Audible Jitter/amirm vs Ethan Winer

amirm

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Well, here's my last word, FWIW. I don't think any music lover should be subjected to dbx. If he wants to employ some informal version, as I do when it is practical, good. But this is about enjoyment of art, and how each of us gets there is pretty personal. If you like vinyl and electrostats, and I like hard discs and active speakers, to each our own bliss, no need for verification. Who would be subject to dbx, in a more perfect world, would be all of those, in positions of self-proclaimed authority, making performance claims that are not otherwise supported outside of their self-proclaimed authority.
Well said :).

Digital audio designers further reducing jitter below levels that are broadly considered to be inaudible and claiming more "inner detail" or a "deeper sound stage.
I have read every paper I have found from AES and elsewhere and yet to see anything that comes close to telling me what jitter is audible . The methodology used in the papers which do exists, are extremely easy to invalidate (the problem set is infinite so even if they wanted to prove this point, they likely couldn't). I could be mean and put you on spot and say "prove this declaration" but I won't :D.

In a better world, these guys, and especially the audiophile press that enables them, would all be subject to rigorous, independent, statistically sound double-blind testing of their claims, their ears and their honesty. But the world is not perfect and I'm not holding my breath.

P
That is not the only way to analyze such claims. We can first look to engineering principals and see if they can explain the design. Only when that fails us, do we need to resort to difficult, expensive and time consuming process of blind testing.

For example, I know MP3 encoders at 128kbps tend to roll off the high frequencies. We don't need double blind test to realize this. Likewise, mathematics shows us the level of jitter we need to get below to reproduce 16 bits of audio samples. Anything less means a degradation. I expect quality hardware to not degrade the audio samples below CD's resolution -- dare I say whether I can hear it or not. It is like buying a car that advertises it can do 150 miles an hour but in reality tests show that it can only do 100. Should I not care because I can't find a road here to test it?
 

Ethan Winer

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I have read every paper I have found from AES and elsewhere and yet to see anything that comes close to telling me what jitter is audible.

Not the AES, but these guys did a bunch of tests and you can download the PDF for free:

Detection Threshold For Jitter

From their conclusion:

Kaoru Ashihara et al said:
The results indicate that the threshold for random jitter on program materials is several hundreds ns for well-trained listeners under their preferable listening conditions. The threshold values seem to be sufficiently larger than the jitter actually observed in various consumer products.

The key is the last part, that even inexpensive consumer gear has jitter low enough to not matter. This makes sense, since jitter can also be expressed as artifacts some number of dB below the music. The graph below from Ken Pohlmann's book Principles of Digital Audio shows a typical level. If artifacts are 100 dB or more below the music, it's a stretch IMO to think that could ever be audible while the music plays.

What I don't understand is why people seem to conclude that whatever shortcomings they may hear in their gear, it's due to jitter. Why jitter? Doesn't it make sense that room acoustic problems - which are at least four or five orders of magnitude larger! - is a far more likely culprit for lack of detail, smearing, and imaging etc?

--Ethan

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Ethan Winer

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What I am not a fan of is subjecting everyday audiophiles to trick tests (sorry Ethan :))

LOL, most of the trick tests I've done were not meant to be that! The last one I did, I screwed up an export of a mix, and two of the three files were bit-identical. Yet, people reported obvious night and day differences between those two files. So I turned lemons into lemonade and made the best of my mistake by pointing that out.

--Ethan
 

amirm

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Not the AES, but these guys did a bunch of tests and you can download the PDF for free:
I have read that paper as it is routinely posted on forums as proof that jitter is inaudible. As much as I admire anyone trying to characterize distortions like this, their approach is exceedingly faulty and the reason I made the comment that I did.

From their conclusion: The results indicate that the threshold for random jitter on program materials is several hundreds ns for well-trained listeners under their preferable listening conditions. The threshold values seem to be sufficiently larger than the jitter actually observed in various consumer products.
I am not going rehash all that they have done wrong as I have written about it many times. But just a few quick points for now:

1. "Random" jitter or for that matter, any other other random distortion turns out to be far more benign than program correlated jitter. If your projector has a fan, I bet you don't hear it most of the time. But imagine if the fan came on every time the screen became bright. I bet you will hear it much more often then. The brain tends to filter non-variant things as a way for you to focus on what is important.

Jitter induced in Audio equipment usually is not random. It could be induced by the power supply ripple voltage, it could be the oscillator for the FL display, it could be the video circuit bleeding into it, etc. Or it could track the source signal in certain ways. All of this generates much more objectionable distortion.

2. They let people play the content through their PCs. Did they profile those PCs first to see what level of jitter they had in them to start? No.

3. You have to pick material that is revealing of the distortion you are trying to find. I can take classical music, encode it at 64kbps and have 99% of the people say it is the same as music. I can then encode some guitar music and have everyone be able to tell the difference no mater how deaf. It almost seems like people assume there is no difference so they don't pay attention to things like this. As I have said before, lack of difference found is more of a fault of the technique than people's perception if the science says there is a difference (and the science definitely does as confirmed in the very article).

On this front, jitter destroys low order bits and can be especially audible where the ear is most sensitive. Knowing this and the fact that the higher the frequency, the more the problem, would lead you to special set of content you would want to use, rather than whatever happens to be on a CD on your desk.

4. As I have noted before, there will never be any proof that jitter is inaudible for the simple reason that jitter has infinite characteristics. It can be caused by one or more modulating signals each of which can have arbitrary frequency, magnitude and waveforms. So no study, no matter how comprehensive, can ever provide sufficient data that jitter cannot be heard in other circumstances.

The key is the last part, that even inexpensive consumer gear has jitter low enough to not matter.
A conclusion which is not supported in any way by their research since they did not show that consumer gear has random jitter. Or even what jitter they have regardless of type.

This makes sense, since jitter can also be expressed as artifacts some number of dB below the music. The graph below from Ken Pohlmann's book Principles of Digital Audio shows a typical level. If artifacts are 100 dB or more below the music, it's a stretch IMO to think that could ever be audible while the music plays.
Is the "music" always at 0db? It has to be for your statement to be true.

What I don't understand is why people seem to conclude that whatever shortcomings they may hear in their gear, it's due to jitter. Why jitter? Doesn't it make sense that room acoustic problems - which are at least four or five orders of magnitude larger! - is a far more likely culprit for lack of detail, smearing, and imaging etc?

--Ethan
In my case it is easy as i use electorstatic headphones to evaluate DACs so I am not impacted by the room.

That aside, it is common fallacy to assume that room distortion masks other types of distortion. Room distortion unless the walls are rattling, is linear in nature. Distortion such as jitter is not. Here is an example that may be easier to understand. If i take a 10 watt amp and drive it at its max volume with 20% THD, would I not hear the distortion due to room acoustics? Of course I would. The non-linearities would be quite obvious despite anything the room might do to the sound.

To be fair, it is true that people talk about jitter all the time as if they really understand it when in reality, it is a complex topic few of us have a good grasp of it, even though we are in the industry :).

Keep in mind that we can show mathematically what must be true for a digital system to reproduce its source. If we wan to regard digital perfect in this ability, we can't in the next breath dismiss the math and say that 8 bits of effective resolution at 20Khz is good enough all of a sudden because subjectively and anecdotally, we think that is OK. One has to either believe in the science or not :).
 

Ethan Winer

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I am not going rehash all that they have done wrong as I have written about it many times.

That's fine. I haven't read everything you've written criticizing their method, and you may not have read all of my writings on this either. So I'll hit just the high points:

1. "Random" jitter or for that matter, any other other random distortion turns out to be far more benign than program correlated jitter.

Sure, but if a noise or buzz or program-related IM-like artifact is 120 dB below the music, nobody can possibly hear it regardless of whether the sound is correlated or not. This is very different from your example of a fan that's clearly loud enough to be audible, but eventually becomes ignored due to being on constantly. One section in my AES Audio Myths video plays music while a very nasty noise is turned on and off at varying levels. Even though the noise goes on and off, and you can see on the play meter where it starts and stops, the noise becomes inaudible at levels far louder than what is typical for jitter.

The only way to know at what level jitter becomes audible is to use a device that lets you vary the amount of jitter while you listen. As far as I know, no such device exists. At least the paper I cited tried to use that approach, even if they had to simulate the jitter using software. For me the bottom line is that jitter artifacts are exceedingly soft. As far as I know, it has never been shown that any human can hear any sound when it's 100+ dB below another sound and thus masked by the louder sound.

2. They let people play the content through their PCs. Did they profile those PCs first to see what level of jitter they had in them to start? No.

On the surface that seems reasonable, but the jitter of any computer sound card is well below the thresholds they measured so that shouldn't matter. Let's use another example. Suppose you wanted to test at what level wow or flutter become audible. This is easy to do on a PC using an FM modulator type audio plug-in, and I've done this a few times. So let's say that when the modulation rate is 10 Hz, the pitch has to vary by 2 cents before anyone can hear it. The PC's own time-based errors are vastly smaller, so I'd be confident accepting the 2 cents result. Does this make sense?

jitter destroys low order bits and can be especially audible where the ear is most sensitive.

Again, how do you know this for sure? Have you done a test where you increased the jitter amount starting from zero and going higher?

there will never be any proof that jitter is inaudible for the simple reason that jitter has infinite characteristics. It can be caused by one or more modulating signals each of which can have arbitrary frequency, magnitude and waveforms. So no study, no matter how comprehensive, can ever provide sufficient data that jitter cannot be heard in other circumstances.

It's not possible to give negative proof. Common sense (as I see it) says that nothing 100+ dB below the music could possibly be audible no matter what the makeup of that sound. So this is why I feel the burden of proof is on those who claim that typical amounts of jitter are ever audible.

Is the "music" always at 0db? It has to be for your statement to be true.

No, but one example (starting at 33:28) in the video linked above has gentle classical music with an average level of -30, peaking around -15 when it gets to the loudest spot. By the time the nasty noise is reduced to -70, it's then only 40 dB softer than the music at -30. Even cranking the volume way up it's difficult or impossible to hear the noise. And that's only a 40 dB difference! If jitter is at -100 it's still 30 dB softer, and only 70 dB below the music, and it's still inaudible. Further, and this is key, the amount of jitter is relative to the music volume. So as the music gets softer the jitter is lower too.

--Ethan
 

Steve Williams

Site Founder, Site Owner, Administrator
Just a few rules for our readers going forward here. this is an INVITED DEBATE between amirm and Ethan Winer. This debate will be moderated by RBFC and any posts other than by the above two mentioned will be deleted. Presently Ethan is involved with some family festivities for the next two days and will not be able to post. Rules are simple each one takes a turn to post and make a point. Depending on how much the debaters get involved there is presently not a time limit however if ideas and posts begin to wane I would call for final arguments and then put up a members poll as to who the winner is and then open the debate for member questions and comments.

So next person to chime in here will be Amir
 

amirm

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Part One

Sorry for the late reply. Just too busy with life outside the forum this weekend :).

BTW, I am assuming the debate is whether there is scientific research in the audibility of jitter not whether jitter is audible or not. While the latter will invariably get discussed in this context, I don’t think a debate alone with prove that. Yes, I can put forth a compelling case than jitter can be audible but I am not going to pretend my personal data rises to proof point. You have to learn the ins and outs of jitter and decide for yourself.
That's fine. I haven't read everything you've written criticizing their method, and you may not have read all of my writings on this either.
I think I have read more about your writing and experiments than you have of mine :). I had for example seen your talk on Audio Myths and listened to your samples. Talking about that conference, JJ sitting next to you used to be an audio architect in my team. I hired him from AT&T. That was before he moved on to a local company which got bought by DTS.
Sure, but if a noise or buzz or program-related IM-like artifact is 120 dB below the music, nobody can possibly hear it regardless of whether the sound is correlated or not.
120 db? Even dedicated high-end DACs have trouble keeping their noise level below that which is roughly equiv. to 20 bits of resolution. A random PC doesn’t have a prayer of even remotely achieving such a spec. More on this below.
One section in my AES Audio Myths video plays music while a very nasty noise is turned on and off at varying levels. Even though the noise goes on and off, and you can see on the play meter where it starts and stops, the noise becomes inaudible at levels far louder than what is typical for jitter.
Asking me about this, is like a magician asking another magician whether he was fooled by his trick! :D I thought your test was cute but quite unfair. There are a million ways to hide an artifact. But as long as there are many other ways where it can rear its ugly head, it doesn’t prove anything. In your example if I remember right, you used loud synthesized music which based on masking effect, does a nice job of hiding the noise.

For a simple counter example, instead of music, use the noise itself as music and run your experiment again. Do you think the noise will be hidden the same as your example? Yes, this is a contrived example but so is yours. You can’t control what constitutes music and hence can’t say that noise itself can’t occur in real life. I could also challenge you to use very quiet music and see how well you can hide it there.

For another example, I can pick music clips where when encoded at 64kbps, untrained listeners cannot tell them from the CD. Does that prove that 64kbps compress is lossless? Of course not. This would be another magician trick as yours :).

The only way to know at what level jitter becomes audible is to use a device that lets you vary the amount of jitter while you listen. As far as I know, no such device exists.
Then this case is open and shut :). Claim was made that there are papers saying we can’t hear the effect of jitter. If the above is the only way and the device does not exist, then everything else is a simulation with its own limitations which must be understood before we trust its conclusions as I tried to note.

BTW, I want to make clear once more that jitter is not a knob you turn and represent all cases of it. Jitter is not one number. Jitter has amplitude, frequency and distribution. The latter alone will have infinite range let alone all three combined. So even if you had the device you wish, you still couldn’t simulate the situation at my home with my DAC and PC for example. Your device and experiment would still represent a theoretical simulation and not representative of all combinations in the field.
At least the paper I cited tried to use that approach, even if they had to simulate the jitter using software.
The paper did no try to use that approach. They could have opted to use a high quality DAC and messed with its clock and have everyone listen to that box alone. Instead, they take a signal, add random jitter to it, and then give the file for people to run on their own hardware. Then they commit another sin and sum up everyone’s results as if the same test condition was applied to all. In all the years of hearing about audio tests I have rarely heard of anyone attempting to test things this way. The number one rule in any such experiment is to control the test variables and having one hardware, with known characteristics is the starting point in that.

Putting that aside, the biggest sin is that the paper is that it ignores it uses random jitter. Let’s bring everyone else into the conversation before I go on.

Jitter is a variation of timing. That variation has a frequency, amplitude, and waveform. Those are the same things your music has by the way. So the way to look at jitter is that an unwanted set of tones are mixed with what you are trying to listen.

Theory and math tells us that if you take a signal and add another one to it in the form of jitter, you get your original signal on the output plus two sidebands that were not there. The frequency of the sidebands is that of jitter. Here is an example of source content being a single 10 KHz tone with jitter that is also a pure tone at 3 KHz:


As you see, we get unwanted sidebands at 7 KHz and 13 KHz. The 3 KHz signal is varying the main signal back and forth in time, resulting in the distortion shown in frequency domain. 3 KHz could be the frequency of the front panel EL display showing you track numbers and such. Or the timer in the DSP in your processor.

As the paper indicates, for those peaks to be ignored, they have to be lower than 250 picoseconds. Only then will they be shorter than the distortion you get from 1 bit of a 16-bit audio not being right. The paper then goes on to conduct an experiment where they claim jitter amplitudes which are thousands of times higher weren’t audible. But was that a hat trick as was Ethan’s test? We start by looking at the spectrum of the jitter they used which they say was random (“Music signals with random jitter were simulated on the digital domain.”).

What is random jitter? It is jitter that is not a pure tone like the example I used above. Instead, it is constantly changing (randomly). Let’s visualize what that looks like when added to a pure (music) tone courtesy of the excellent tutorial by late Julian Dunn written for Audio Precision (company who makes my audio analyzer):

The source signal is at 12 KHz this time which is not material. What is material is that the sidebands have disappeared! Instead, what we have is the elevated noise floor. Why? Imagine taking the two sidebands in the first graphics and move them back and forth randomly very fast. What do you get? The second graphic! Instead of one sideband, you get infinite number of them.

Let me ask you this: which artifact do you think is more audible? The situation where the sidebands stand out like the first example or when it resembles noise? I suspect the answer is obvious and hence the reason we care more about non-random distortion.

Now let’s be careful as another hat trick could be in the works in this area. And that is, there are scenarios where jitter is not audible due what we call the masking effect. Masking is where a louder signal overshadows a fainter one close to it. The ear simple ignores the other. Here it is graphically:

As you see, the “masker” which in our situation could be the music, is able to cover the “masked sound” which could be the jitter sideband. Since jitter has much lower volume typically than music, it is very important to pay attention to this factor and not conduct experiments where we know in advance jitter won’t be audible. For this reason, jitter frequencies below 100 Hz (such as those caused by 50/60Hz of the power supply) can be safely ignored as their sidebands hug the main frequency very closely and hence are not audible.

The paper did not make the above mistake. But I wanted to mention it as an example of deep understanding one needs to have of the subject matter at hand before attempting to create an experiment. Jitter is a complex field and requires much knowledge of math, theory and perceptual effects before one should drill into conducting experiments around it.
 

amirm

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For me the bottom line is that jitter artifacts are exceedingly soft. As far as I know, it has never been shown that any human can hear any sound when it's 100+ dB below another sound and thus masked by the louder sound. .. On the surface that seems reasonable, but the jitter of any computer sound card is well below the thresholds they measured so that shouldn't matter.
“Any computer sound card?” How do you know that the performance of any combination of sound card and a PC is let alone it being better than 100 db? For others watching, CD’s signal to noise ratio is 96db. Ethan is saying any arbitrary combination of PC sound card and PC betters the CD’s maximum theoretical performance. Nothing could be further than the truth!

Ethan, you are talking to a guy who in former life ran the digital media division at Microsoft and his group was responsible for setting the standards for audio fidelity on the PC and testing the same. JJ, who sat next to you was tasked with trying to improve audio fidelity in Windows. You would not believe the number of times he was threatened with his job and I had to become his human shield because he dared to suggest that WHQL audio hardware performance goals for OEMs shipping Windows should be better than a cassette tape at 75 db! This, despite the huge cash incentives for these companies to comply with these goals and they still pushed back.

The specs published for a soundcard are likewise nonsense even if they rise up to the level you mention. A PC sound card doesn’t do anything until you plug it into a PC. As soon as you do, it gets polluted by the noise in there. Take my very high-end Dell workstation. It shipped with a Sound Blaster X-Fi which claims 109 db signal to noise ratio. On said PC, I also have an HP scanner/fax/printer hooked up using USB. As soon as I turn on the HP, I hear every action it takes through the output of my sound card! I am taking about nice buzzing sound as soon as I turn on the HP. I am talking about the buzzing sound changing in tone and character as the HP does its self-test. You want to tell me I an getting 109 db of signal to noise ratio? I think not :). I am lucky if I am getting half of that.

Using my old Dell laptop, I can hear the CD ROM spin up as I access it through the headphone jack. Ditto for the hard disk turning on and off. On my son’s desktop PC, using a discreet sound card, the same thing happens.

This is the kind of precision attention to details like this to give credibility to the data being presented. When I read that someone let people use a laptop to connect with any sound card they so choose and then try to sum up all the data together, I lose all interest in them understanding what they are working with.
Again, how do you know this for sure? Have you done a test where you increased the jitter amount starting from zero and going higher?
Let me first repeat again that topic was whether there is a paper that tells us jitter is not audible. And that I already explained (and yet again in this post) that there is no one knob you can change this way.

That said, I have done approximations of this. For example, I have done blind testing of what happens if you turn off the video and front panel circuits in a source and whether when feeding an external high-quality DAC, it makes a difference. And it did. The only thing that the source feeds the DAC is the data samples and clock. We both accept that data samples do not change. So something made the timing better in the source. Could I have imagined the change and got lucky even though it was a blind test? Sure. That is why I said I am not here to prove that jitter is audible – only to point out that overly simplistic test like the one sited don’t do the opposite either.
Common sense (as I see it) says that nothing 100+ dB below the music could possibly be audible no matter what the makeup of that sound. So this is why I feel the burden of proof is on those who claim that typical amounts of jitter are ever audible.
We are not debating if someone can hear noise at levels below 100db. We are debating if the other side did a good job of proving jitter is inaudible below certain rate. They have not. They were super sloppy in not controlling the listening stations. I have never ever heard of anyone running such tests and letting users build their own players with whatever configurations they wanted. And out of a PC configuration circa 2003 no less.

As for proof, I can show you the math that the distortion exists. It is not then my job to prove that the distortion is audible but yours and others to say it doesn’t! If the math didn’t exist, it would indeed be my job but that is not the case.
Further, and this is key, the amount of jitter is relative to the music volume. So as the music gets softer the jitter is lower too.
--Ethan
Not so my friend. Not so. You are right that jitter distortion amplitude is proportional to the music level. The key to understand is that jitter does NOTHING to the signal it is attacking. Instead, we have to worry about where the distortion lands. Let me explain.

Unlike the examples I have used here, music is not a single tone but many combined at varying frequencies. When jitter acts on them, it creates as many sidebands as there are frequencies in the music.

The music spectrum varies in signal amplitude and in general, its level tends to get lower as the frequencies get higher. Here is an example picture I happen to have around which is a Blu-ray Disc music capture:


Here we see that the mid-band peaks are at about -35 db. But higher frequencies are at -70 to -80db. Why is that important? Well, recall that jitter creates distortions sidebands whose distance from the original frequency is the same as jitter. So let’s say a 6Khz jitter exists in the system. If you apply that to the peak at 2 KHz (-40db), you get sideband on the right that lands at 8Khz (2+6 = 8 KHZ). Let me repeat that again: the distortion is at 8 KHz not at 2 KHz where the music was. The louder the 2 KHz tone gets, the louder the distortion shadow at 8 KHz gets.

If you look at 8 KHz region in the above graph, you see that the signal amplitude is at -80db or a whopping 40db lower than the peaks at lower frequencies. The signal there is not nearly as strong to mask the jitter distortion. And as a result, the music has the potential to get “brighter” as its high frequencies are accentuated by the effect of jitter. If you are going to learn one thing about how to hear jitter is to understand this very observation. You need to have content where you can hear the increased distortion in the high frequencies. There is another case where it can be audible which I will get into later given the length of this post :).
 

Ethan Winer

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Dammit Steve, now look how much work you made me do! :D

Amir, I read both of your posts twice, but I'll hit only the high points in my reply. If you think I'm skipping or ducking something important let me know. And I think it's very cool that you were JJ's boss!

I am assuming the debate is whether there is scientific research in the audibility of jitter not whether jitter is audible or not.

Actually, I'm far more interested in knowing if jitter is ever audible in usual amounts. Knowing about research is fine, but as you pointed out a lot of research is suspect. Personally I don't trust anyone's research unless I was there in person! Well, almost, anyway.

120 db? Even dedicated high-end DACs have trouble keeping their noise level below that which is roughly equiv. to 20 bits of resolution ... The specs published for a soundcard are likewise nonsense even if they rise up to the level you mention. A PC sound card doesn’t do anything until you plug it into a PC. As soon as you do, it gets polluted by the noise in there.

If anything, that proves my point that jitter is a non-issue since it will always be masked by other, louder sounds. Remember, my interest is whether jitter ever matters in practice. For example, it's easy to "prove" that dither reduces distortion when going from 24 bits to 16 bits, but in practice it's rarely / never audible. As further background, my motivation for caring about jitter is the silliness I read in magazines and web forums, where some "expert" tells a newbie the reason his system sounds poor is due to jitter so he should buy a new sound card. Jitter is never the reason someone's system sounds poor. At best, and I'm still not convinced, jitter might be audible in certain contrived situations when played back at near-pain volume levels.

if I remember right, you used loud synthesized music which based on masking effect, does a nice job of hiding the noise.

Great, you acknowledge that for pop music recorded at sensible levels jitter is not a problem. We're halfway there! But that was only one of the examples. As I mentioned in Post #5 above:

one example (starting at 33:28) in the video linked above has gentle classical music with an average level of -30, peaking around -15 when it gets to the loudest spot. By the time the nasty noise is reduced to -70, it's then only 40 dB softer than the music at -30. Even cranking the volume way up it's difficult or impossible to hear the noise. And that's only a 40 dB difference!

So even with gentle soft music, and a harsh nasty buzzing noise only 40 dB softer, it's difficult or impossible to pick out the noise.

Folks, all anyone has to do to convince me that a typical amount of jitter is ever audible is to post a Wave file proving the point!

That's all that's needed. Make an example like my AES video where jitter or a simulation is turned on an off, or however you'd like to present it, and let's hear it. Note I don't mean this at you Amir as much as at everyone who believes jitter is ever audible in practice. And I further mean with normal music or speech, not contrived test signals.

I can pick music clips where when encoded at 64kbps, untrained listeners cannot tell them from the CD. Does that prove that 64kbps compress is lossless? Of course not.

Great point, and I agree. But again, all I ask for is one example.

I want to make clear once more that jitter is not a knob you turn and represent all cases of it. Jitter is not one number. Jitter has amplitude, frequency and distribution. The latter alone will have infinite range let alone all three combined. So even if you had the device you wish, you still couldn't simulate the situation at my home with my DAC and PC for example. Your device and experiment would still represent a theoretical simulation and not representative of all combinations in the field.

Another good point, and again all I need is one example proving that jitter is ever audible in any form having any spectrum.

Let me ask you this: which artifact do you think is more audible? The situation where the sidebands stand out like the first example or when it resembles noise? I suspect the answer is obvious and hence the reason we care more about non-random distortion.

As soon as someone posts an example file I'll be able to tell you which is more audible. Though obviously the spectrum of the noise is a factor, with emphasis around 2 to 4 KHz being far more audible than noise having mostly bassy content. If one believes that correlated artifacts - which are like harmonic distortion - are audible, they need to see typical THD specs for loudspeaker drivers!

Using my old Dell laptop, I can hear the CD ROM spin up as I access it through the headphone jack. Ditto for the hard disk turning on and off. On my son’s desktop PC, using a discreet sound card, the same thing happens.

Depending on the sound card, you can often get rid of those noises by muting or turning down the volume of all unused inputs such as "TAD" and microphone in etc. The problem with some software mixers is they by default hide some volume controls. So you don't even see them and know to turn them off!

We are not debating if someone can hear noise at levels below 100db. We are debating if the other side did a good job of proving jitter is inaudible below certain rate.

I'm not a trained scientist so I'm not qualified to say if that test was valid or not. It convinced me! But more important, my own experiments confirm that anything that soft is simply not audible no matter the spectrum of the artifacts, or whether the artifacts are correlated or uncorrelated.

It is not then my job to prove that the distortion is audible but yours and others to say it doesn’t! If the math didn’t exist, it would indeed be my job but that is not the case.

I disagree. One basic rule of logic is you can't demand proof of a negative. We all know that green cheese exists, but it's not my job to prove the moon doesn't contain green cheese. So logically speaking, in this case, it's not my job to prove that jitter can't be audible either. All logic and common sense - and experiments about the audibility of soft artifacts - conclude that typical amounts of jitter can't possibly be audible. So the burden of proof is on those who say it can be audible. Again, all that side has to do is make an example Wave file proving their point! I've asked for this for years now, but so far nothing.

--Ethan
 

amirm

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Apr 2, 2010
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Thanks Ethan for a measured response :). Hitting on a few simple points for now:

1. The fact that PC music is not of high fidelity doesn't mean for the people in this forum, we get to dismiss low level distortion when majority of the people here spend lots of dollars to do a lot better. Take me. I have an $8K Mark Levinson DAC. The box is getting long in the tooth but that was the only way I could get the best fidelity (combined with a Pro Audio sound card). I agree with you that with the general public, they have no business worrying about things like jitter. CD quality in all forms recreated for them is plenty good. Again, that doesn't fly here were people want to know if they had all the money, could they do better.

2. I can't give a wav file to hear jitter. Jitter is induced by the timing of the source. The Wave file is devoid of that timing. I could try to mess with the samples as the research paper did but then I still wouldn't know if your setup is transparent to it. Finally, I don't know if you are able to hear jitter. I would say even amongst the high-end audiophiles, only a fraction would be able to detect it.

3. On the issue of hearing jitter, the best way to get there is to actually listen to compressed music and see if you can hear its artifacts! OK, sit back on the chair. Didn't mean to make you fall off of it :D. Reason for this is that compressed music is all about reducing the effective bit depth of music samples. We convert music to frequency domain and then selective represent each band at lower and lower quantization levels (resolution). When we do that too much, then you hear the artifacts. So see if you can spot artifacts in 320kbps AAC (not MP3 -- that codec is never transparent no matter what the bit rate). Keep listening until you can hear and spot where it fails. Then use the same clips to test your digital audio gear. No guarantees of course but you will at least be able to take a step in the right direction.

4. On the last point I am puzzled. You say you don't hear jitter and that is proof enough for you that it must be inaudible. Isn't that proving a negative by itself? If you don't believe in proving a negative, then you shouldn't believe any study that says they couldn't hear jitter either!

At the end, this is my view regarding jitter and digital audio reproduction. To the extent possible, you should try to get a system that is at least transparent to CD audio resolution. That is not an exceedingly high bar. It means having an honest, 16-bits of resolution without distortion and noise. And a frequency response to 22Khz. The cost of doing that is less than any high-end speaker these days. I can't assure you that your ears can hear the difference for sure. But you don't sit there wondering if you are missing something either.
 

Ethan Winer

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Jul 8, 2010
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1. The fact that PC music is not of high fidelity doesn't mean for the people in this forum, we get to dismiss low level distortion when majority of the people here spend lots of dollars to do a lot better.

I agree that just because some people don't have a good system - and probably less than 1 percent even have minimal room treatment - this doesn't mean we shouldn't strive for the highest fidelity possible. However, I disagree that all computer-based systems are lacking. My Dell computer has an M-Audio Delta 66 sound card, and it's definitely clean for both recording and playback down to the noise floor of 16 bits.

I agree with you that with the general public, they have no business worrying about things like jitter.

Yet look at how many people read that jitter is a problem, and then spend ten times more than needed to buy a converter that some ignorant reviewer claims sounds better than usual due to its low jitter.

2. I can't give a wav file to hear jitter. Jitter is induced by the timing of the source. The Wave file is devoid of that timing. I could try to mess with the samples as the research paper did but then I still wouldn't know if your setup is transparent to it.

The task isn't to worry about the listener's playback system, only to create a file with jitter-like artifacts at a level that's typical. This is the crux of it. If you create a pair of files and run a poll asking people to choose which file they think has the added jitter, the poll results will hold the answer. If out of 100 people over multiple trials nobody can reliably pick the file with the added artifacts, that proves to my satisfaction that jitter is not an audible problem.

I don't know if you are able to hear jitter. I would say even amongst the high-end audiophiles, only a fraction would be able to detect it.

I'm sure you think you can hear jitter, but how do you know that for sure? As I explained in my AES video, the only way to know at what level jitter is audible to you is with a box that let's you add from zero to a lot. Better, someone else would turn the knob while you listen blind, and you say when you think you can just hear it. Do that ten times and see if the thresholds are consistent, and below the noise floor of a CD. If so, you win. If not, I win. :D

3. On the issue of hearing jitter, the best way to get there is to actually listen to compressed music and see if you can hear its artifacts! OK, sit back on the chair. Didn't mean to make you fall off of it :D. Reason for this is that compressed music is all about reducing the effective bit depth of music samples. We convert music to frequency domain and then selective represent each band at lower and lower quantization levels (resolution). When we do that too much, then you hear the artifacts.

I can't see how reducing the bit-rate of lossy-compressed audio until you hear it is related to increasing jitter until you hear it. One is added noise, the other creates holes in the frequency response. To me, low bit-rate compressed audio sounds like comb filtering. And that's because it is more or less. That's not at all like jitter which is the addition of artifacts. You can express jitter as being at a level equal to a particular low bit (16th bit, 18th bit), but that's not the same as carving out parts of the spectrum!

You say you don't hear jitter and that is proof enough for you that it must be inaudible. Isn't that proving a negative by itself? If you don't believe in proving a negative, then you shouldn't believe any study that says they couldn't hear jitter either!

Not quite. I say that a typical amount of jitter is so soft that nobody can possibly hear it. This is a far more sensible way to approach audibility testing! That's why I created the nastiest buzzing noise I could muster for my AES test. Since that noise disappears at levels far higher than jitter, that is what convinces me that jitter is never audible.

you should try to get a system that is at least transparent to CD audio resolution. That is not an exceedingly high bar. It means having an honest, 16-bits of resolution without distortion and noise. And a frequency response to 22Khz.

I have that. Two systems in fact. Well, except for the 22 KHz part. But at the age of 61 I can't hear much past 14 KHz anyway so its moot. But my ability to hear detail and distortion is pretty good.

Not to turn this into a pitch for room treatment, but it kills me when some audiophiles talk about clarity and detail, while listening in a room where the early reflections are uncontrolled and only a few dB below the direct sound. That obscures clarity far more than a cheap sound card, and even cheap speakers. I'd rather have $100 bookshelf speakers in a treated room than [whatever expensive brand] in a room with bare walls and no bass traps. I guarantee the sound will be clearer with the $100 speakers.

--Ethan
 

DonH50

Member Sponsor & WBF Technical Expert
Jun 22, 2010
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I can set up test files showing the impact of various jitter on various simple signals (mathematical test cases). At least that would let you play with magnitudes and look at the FFTs to see if it makes any sense at all at typical jitter levels. I am pretty sure I can generate .wav files but have to check what toolboxes I have at work (I am off this week, hurray!) Since it is system-dependent, what adding jitter would do is let people hear what it does more so than allowing conclusions about it's audibillity in the test files, since it makes sense (to me anyway) to run the jitter high enough so that you can hear it (maybe a series of runs with various levels?) Might help, but probably won't settle the debate. All it takes is time, and lots of it!

Small amounts of random jitter can actually improve the noise floor by decorrelating other spurs. Colored noise also helps and does so without corrupting the in-band noise floor (SNR). Deterministic jitter adds discrete tones, as has been noted, and those are (imo) the main culprit/basis for any claim to audible signal degradation.

I agree deterministic jitter is by far the largest culprit. And that room response swamps other error sources for most of us. I'd still stick with my expensive speakers even in a bare room, though, Ethan -- I can treat later! :)

Great debate! - Don

Sorry -- I missed that this was to be only Amir and Ethan! I tried to delete this post but it still cropped up -- hopefully an admin can wipe it. Senility, or maybe just getting back from vacation blues - Don
 

amirm

Banned
Apr 2, 2010
15,813
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Seattle, WA
However, I disagree that all computer-based systems are lacking. My Dell computer has an M-Audio Delta 66 sound card, and it's definitely clean for both recording and playback down to the noise floor of 16 bits.
Hmmm. The discussion started with you saying "any PC sound" card is that good. To the extent you now are saying that your specific configuration sounds good then we have made a ton of progress!

That said, how do you know your configuration resolves 16-bits? How did you measure it? And can you post those results? Delta makes really good cards but once you plug it into a random PC, you are never assured of its performance.
Yet look at how many people read that jitter is a problem, and then spend ten times more than needed to buy a converter that some ignorant reviewer claims sounds better than usual due to its low jitter.
I would say that the other side has not found an effective way to communicate its message then :). Besides your side is guilty too. This very same argument over this paper is going on in another forum as I type this. People are quoting that paper as gospel and when I asked the poster what the conclusion meant, he couldn't even spell it alone know what it meant!

I suggest we put aside solving for world peace and continue on with the merits of the matter. Whether someone should or should not be worrying about jitter is not something that changes the facts. If you and I do a good job of discussing it, maybe there will be less of it :).
The task isn't to worry about the listener's playback system, only to create a file with jitter-like artifacts at a level that's typical. This is the crux of it. If you create a pair of files and run a poll asking people to choose which file they think has the added jitter, the poll results will hold the answer. If out of 100 people over multiple trials nobody can reliably pick the file with the added artifacts, that proves to my satisfaction that jitter is not an audible problem.
I think we are still talking past each other. I already explained how Jitter can have infinite variations. Which profile would I simulate and why? And how do you enable people to use their home systems rather than clicking on a file on their laptop and testing that way? Burning CDs and such is a lot of work to ask people in the process.
I'm sure you think you can hear jitter, but how do you know that for sure?
Well, let me say that I am more sure of hearing it than you are that you can't hear it :). At least I have positive proof for something. You are relying on negative logic that if I have not heard it in my limited testing (or worse, assumptions), then it must not be audible.

As I explained in my AES video, the only way to know at what level jitter is audible to you is with a box that let's you add from zero to a lot. Better, someone else would turn the knob while you listen blind, and you say when you think you can just hear it. Do that ten times and see if the thresholds are consistent, and below the noise floor of a CD. If so, you win. If not, I win. :D
Ethan, this is getting a bit frustrating. I have said repeatedly that there is no one number you can dial this way. It is not frequency response or volume where you could just have one dial to affect it. As a minimum, you would have to have three dials and the last dial, would have to have infinite dials itself to represent all the spectrum distributions.

Let's put that aside for now. Please explain why I heard what I heard. Aren't there only two explanations?

1. Jitter is audible.

2. I imagined it, even though I am schooled in objective and scientific testing of audio and have an engineering background to boot, with an opinion prior to testing that all of this was non-sense as you, and that the test was run blind?

Remember, my goal here is not to prove audibility of jitter. But rather, giving you a perspective that it is not just ordinary audiophiles who worry about jitter but people with extensive experience in the science of audio (especially the perceptual aspects of it) solid enough knowledge of the engineering behind it. In other words, as long as you accept that the case is not open and shut as you thought, then I am golden :). I have no higher aspirations.
I can't see how reducing the bit-rate of lossy-compressed audio until you hear it is related to increasing jitter until you hear it. One is added noise, the other creates holes in the frequency response.
That's incorrect in the way you are stating it. If you look at compressed music, it will appear to have full response (at high enough data rates and with modern codecs than MP3). There will be no holes in its spectrum. Let's review again what how we perform lossy audio compression:

1. Signal is transformed from time domain (PCM samples) into frequency domain. Think of dancing bar graphs in an equalizer. For now, assume if the samples are 16 bits each, the frequency "bins" are also 16 bits.

2. Those frequencies are analyzed and based on perceptual model of the ear, we reduce the number of bits we assign to each frequency. So for example, if there is a loud sound at 1 KHz, we will leave that band alone but take bits away from another sound at 900 Hz since that is in the "masking" shadow of the 1 KHz sound.

The reduced bits allocated to the 900 Hz band causes increased distortion which we call "quantization noise." If for example, we chop that band down to 8 bits, then we have created a much coarser signal and added distortion because of it. However, since this sound is likely to not be audible anyway, then butchering it this way is probably not harmful.

3. We then take all of those frequency bins values which are just a bunch of numbers and apply a *lossless* compression algorithm to it as we would with zipping a file on your computer. This is called an "entropy coder" if you want to impress your friends at the next holiday party :D.

4. The output of #3 is your compressed file. The player takes that file, applies the reverse of entropy coder and gets the frequency bins. It then does the reverse transform of that and gets the PCM samples. Data rate is reduced because we have reduced the bandwidth allocated to each bin variably and the entropy coder is optimized in compressing those values further.

While it is true that if you heavily compress a file, some of the bins may get zeroed out, in the high data rates that I mentioned, that does not occur. Instead, what we get is increased quantization noise at certain frequencies. The increased noise tends to rob the signal its ambiance which is at much lower level than the music and increases its high frequency content (symptom of over quantizing a signal). Both of these occur in a very similar way with jitter. Jitter reduces system resolution and as such, it can also obliterate low level detail and help increase the high level frequency energy of the signal.

Is the effect identical? No. But to the extent you learn to hear the artifacts of lossy music, you are a long way toward hearing artifacts rather than just hearing music. And that is key: there is a big difference in using your ears to enjoy something than to use it as an instrument, trying to find a flaw. Most people are not good at the latter. By practicing to hear lossy compression artifacts at high data rates, they learn what it takes to do that. If I may use a crude analogy, it is the difference between reading lips to understand someone in addition to hearing their voice or doing the latter alone. It takes practice to read lips, it doesn’t come naturally to people.
You are ahead of most members here in hearing acoustic flaws. I am sure you don’t take that skill for granted or there or there would be no need to do the demo videos you have done. Likewise, I feel that more experience is needed to hear digital artifacts than what a person may already know.
To me, low bit-rate compressed audio sounds like comb filtering. And that's because it is more or less.
You have a tendency Ethan to read everything through the lens of acoustic problems :). There are certain artifacts at very high levels of compression that might sound like comb filtering but that is not a correct generalization. Encode some speech and listen to it at 32kbps. It won’t sound like comb filtering but rather, will have a coarseness to it that is created by the effects I mentioned (more commonly called “pre-echo”).
I say that a typical amount of jitter is so soft that nobody can possibly hear it.
Have you ever heard jitter at any level Ethan? Do you have an external DAC and does it support multiple interfaces? If so, can you hook them up to your sound card and then switch between them and tell us whether they all sound identical? Try comparing S/PDIF, AES and Toslink. Then for the former two, try both a high quality short cable and long, crappy cable. Again, with every change, do an A/B against the other.

I have ran the above test and I was shocked to hear the differences. It was totally unexpected for me. I would say you owe doing tests like this way before we owe you getting 100 people to vote whether they hear jitter or not :p.
Not to turn this into a pitch for room treatment, but it kills me when some audiophiles talk about clarity and detail, while listening in a room where the early reflections are uncontrolled and only a few dB below the direct sound. That obscures clarity far more than a cheap sound card, and even cheap speakers. I'd rather have $100 bookshelf speakers in a treated room than [whatever expensive brand] in a room with bare walls and no bass traps. I guarantee the sound will be clearer with the $100 speakers.
Again, that is neither here nor there. People here want the best so you should assume that they have done all they can to their room and are now wanting to understand two things:

1. If they believe in digital, what is the best system for them.


2. If they believe analog sounds better, what would explain that.

I am trying to provide data for both. People natively understand analog concepts of distortion. They hear it readily. Turn up the amp too much and you hear clipping distortion. Get a speaker that has too much highs or is boomy and they hear that too. But where would they even begin to know what jitter sounds like? That is what I am trying accomplish here. Teach them the science as you do with acoustics. Tell them how it *could* be audible. And then let them “go fish.” We don’t control their pocketbook nor can we ever get into their heads to know what they hear. So let’s not keep debating where they should spend their money and what is good for them…
 

rblnr

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May 3, 2010
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darn but this is the makings of something good.

Im watching from the sidelines as is everyone else. Thanks Amir and Ethan

Yep, thanks guys. Really appreciating this.
 

Ethan Winer

Banned
Jul 8, 2010
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The discussion started with you saying "any PC sound" card is that good.

I did? I thought I said that with any sound card, for whatever reason, jitter will be too soft to hear.

That said, how do you know your configuration resolves 16-bits? How did you measure it? And can you post those results? Delta makes really good cards but once you plug it into a random PC, you are never assured of its performance.

I know I can hear down to the last bit by playing very soft music or other sounds at high levels. I just did a test by creating a wave file generated in Sound Forge having one second of silence, followed by one second of 250 Hz at -90 dB, followed by another second of 250 Hz at -84. When I played it back at full volume I could easily hear when each level of 250 Hz started. Now, I admit there was some very soft hum and noise in the background. So you could argue that I was hearing the tones below the noise. I'd have to look further to see if that's true. Unless you'd like to visit me and we'll do it together. :D

For anyone else who would like to test their system's playback, I put the file (about 260 KB) on my web site:

250hz.wav

Besides your side is guilty too.

All I'm guilty of is asking for proof that artifacts 100 dB below the music are ever audible while the music plays, regardless of the makeup of those artifacts.

I already explained how Jitter can have infinite variations. Which profile would I simulate and why?

You would apply whichever profile you believe best makes your case. Do whatever tests you want, then pick the one you believe shows the (simulated) jitter at its worst. All I ask is that you do this at a level that's typical for jitter, not at -60 or whatever.

And how do you enable people to use their home systems rather than clicking on a file on their laptop and testing that way? Burning CDs and such is a lot of work to ask people in the process.

If you post a Wave file, people can make their own CD. In fact, that would be a nice addition for this forum, allowing Wave file attachments. I understand this takes up server space, but even if you limited the files to 1 MB that would be big enough for short tests like this. A 1 MB mono Wave file at 44/16 is about 12 seconds long.

Ethan, this is getting a bit frustrating. I have said repeatedly that there is no one number you can dial this way. It is not frequency response or volume where you could just have one dial to affect it. As a minimum, you would have to have three dials and the last dial, would have to have infinite dials itself to represent all the spectrum distributions.

Again, simulate jitter using whatever "dial" settings you like. As long as its truly representative of actual jitter I'll be happy.

Please explain why I heard what I heard.

What did you hear, and under what conditions? Was it a blind test? Double-blind?

I've seen people say they switched sound cards to one that claims lower jitter, heard an improvement, and concluded the lower jitter must be the reason. But that ignores a dozen other possible differences between two brands of sound card.

my goal here is not to prove audibility of jitter.

Then I guess we're done!

Seriously, I thought your point was that in some situations typical amounts of jitter can be audible. If you don't think that jitter is ever audible in usual amounts, then it's just an intellectual curiosity, and maybe an engineering goal, so we're done. I agree that design engineers should always strive for the highest performance possible, even if the result is not audible. For example, some things that are not immediately audible can become audible after several generations of copying or processing.

If for example, we chop that band down to 8 bits, then we have created a much coarser signal and added distortion because of it. However, since this sound is likely to not be audible anyway, then butchering it this way is probably not harmful.

I did not know that. I ASSumed that lossy compression was similar to digital noise reduction, where a many-band gate is applied to remove content below a threshold. Though in this case I figured the threshold for any given band was dependant on whatever else is going on in other bands at the time. Noise reduction and low bit-rate lossy compression both have a "swirly" phase-shifter comb filtered type sound.

While it is true that if you heavily compress a file, some of the bins may get zeroed out, in the high data rates that I mentioned, that does not occur.

Ah, great explanation. That explains the swirly sound. Thanks!

Have you ever heard jitter at any level Ethan?

How could I know? As I said, I don't know of any scenario to test this other than a black box that lets you just jitter from zero through a lot. That's why I asked about the tests you've done, and how you concluded that jitter is ever audible.

Do you have an external DAC and does it support multiple interfaces?

My Delta 66 can be slaved to an external clock via SMPTE, but I don't have anything other than an old Sony DAT recorder that can send SMPTE.

I have ran the above test and I was shocked to hear the differences.

But how do you know the difference you heard was due only to jitter? And was this a blind test someone else administered? As I'm sure you heard JJ say in our AES video, even he needs someone else to test him blind with stuff like this.

--Ethan
 

amirm

Banned
Apr 2, 2010
15,813
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But how do you know the difference you heard was due only to jitter? And was this a blind test someone else administered? As I'm sure you heard JJ say in our AES video, even he needs someone else to test him blind with stuff like this.

--Ethan
That specific test was not run blind. I was simply suggesting that as an example of something I thought you could try. The more rigorous tests I ran were all double-blind. For example, I would test the effect of turning off the video circuit in a digital source (e.g. DVD-A) by turning away from the gear, hitting the switch many times in a row so that I didn't know what state it was in at the end. And then toggle back and forth while listening. If I could hear a difference, I would go back and forth a couple of times to be sure. Then I would turn and look at the display to see what mode the device was in. Often, I would repeat the test a second time to be sure there was not a pattern that was effecting me. I had done similar tests by turning off the front panel.

As to how I know it was jitter, I can't explain it any other way. What other things could cause a source driving a DAC through three separate digital connections sound different? OK, there could be a reason for one of them being different but I let that be an exercise for the reader :D.

I am open to an explanation why the above small modifications to the setup would create audible distortion or differences but jitter would not.
 

amirm

Banned
Apr 2, 2010
15,813
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Seattle, WA
I know I can hear down to the last bit by playing very soft music or other sounds at high levels. I just did a test by creating a wave file generated in Sound Forge having one second of silence, followed by one second of 250 Hz at -90 dB, followed by another second of 250 Hz at -84. When I played it back at full volume I could easily hear when each level of 250 Hz started. Now, I admit there was some very soft hum and noise in the background. So you could argue that I was hearing the tones below the noise. I'd have to look further to see if that's true.
Beyond noise, you don't know if what you heard was true to the original. It could have been super distorted and you wouldn't know it. The only way to make sure you are linear down to the last bit is to measure it.

All I'm guilty of is asking for proof that artifacts 100 dB below the music are ever audible while the music plays, regardless of the makeup of those artifacts.
I don't know why you keep mentioning 100db. I have explained why that is not the right way to look at this. Let's review what the paper said:

"It was shown that the detection threshold for random jitter was several hundreds ns for well-trained listeners under their preferable listening conditions."

Now let's look at the AP graph I posted which was for a periodic jitter of just 7 nanoseconds, not "several hundreds" mentioned in the article:


So as you see, even at much lower levels, the sidebands peak up to -80db.

Now let's again look at real music capture using my audio precision:



So does it not look like even at 7ns the distortion could rise up to the level of high frequency content in the music?

Do whatever tests you want, then pick the one you believe shows the (simulated) jitter at its worst. All I ask is that you do this at a level that's typical for jitter, not at -60 or whatever.
Per above, sounds like you want a situation where on purpose, jitter would not be audible. Show me where high frequency content is at 0db and I buy your arguments. Otherwise, we don't have meeting of minds on where jitter causes distortion and could be audible.
 

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