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Audibility of a CD-Standard A/DA/A Loop Inserted into High-Resolution Audio Playback

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[Engineering Report] 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.

JAES Volume 55 Issue 9 pp. 775-779; September 2007
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Jon Boley

Comment posted August 27, 2008 @ 11:23:47 UTC (Comment permalink)

My compliments to the authors for performing this study. The result that immediately struck me as odd was the fact that female subjects scored 37.5% correct... well below chance. I wonder, was the variability greater for this subject pool?
Also, were the results analyzed only in terms of correct/incorrect? Or were the results also analyzed for false positives and false negatives?
For example, it would be interesting if the female subjects, or anyone else, tended to be biased toward answering either A or B, thus skewing the results. (This would be a great opportunity to apply Signal Detection Theory.)

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Douglas Rife

Comment posted August 31, 2008 @ 00:40:34 UTC (Comment permalink)

I read this paper with great interest when first published almost 1 year ago. Considering recent efforts promoting the supposed superiority of high resolution digital audio formats (24 bit, 96 kHz PCM as well as DSD) the authors' conclusion of no detectable audio quality difference between these formats compared to ordinary CD resolution (16 bit, 44.1 kHz PCM) would indeed be expected to create controversy. While I'm sympathetic to the authors' findings, it would have been a much stronger paper if the authors had revealed exactly what "well regarded CD recorder with real-time monitoring" was used to reduce the resolution of the audio signal to the CD standard. The authors claim their mystery CD recorder performed an A/D conversion using only 16-bit resolution and a 44.1 kHz sampling rate, followed by an immediate conversion back to the analog domain using a 16-bit, 44.1 kHz DAC (see Figure 1). But readers have no way to verify this claim without knowing the exact equipment the authors used in their tests and that they verified by correspondence with the manufacturer of this mystery CD recorder that the A/D/A conversion block shown in Figure 1 of their paper was actually being implemented. In other words, the authors need to leave no doubt that this block is really an A/D/A function at 16-bit resolution and not, for example, a purely analog monitor function or, one made at a higher bit resolution or sample rate.

Drew Daniels
Drew Daniels

Comment posted September 14, 2008 @ 00:16:48 UTC (Comment permalink)

[Recently an article on audio quality by Meyer and Moran (J. Audio Eng. Soc. vol. 55, pp. 775-779, [2007 Sep.]) has given rise to a number of letters sent to the Journal. There is a lot of intense debate and opinion on this topic that probably would be interesting to many AES members, and this might actually help to bring out the key aspects of the issue. Accordingly, we feel that an AES online forum that allows members to give their opinions and experiences will be valuable.]—JAES e-mail to members.

That debate about this issue is intense there is no doubt. Can such debate be borne entirely of electroacoustical differences which might actually be impossible to explain, demonstrate and reproduce in a statistically significant way, or is it perhaps possible that financial interests could be at risk if some ultimate determination of audibility suggested that a large number of digital audio products were overbuilt, overpriced, or even entirely unnecessary?

This author deplores fear-mongering and considers its use as a marketing tool on people with insecurities about their technical knowledge, a crime of fraud. The audio marketplace, its many commercially oriented magazines, and the manufacturers who advertise in these magazines, now rarely if ever, supply graphical engineering data or even detailed and accurate print information that might be used to make purchasing decisions. The vast majority of equipment manuals are a list of sales points and usually not helpful. More and more, manufacturers rely on convincing potential customers of the superiority of their products by subjective means and anecdotal comments from celebrities. This is marketeering—not engineering--and it harms consumers everywhere. Those who are forced to purchase manufactured products to remain competitive in their services to clients, products which can not be tested in personal labs before purchases are made, should speak up about what has become an excessively greedy and cynical marketplace where increasingly, we are victims of marketeers rather than beneficiaries of good engineering.

Drew Daniels

Mark Jay
Mark Jay

Comment posted December 8, 2014 @ 18:09:58 UTC (Comment permalink)

I wholly applaud and agree with the reply posted by Drew Daniels. It is both eloquent and succinct; I also believe it to be in the proper spirit of the debate and solidly framed by reality. I would be remiss were I not to thank Messrs Meyer and Moran for having published this paper in the first place.

While I think there may be debate as to the 'precision' of the various approaches taken, the only thing of import is whether or not one method results in noticeably improved sound, and if said method can consistently be identified. If it cannot, then the rest is moot.
Again, Thank You Mr. (Dr.?) Daniels for your post.
Mark A. Jay

Hans Beekhuyzen
Hans Beekhuyzen

Comment posted March 31, 2015 @ 17:17:31 UTC (Comment permalink)

I also applaude the initiative, although I also agree with Douglas Rife's arguments. I personally don't believe that the extended frequency respons offered by higher sample rates is of any consequence. Many people that claim to hear differences (including myself), are at an age where even 15 kHz might be of little interest. And let's not forget that 10kHz to 20 kHz is only one of the ten octaves of the DIN 45.500 hifi standard (20 to 20,000Hz). Let's look beyond the obvious. Some listeners might be wrong in claiming they can hear the difference between 44.1/16 and 192/24, but can they all be wrong?

I have experienced a comparable judgement when in the eighties I claimed to hear differences between two S/PDIF cables between a CD-player and DAC. I was put down like now is done with those who claim to hear higher fs. It was even 'proven' that I was wrong: a file was copied via two different S/PDIF cables and then compared at bit level on a Sonic Solutions system. Both files were identical, so I was wrong. We now know that incorrect digital cables cause jitter that can be corrected by buffering (storing in the case of the Sonic system) but can't be corrected when feeding a DAC.

Regarding the high sample rates: research done by Bob Stuart and Peter Craven might go in the right direction. They found literature that indicate our auditory system has a time resolution  of 7 µs, which would need a sample rate of 141 kHz or better. They also work on a codec that reduces the time smearing (caused by anti-aliasing and reconstruction filters) in the total record-reproduce chain. Both seem credible to me and would make the research on whether we directly or indirectly) hear content above 20 kHz less relevant.

Regardless the above I thank the researchers and members of the BAC for persuiing the thruth by putting large amounts of time in this project.   

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Stefan Heinzmann

Comment posted April 2, 2015 @ 21:28:30 UTC (Comment permalink)

I don't see a good reason why the listeners claiming a difference between 44.1/16 and 192/24 couldn't all be wrong. This is not a question that is decided by majority, or by size of minority. Meyer and Moran themselves offer an explanation in their chapter 4. Essentially, people misattribute the difference they hear. They believe it originates in the sampling rate or wordlength, when it reality it has a completely different root cause. I find this argument plausible. Most people who claim that they hear differences between the formats do not appear to have ensured that alternative explanations can be ruled out.

Your anecdote with the SPDIF cable could serve to demonstrate that point: Accepting the explanation that appears to be the most obvious one is not always right, and if you want to know the truth of the matter, you have to conduct experiments which allow to confirm or reject competing explanations.

Unfortunately, your conclusion contains another flaw: You state that jitter caused by the cable connection can't be corrected when feeding a DAC. In fact it can, and many DACs you can buy do that quite successfully. It is a straightforward application of PLL technology that has been in use for decades in the audio industry as well as other industries.

Yet another flaw seems to be a recurring theme when discussing higher sampling rates: The link that is being made between the "time resolution of the auditory system" and the sampling rate is wrong. This has been refuted multiple times and keeps coming back. The alleged time resolution is not present in such a generalized way. It is a binaural property that applies to phase differences between the two ears, and has nothing to do with the time resolution of a single signal. The sampling rate or signal bandwidth does not limit the resolution of phase differences between two signals, hence it is invalid to link the two.

One more word regarding Mr. Rife's contribution, since you agree with him: I find it incomprehensible how one can cast doubt on the A/D - D/A conversion in the way Mr. Rife has. It is true that the paper by Meyer and Moran does not explicitly state what model was used. It is clearly visible on the photograph shown in their article, however, and additional explanations can be found on the BAS website. Furthermore, it is clear from the text that the conversions must have taken place, because at elevated gain the noise floor of the 16-bit quantisation was audible. Quite how one can still harbor doubts in such a situation is beyond me. There is value in questioning an experimental setup in order to uncover flaws that may have escaped the authors, but there is also a point beyond which such questioning becomes unconstructive and perhaps even malicious.

Kind regards

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Sergei Lopatin

Comment posted April 24, 2015 @ 20:33:31 UTC (Comment permalink)

Do you know of efforts to replicate this result while using contemporary high-resolution sources, and also moninors with upper bandwidth limit around 40 KHz (e.g. Adam, Focal) ? To the best of my knowledge, such monitors only started to appear around 2010.

In my personal experience, I only started noticing significant advantages of SACD when I switched to Adam. To me, it mostly manifests itself as a strong "presense effect", especially on classical recordings. I've never experienced it to such degree with any CD recordings.

It is not about the frequency range, naturally - can't hear much above 13 KHz - but about the perception of simultaneous presense of non-interfering sound sources of wildly differing intensities in different spacial locations, including more realistic delayed reflections. I can see - mathematically - how longer words and higher rates may contribute to that.

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