the Block Error Rate, or BLER. BLER is the number of blocks per second that contain errant data, before error correction. The raw data stream from a CD (called "channel bits") contains 7350 blocks per second, with a maximum allowable BLER (as specified by Philips) of 220. A disc with a BLER of 100 thus has 100 blocks out of 7350 with errant or missing data. In these experiments, Block Error Rate is the primary indicator of a particular tweak's effect on error-rate performance.
In addition to measuring the effects of CD tweaks on BLER, I explored their potential to reduce interpolations. To do this, I used the Pierre Verany test CD that has intentional dropouts in the spiral track. The disc has a sequence of tracks with increasingly long periods of missing data.
First, I found the track that was just above the threshold of producing an uncorrectable error (called an "E23 error") as analyzed by the Design Science CD Analyzer (see Sidebar). The track was played repeatedly to assure consistency, thus avoiding the ascription to chance of any subsequent change. Then, the same track was played and analyzed, this time after the addition of a CD treatment or device. This twofold approach—measuring a tweak's effect on both BLER and interpolations—would seem to cover the gamut of error-reduction potential.
There are two general misconceptions about CD errors and sound quality: 1) errors are the primary source of sonic degradation; and 2) if there are no uncorrectable errors, there can be no difference in sound.
The first conclusion is largely due to the marketing programs of CD-accessory manufacturers who claim their products reduce error rates. Many of the devices tested claim to improve sound quality by reducing the amount of error concealment performed by the CD player. In fact, interpolations (error concealment) rarely occur. In the unlikely event that concealment is performed, it will be momentary and thus have no effect on the overall sound. At worst, a transient tick or glitch would be audible.
To better understand the nature of data errors, a look at CD Read-Only Memory (CD-ROM) is useful. A CD-ROM is manufactured just like an audio CD, but contains computer data (text, graphics, application software, etc.) instead of music. The data retrieved from a CD-ROM must be absolutely accurate to the bit level, after error correction. If even a single wrong bit gets past the error correction, the entire program could crash. The errant bit may be within instructions for the host computer's microprocessor, causing the whole application to come to an instant halt, making the disc useless.
To prevent this, a quality-control procedure is routinely used at the mastering and pressing facility to assure 100% error-free performance. Samples of the finished CD-ROM are compared, bit for bit, to the original source data. For high-reliability applications, each replicated disc undergoes this process. This rigorous testing reveals much about the error-correction ability of the CD's Cross Interleaved Reed-Solomon encoding (CIRC). Throughout dozens of hours of this verification procedure, I cannot remember even a single instance of one wrong bit getting through.
It could be argued that CD-ROM has additional error-correction ability not found on CD audio discs. This is true, but the additional layer of error correction is almost never invoked. Furthermore, in all the hours of error-rate measuring for this project, I never encountered an E23 error, the first and most sensitive indication of an interpolation (except on the Pierre Verany disc, which has intentional errors). In fact, I saw only one E22 error, the last stage of correction before concealment. In retesting the disc, the E22 error disappeared, indicating it was probably due to a piece of dirt on the disc. Finally, the unlikely occurrence of an uncorrectable error is exemplified by the warning system in the Design Science CD Analyzer. The system beeps and changes the computer's display color to red to alert the operator if even an E22 error (fully corrected) is detected.
http://www.stereophile.com/reference/590jitter/
The Absolute Sound of What?
One of the things that distinguishes a dedicated audiophile from Joe Q. Public is that he has some notion of what audio fidelity is all about.
The typical buyer of a "steeryo" is seeking nothing more than pleasant or exciting sounds, and is easily satisfied because he has no greater expectation of audio than this. The audiophile, however, is aware that reproduced sound can resemble (more or less) real, live sound, and he is driven in a continual search for that ultimate truth ("fidelity to the original") even while realizing, intellectually at least, that it is unattainable.
Because he understands what the word "reproduction" means, the audiophile thinks in terms of a relationship to an original sound. This original is, of course, the sound of live music, and the touchstone for its reproduction is accuracy. Unfortunately, though, we don't really compare the reproduction with the real thing—because we can't. Only a recording engineer can saunter back and forth between the real thing (which takes place in a studio or hall) and the reproduction of it (in the control room with its monitor system). We audiophiles must be content to compare the reproduction with what we remember to be the sound of live music. Even the amateur recordist must carry the memory of that original sound home with his tapes in order to evaluate them.
And that memory may not serve us that well. Few of us have learned to listen with enough attention and skill to be able to break live sound down into its components and to observe what each sounds like. Most of us remember only an overall impression—the gestalt of the thing. And many of us must admit, to ourselves at least, that we have not heard live music for years or, worse, never at all. For the vast majority of audiophiles then, the reference standard is not the absolute sound of live music, but an imagined ideal—a mental picture of how we remember its having sounded or how we would like it to sound. At this point, accuracy becomes a dubious criterion because of the vagueness of the original to which we compare the copy. System evaluation becomes a (simple?) matter of "it's good if it sounds good."
The problem with this is that one man's good is another man's distortion. Different people listen to and assign different orders of importance to different aspects of reproduced sound. Thus, while two very picky listeners may agree that a system has good bass, good highs, and a colored middle range, they will disagree as to how good the system is if one happens to be critical of highs and lows while the other is critical of the middle range.
In short, we really don't have any way of reliably assessing the accuracy of reproduced sound. Even a recording engineer cannot be confident of the sound of his own recording, because what he hears in the control room depends on his monitoring equipment, which is no more—and is often less—accurate than a home system. (Many pros do not, in fact, aim for realism at all, but for what they call a "commercial sound"—one that will sell. Thus a recording may not even have the potential for sounding realistic.)
All this does not, however, discourage audiophiles in their search for the Holy Grail of musical accuracy. There are a couple of approaches from which to select. The casual audiophile, who has more interest in music than the ultimate in fi, will usually choose a record label whose releases he favors for their musical values, and will tailor his system to sound best with most of that label's recordings. Discs from other labels may sound good on this system too, but it will be a matter of luck, and bear little relationship to accuracy.
Perfectionist audiophiles, on the other hand, usually aim for maximum accuracy in the playback system itself. The idea here is that, if the system accurately reproduces what is on the recording, the best recordings will yield the most natural sound. (This philosophy has the added benefit of rewarding those record manufacturers who strive hardest for realism.)
This seems like an elegantly simple solution, but there's a flaw. In order to ascertain the accuracy of a disc's reproduction, we must have an original to compare it to. But we can't compare it to the sound that was fed into the master-tape recorder, because that sound was gone forever when the recording session ended. The closest we can get to that original signal is the one that comes from the recorder when the tape is played back. That, after all, is the signal that was used to cut the disc, and if the disc sounds the same as the tape, then we know our record-playing system (the arm, cartridge, and preamp) is accurate. Right? Not necessarily—the record cutting and pressing system was optimized based on a comparison to the original sound, but with probably a completely different phono system than the one you use at home.
Before approving a new release, a record producer is sent a test pressing of it (footnote 1), which he then plays through his reference system and compares with what he hears directly from the master tape. If they don't sound alike, he tells the cutting engineer to make appropriate equalization corrections for the final release cut, or to simply re-cut the disc with the same equalization.
Wouldn't this ensure that his disc sounds like the original tape? Not quite, because it is more than likely that his phono system and preamp have significant colorations, which will make the disc sound different from the way it "actually" sounds. Why, then, should our perfectionist record producer trust his playback system? Because he carefully chose it to make his records sound as much as possible like his tapes!
We've all heard of Catch 22, but in case you're unsure of its meaning, it is about circularity—in reasoning, causality, and Ultimate Truth. Circularity exists when A is a function of B, while B is determined by A. A popular example of circularity is the chicken-or-the-egg question. Then there's the apocryphal "Timbuktu Paradox," which relates the story of the retired sea captain who fires a cannon every day at the precise moment the town hall clock says 12 noon, while the town-hall custodian checks his clock every day by the sound of the 12 o'clock cannon.
What in fact does a record sound like? Think for a moment before answering. It has no sound at all. Hold one up to your ear, and what do you hear? Nothing, of course. To hear what's "on" a recording, you have to reproduce it through a phono system. And what does that phono system really sound like? It sounds like the record with various things added or subtracted. And the music goes 'round and 'round..
There really isn't any way of knowing precisely what is the sound of a record or its player. This is one reason why, in this age of high technology, audio continues to be such a cabalistic field. Where knowledge fails, mysticism moves in.
But just because we cannot make absolute assessments of disc-reproduction accuracy doesn't mean we should abandon the accuracy criterion altogether, any more than we should all stop trying to be good people just because we can't be perfect. There is, in fact, a way we can get reasonably close to the ultimate truth about an analog disc and its player, and that way believe it or not is through the Compact Disc.
The CD has all along been touted as an absolutely accurate recording/playback medium, no doubt to the embarrassment of those manufacturers who so promoted it. Even the mass circulation hi-fi magazines have been reporting that some players sound better than others, and that the best are getting better as time goes on. But another question that has assumed growing importance is just how good the Compact Disc system actually is, because the answer to that question will determine how far the CD can go towards needs of the audiophile who cares about accuracy (footnote 2).
Numbers of audiophile-oriented record manufacturers have been claiming that the CD sound is "virtually indistinguishable from" the sound of the original master tapes. Even allowing for a certain amount of hyperbole (footnote 3), this would seem to indicate that a CD may offer us the most direct path back to the sound of the original master recording. But how much does a CD sound like its master?
To my knowledge, the only investigation of this was done a couple of years ago by England's Hi-Fi News & Record Review. Those listening tests involved direct comparisons between the sound of some Decca CDs and their digital master tapes. The test results were not felt to be entirely conclusive. While there was agreement that the CDs sounded pretty much like the original masters, there was some disagreement as to how important were the minor differences noted.
HFN/RR's experiment is already outdated anyway. Since that time the audio quality of the best CD players has improved dramatically, while many of professional recorders have stayed the same. And the conditions of HFN/RR's tests were not quite the same as an analog disc/tape comparison, because a set of spurious electronics were introduced into the "original" signal: the digital recorder's playback circuitry.
When mastering from analog, the original signal—that is, the signal feeding the cutting system—is already in analog form and can be auditioned directly. But in CD mastering, the original is in digital form, and stays that way up until the time the disc is played in your home. In order to compare the original (digital) with the playback (analog), D/A conversion must occur at the output of the recording deck. And there's the catch. That D/A converter and audio section was not present in the chain that delivered the original signal to the CD. In other words, when we make a CD/master-tape comparison, the "original" sound is being processed by electronics which are different from those used for the CD playback, and the former may not be as good as the latter.
Professional recording equipment is notorious for having less than perfectionist-quality audio circuitry and parts. That's why every recording studio that aims for the best sound customizes its tape decks. Some consumer CD players, such as the Meridian and Mission units, probably produce a better sound from CD digital than do the decks used to master those CDs. So it is more than likely that, if HFN/RR were to repeat those tests today, the CD sound would emerge as the clearcut winner, and would actually be judged better than the "original tape."
Under the circumstances, though, it is likely that such comparisons between the master and the consumer product are more reliable for digital recordings than for analog ones, because there are no mechanical transducers involved. Bad electronics can do some nasty things to digital sound, but they tend to have relatively little effect on the spectral balance of the sound—the balance between bass and treble, and the absolute high-end content. Thus, while we may still quibble over other aspects of CD sound, there is little doubt but that what we hear from a CD is much closer in spectral balance to the master tape than what we hear from an analog reproduction of the same recording.
This is why I adopted CD as my "standard" for judging most aspects of the sound of analog signal sources. Where CDs contrast consistently with what I hear from analog, I assume (on faith, you might say) that the CD sound is closer to the truth in spectral balance and low-frequency quality. If that CD sound is not "good," I do not assume that the better analog sound is "right." Instead, I adjust the other components in my system—the loudspeakers, in particular—until the sound I hear from CD in the listening room is as close as possible to what I remember of live music. This then becomes my standard for evaluating analog sources. The sound I get from analog is of a very high standard, and it has very similar spectral balance to digital sources.
The CD is still not what I consider to be anything like an "absolute" standard, but I do believe it is the closest approach to such an absolute that we're likely to find. It's certainly better than wondering whether the lovely sounds I get from some analog discs are the result of almost-perfect everythings in the chain, or merely a fortuitous mating between gross system colorations all the way from the microphones to the loudspeakers.
http://www.stereophile.com/asweseeit/363/
The typical buyer of a "steeryo" is seeking nothing more than pleasant or exciting sounds, and is easily satisfied because he has no greater expectation of audio than this. The audiophile, however, is aware that reproduced sound can resemble (more or less) real, live sound, and he is driven in a continual search for that ultimate truth ("fidelity to the original") even while realizing, intellectually at least, that it is unattainable.
Because he understands what the word "reproduction" means, the audiophile thinks in terms of a relationship to an original sound. This original is, of course, the sound of live music, and the touchstone for its reproduction is accuracy. Unfortunately, though, we don't really compare the reproduction with the real thing—because we can't. Only a recording engineer can saunter back and forth between the real thing (which takes place in a studio or hall) and the reproduction of it (in the control room with its monitor system). We audiophiles must be content to compare the reproduction with what we remember to be the sound of live music. Even the amateur recordist must carry the memory of that original sound home with his tapes in order to evaluate them.
And that memory may not serve us that well. Few of us have learned to listen with enough attention and skill to be able to break live sound down into its components and to observe what each sounds like. Most of us remember only an overall impression—the gestalt of the thing. And many of us must admit, to ourselves at least, that we have not heard live music for years or, worse, never at all. For the vast majority of audiophiles then, the reference standard is not the absolute sound of live music, but an imagined ideal—a mental picture of how we remember its having sounded or how we would like it to sound. At this point, accuracy becomes a dubious criterion because of the vagueness of the original to which we compare the copy. System evaluation becomes a (simple?) matter of "it's good if it sounds good."
The problem with this is that one man's good is another man's distortion. Different people listen to and assign different orders of importance to different aspects of reproduced sound. Thus, while two very picky listeners may agree that a system has good bass, good highs, and a colored middle range, they will disagree as to how good the system is if one happens to be critical of highs and lows while the other is critical of the middle range.
In short, we really don't have any way of reliably assessing the accuracy of reproduced sound. Even a recording engineer cannot be confident of the sound of his own recording, because what he hears in the control room depends on his monitoring equipment, which is no more—and is often less—accurate than a home system. (Many pros do not, in fact, aim for realism at all, but for what they call a "commercial sound"—one that will sell. Thus a recording may not even have the potential for sounding realistic.)
All this does not, however, discourage audiophiles in their search for the Holy Grail of musical accuracy. There are a couple of approaches from which to select. The casual audiophile, who has more interest in music than the ultimate in fi, will usually choose a record label whose releases he favors for their musical values, and will tailor his system to sound best with most of that label's recordings. Discs from other labels may sound good on this system too, but it will be a matter of luck, and bear little relationship to accuracy.
Perfectionist audiophiles, on the other hand, usually aim for maximum accuracy in the playback system itself. The idea here is that, if the system accurately reproduces what is on the recording, the best recordings will yield the most natural sound. (This philosophy has the added benefit of rewarding those record manufacturers who strive hardest for realism.)
This seems like an elegantly simple solution, but there's a flaw. In order to ascertain the accuracy of a disc's reproduction, we must have an original to compare it to. But we can't compare it to the sound that was fed into the master-tape recorder, because that sound was gone forever when the recording session ended. The closest we can get to that original signal is the one that comes from the recorder when the tape is played back. That, after all, is the signal that was used to cut the disc, and if the disc sounds the same as the tape, then we know our record-playing system (the arm, cartridge, and preamp) is accurate. Right? Not necessarily—the record cutting and pressing system was optimized based on a comparison to the original sound, but with probably a completely different phono system than the one you use at home.
Before approving a new release, a record producer is sent a test pressing of it (footnote 1), which he then plays through his reference system and compares with what he hears directly from the master tape. If they don't sound alike, he tells the cutting engineer to make appropriate equalization corrections for the final release cut, or to simply re-cut the disc with the same equalization.
Wouldn't this ensure that his disc sounds like the original tape? Not quite, because it is more than likely that his phono system and preamp have significant colorations, which will make the disc sound different from the way it "actually" sounds. Why, then, should our perfectionist record producer trust his playback system? Because he carefully chose it to make his records sound as much as possible like his tapes!
We've all heard of Catch 22, but in case you're unsure of its meaning, it is about circularity—in reasoning, causality, and Ultimate Truth. Circularity exists when A is a function of B, while B is determined by A. A popular example of circularity is the chicken-or-the-egg question. Then there's the apocryphal "Timbuktu Paradox," which relates the story of the retired sea captain who fires a cannon every day at the precise moment the town hall clock says 12 noon, while the town-hall custodian checks his clock every day by the sound of the 12 o'clock cannon.
What in fact does a record sound like? Think for a moment before answering. It has no sound at all. Hold one up to your ear, and what do you hear? Nothing, of course. To hear what's "on" a recording, you have to reproduce it through a phono system. And what does that phono system really sound like? It sounds like the record with various things added or subtracted. And the music goes 'round and 'round..
There really isn't any way of knowing precisely what is the sound of a record or its player. This is one reason why, in this age of high technology, audio continues to be such a cabalistic field. Where knowledge fails, mysticism moves in.
But just because we cannot make absolute assessments of disc-reproduction accuracy doesn't mean we should abandon the accuracy criterion altogether, any more than we should all stop trying to be good people just because we can't be perfect. There is, in fact, a way we can get reasonably close to the ultimate truth about an analog disc and its player, and that way believe it or not is through the Compact Disc.
The CD has all along been touted as an absolutely accurate recording/playback medium, no doubt to the embarrassment of those manufacturers who so promoted it. Even the mass circulation hi-fi magazines have been reporting that some players sound better than others, and that the best are getting better as time goes on. But another question that has assumed growing importance is just how good the Compact Disc system actually is, because the answer to that question will determine how far the CD can go towards needs of the audiophile who cares about accuracy (footnote 2).
Numbers of audiophile-oriented record manufacturers have been claiming that the CD sound is "virtually indistinguishable from" the sound of the original master tapes. Even allowing for a certain amount of hyperbole (footnote 3), this would seem to indicate that a CD may offer us the most direct path back to the sound of the original master recording. But how much does a CD sound like its master?
To my knowledge, the only investigation of this was done a couple of years ago by England's Hi-Fi News & Record Review. Those listening tests involved direct comparisons between the sound of some Decca CDs and their digital master tapes. The test results were not felt to be entirely conclusive. While there was agreement that the CDs sounded pretty much like the original masters, there was some disagreement as to how important were the minor differences noted.
HFN/RR's experiment is already outdated anyway. Since that time the audio quality of the best CD players has improved dramatically, while many of professional recorders have stayed the same. And the conditions of HFN/RR's tests were not quite the same as an analog disc/tape comparison, because a set of spurious electronics were introduced into the "original" signal: the digital recorder's playback circuitry.
When mastering from analog, the original signal—that is, the signal feeding the cutting system—is already in analog form and can be auditioned directly. But in CD mastering, the original is in digital form, and stays that way up until the time the disc is played in your home. In order to compare the original (digital) with the playback (analog), D/A conversion must occur at the output of the recording deck. And there's the catch. That D/A converter and audio section was not present in the chain that delivered the original signal to the CD. In other words, when we make a CD/master-tape comparison, the "original" sound is being processed by electronics which are different from those used for the CD playback, and the former may not be as good as the latter.
Professional recording equipment is notorious for having less than perfectionist-quality audio circuitry and parts. That's why every recording studio that aims for the best sound customizes its tape decks. Some consumer CD players, such as the Meridian and Mission units, probably produce a better sound from CD digital than do the decks used to master those CDs. So it is more than likely that, if HFN/RR were to repeat those tests today, the CD sound would emerge as the clearcut winner, and would actually be judged better than the "original tape."
Under the circumstances, though, it is likely that such comparisons between the master and the consumer product are more reliable for digital recordings than for analog ones, because there are no mechanical transducers involved. Bad electronics can do some nasty things to digital sound, but they tend to have relatively little effect on the spectral balance of the sound—the balance between bass and treble, and the absolute high-end content. Thus, while we may still quibble over other aspects of CD sound, there is little doubt but that what we hear from a CD is much closer in spectral balance to the master tape than what we hear from an analog reproduction of the same recording.
This is why I adopted CD as my "standard" for judging most aspects of the sound of analog signal sources. Where CDs contrast consistently with what I hear from analog, I assume (on faith, you might say) that the CD sound is closer to the truth in spectral balance and low-frequency quality. If that CD sound is not "good," I do not assume that the better analog sound is "right." Instead, I adjust the other components in my system—the loudspeakers, in particular—until the sound I hear from CD in the listening room is as close as possible to what I remember of live music. This then becomes my standard for evaluating analog sources. The sound I get from analog is of a very high standard, and it has very similar spectral balance to digital sources.
The CD is still not what I consider to be anything like an "absolute" standard, but I do believe it is the closest approach to such an absolute that we're likely to find. It's certainly better than wondering whether the lovely sounds I get from some analog discs are the result of almost-perfect everythings in the chain, or merely a fortuitous mating between gross system colorations all the way from the microphones to the loudspeakers.
http://www.stereophile.com/asweseeit/363/
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