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Thread: Audible Jitter/amirm vs Ethan Winer

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    Quote Originally Posted by Phelonious Ponk View Post
    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 .

    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?

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    Quote Originally Posted by amirm View Post
    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:

    Quote Originally Posted by Kaoru Ashihara et al
    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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    Quote Originally Posted by amirm View Post
    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

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    Quote Originally Posted by Ethan Winer View Post
    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 .

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    Quote Originally Posted by amirm View Post
    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

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    Site Founder And Administrator Steve Williams's Avatar
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    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
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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.
    Quote Originally Posted by Ethan Winer View Post
    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! 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.

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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.
    ďAny computer sound card?Ē 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, CDís signal to noise ratio is 96db. Ethan is saying any arbitrary combination of PC sound card and PC betters the CDís 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 doesnít 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 sonís 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 Ė only to point out that overly simplistic test like the one sited donít 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 doesnít! If the math didnít 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 letís 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 ďbrighterĒ 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 .

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    Damn it Steve. Look at how much work you made me do!

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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
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