For the CD loop we used a well-regarded professional CD recorder with real-time monitoring. Levels in both channels were matched to within 0.1 dB using a very high-performance adjustable analog gain stage, which was always in the 16/44.1 signal path. Audio switching was handled by an ABX CS-5 double-blind comparator (see Fig. 3).
High-resolution audio offers a lower digital noise floor, so playback levels are a significant factor. Does the lower noise have any practical consequence, given modern compression techniques and existing noise floors in microphones, preamplifiers, and mixing consoles? We found that most of the SACD and DVD-A recordings produced what might be termed realistic playback (that is, the subjects heard the sources loudly and clearly, with natural timbres and appropriate scale but without discomfort) at a system gain such that a 1-kHz octave band of noise recorded at an average level of −16 dBFS produced an SPL at the listening position of 85 dB, unweighted. For some classical recordings of very wide dynamic range, listening levels 5?7 dB higher than this were used from time to time. The test signal we used to produce 85 dB SPL at our standard gain is available on the Boston Audio Society Web site. The downward frequency sweep, used at the same playback level as a quick test of high-frequency hearing limits in our subjects, can be found on the same page, at www.bostonaudiosociety.org/media
2 THE RESULTS
The test results for the detectability of the 16/44.1 loop on SACD/DVD-A playback were the same as chance: 49.82%. There were 554 trials and 276 correct answers. The sole exceptions were for the condition of no signal and high system gain, when the difference in noise floors of the two technologies, old and new, was readily audible. As the tests progressed, we repeatedly sorted the data for correlations with age, sex, upper frequency hearing limit, or experience. No such correlations have emerged. Specifically, on music at normal levels as defined here, audiophiles and/or working recording-studio engineers got 246 correct answers in 467 trials, for 52.7% correct. Females got 18 in 48, for 37.5% correct. Those subjects able to hear tones above 15 kHz got 116 in 256 trials, for 45.3% correct; listeners aged 14?25 years old (who were, as it turned out, the same group), also got 116 correct in 256 trials, 45.3%. The ?best? listener score, achieved one single time, was 8 for 10, still short of the desired 95% confidence level. There were two 7/10 results. All other trial totals were worse than 70% correct. Furthermore, none of the more elaborate and expensive playback systems (for which the subjects were all dedicated amateur audiophiles, active students in a professional recording program, and/or experienced working professionals) revealed detectable differences on music, again at levels as defined previously.
In one brief test with two subjects we added 14 dB of gain to the reference level quoted and tested the two sources with no input signal, to see whether the noise level of the CD audio channel would prove audible. Although one of the subjects was uncertain of his ability to hear the noise, both achieved results of 10/10 in detecting the CD loop. (We have not yet determined the threshold of this effect. With gain of more than 14 dB above reference, detection of the CD chain?s higher noise floor was easy, with no uncertainty. Tests with other subjects bore this out.)
The high-resolution sources when played back at the +14-dB level were unpleasantly (often unbearably) loud, and modern, aggressively mastered CDs even more so. Room tone and/or preamplifier noise in almost all recordings masked the 16/44.1 noise floor, though we did find one or two productions in which there was a detectable difference in room tone at gain settings of +20 dB or more above the reference level. At these very high gains we could also hear subtle low-level decoding errors in all but the most expensive of the high-resolution players. From the many different recordings we used it emerged that almost no music or voice program, recording venue, instrument, or performer exceeds the capabilities of a well implemented CD-quality record/playback loop. The CD has adequate bandwidth and dynamic range for any home reproduction task, and it is a rare playback venue that is quiet enough to reveal the 16-bit noise floor of our A/D/A loop?which has no noise shaping and was therefore less than optimal in this regard?even at gains above our reference.
We have analyzed all of the test data by type of music and specific program; type of high-resolution technology; age of recording; and listener age, gender, experience, and hearing bandwidth. None of these variables have shown any correlation with the results, or any difference between the answers and coin-flip results. The previous work cited, some of it at the very beginning of the CD era and some more recent, pointed toward our result. With the momentum of widespread ?high-rez? anecdotes over the last decade, culminating in the Stuart assertions, we felt the need to go further and perform a thorough, straightforward double-blind level-matched listening test to determine whether 16/44.1 technology would audibly degrade the sound of the best high-resolution discs we could find. We used a large and varied sample of serious listeners; we conducted our tests using several different types of high-quality playback systems and rooms; and we took as much time as we felt necessary to establish the transparency of the CD standard.
Now, it is very difficult to use negative results to prove the inaudibility of any given phenomenon or process. There is always the remote possibility that a different system or more finely attuned pair of ears would reveal a difference. But we have gathered enough data, using sufficiently varied and capable systems and listeners, to state that the burden of proof has now shifted. Further claims that careful 16/44.1 encoding audibly degrades high resolution signals must be supported by properly controlled double-blind tests.
4 A NOTE ON HIGH-RESOLUTION RECORDINGS
Though our tests failed to substantiate the claimed advantages of high-resolution encoding for two-channel audio, one trend became obvious very quickly and held up throughout our testing: virtually all of the SACD and DVD-A recordings sounded better than most CDs? sometimes much better. Had we not ?degraded? the sound to CD quality and blind-tested for audible differences, we would have been tempted to ascribe this sonic superiority to the recording processes used to make them.
Plausible reasons for the remarkable sound quality of these recordings emerged in discussions with some of the engineers currently working on such projects. This portion of the business is a niche market in which the end users are preselected, both for their aural acuity and for their willingness to buy expensive equipment, set it up correctly, and listen carefully in a low-noise environment. Partly because these recordings have not captured a large portion of the consumer market for music, engineers and producers are being given the freedom to produce recordings that sound as good as they can make them, without having to compress or equalize the signal to suit lesser systems and casual listening conditions. These recordings seem to have been made with great care and manifest affection, by engineers trying to please themselves and their peers. They sound like it, label after label. High-resolution audio discs do not have the overwhelming majority of the program material crammed into the top 20 (or even 10) dB of the available dynamic range, as so many CDs today do.
Our test results indicate that all of these recordings could be released on conventional CDs with no audible difference. They would not, however, find such a reliable conduit to the homes of those with the systems and listening habits to appreciate them. The secret, for two-channel recordings at least, seems to lie not in the high-bit recording but in the high-bit market.