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Claims both published and anecdotal are regularly made for audibly superior sound quality for two-channel audio encoded with longer word lengths and/or at higher sampling rates than the 16-bit/44.1-kHz CD standard. The authors report on a series of double-blind tests comparing the analog output of high-resolution players playing high-resolution recordings with the same signal passed through a 16-bit/44.1-kHz "bottleneck." The tests were conducted for over a year using different systems and a variety of subjects. The systems included expensive professional monitors and one high-end system with electrostatic loudspeakers and expensive components and cables. The subjects included professional recording engineers, students in a university recording program, and dedicated audiophiles. The test results show that the CD-quality A/D/A loop was undetectable at normal-to-loud listening levels, by any of the subjects, on any of the playback systems. The noise of the CD-quality loop was audible only at very elevated levels.

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The test results for the detectability of the 16/44.1 loopon 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 signaland high system gain, when the difference in noise floorsof the two technologies, old and new, was readily audible.As the tests progressed, we repeatedly sorted the datafor correlations with age, sex, upper frequency hearinglimit, or experience. No such correlations have emerged.Specifically, on music at normal levels as defined here,audiophiles and/or working recording-studio engineers got246 correct answers in 467 trials, for 52.7% correct. Fe-males got 18 in 48, for 37.5% correct. Those subjects ableto hear tones above 15 kHz got 116 in 256 trials, for 45.3%correct; listeners aged 14–25 years old (who were, as itturned out, the same group), also got 116 correct in 256trials, 45.3%. The “best” listener score, achieved onesingle time, was 8 for 10, still short of the desired 95%confidence level. There were two 7/10 results. All othertrial totals were worse than 70% correct.Furthermore, none of the more elaborate and expensiveplayback systems (for which the subjects were all dedi-cated amateur audiophiles, active students in a profes-sional recording program, and/or experienced workingprofessionals) revealed detectable differences on music,again at levels as defined previously.In one brief test with two subjects we added 14 dB ofgain to the reference level quoted and tested the twosources with no input signal, to see whether the noise levelof the CD audio channel would prove audible. Althoughone of the subjects was uncertain of his ability to hear thenoise, both achieved results of 10/10 in detecting the CDloop. (We have not yet determined the threshold of thiseffect. 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.)Fig. 2. Critical listening position for majority of tests.ENGINEERING REPORTSAUDIBILITY OF CD-STANDARD A/D/A LOOPJ. Audio Eng. Soc., Vol. 55, No. 9, 2007 September777
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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 record-ings masked the 16/44.1 noise floor, though we did findone or two productions in which there was a detectabledifference in room tone at gain settings of +20 dB or moreabove the reference level. At these very high gains wecould also hear subtle low-level decoding errors in all butthe most expensive of the high-resolution players.From the many different recordings we used it emergedthat almost no music or voice program, recording venue,instrument, or performer exceeds the capabilities of a well-implemented CD-quality record/playback loop. The CDhas adequate bandwidth and dynamic range for any homereproduction task, and it is a rare playback venue that isquiet enough to reveal the 16-bit noise floor of our A/D/Aloop—which has no noise shaping and was thereforeless than optimal in this regard—even at gains above our reference.

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"Virtually all of the SACD and DVD-A recordings sounded better than most CDs — sometimes much better... 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... 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."

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