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Jitter Correlation to Audibility

Discussion in 'Sound Science' started by robertsong, Jun 19, 2013.
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  1. robertsong
    From Steve Nugent (a leading authority of digital audio). It's a little dated, but there's some fantastic info here. I "bolded" what I found to be the most significant parts.
    Jitter Correlation to Audibility
    The correlation of jitter measurements to audibility is in its infancy IME. The problems start with the characterization of jitter. Generally, manufacturers of crystal oscillators specify jitter in terms of RMS jitter amplitude. The problem is that they often neglect to state that this is specified at 10kHz and higher. There is also no spectral or frequency content information specified. This makes it very difficult to tell which oscillators will have audible jitter or objectionable jitter.
    For instance, Empirical Audio uses two oscillators that are both specified at 2psec RMS jitter. The two oscillators sound radically different to me when used in a re-clocker in a resolving audio system. This leads me to believe that the spectrum, or frequency content of the jitter is as important or maybe even more important than the amplitude. I also believe that correlated jitter or jitter with a relationship to the data pattern or audio signal is also more audible than random jitter. This seems to be the consensus in a number of AES papers.
    Studies by the AES (analysis, not human testing) conclude that these are the thresholds of audibility:
    [1] 120psec P-P jitter audibility threshold for 16-bit DAC and 8psec P-P jitter audibility threshold for 20-bit DAC
    [2] 20psec P-P of data-correlated jitter audibility threshold at certain frequencies and "A simple model of jitter error audibility has shown that white jitter noise of up to 180psec P-P can be tolerated in a DAC, but that even lower levels of sinusoidal jitter may be audible"
    Since many measurements (that don't specify any particular frequency content) performed by Stereophile in [3] are above 150psec or close to this, I do not believe that we have reached the limits of jitter audibility yet. I suspect that P-P jitter needs to be almost an order of magnitude smaller, or around 15psec to be inaudible in all systems.
    I believe the ability of the human ear/brain, particularly the trained ear, to hear minute differences, particularly data-correlated jitter, is grossly underestimated. The live listening AB/X studies published to date (that I have read) are inconclusive IMO. The systems used were not resolving enough IMO, the recording quality was not good enough and the test signals were random and not correlated and therefore inadequate to properly test for jitter audibility. I tend to believe the numbers arrived at by the AES analytical studies rather than the A/BX listening tests.
    There are a series of double-blind tests being performed by many audiophiles using synthetic jitter tracks provided by HDTracks. These may shed some new light on true audibility. Again, the effectiveness of these experiments is only as good as the quality of the tracks provided, the jitter that was synthesized and the audio systems that are used for testing. The results from the first set of jitter tracks shows just how unresolving most audiophile systems are. There are couple that could pick out the majority of the tracks by increasing jitter, but the majority could not hear any difference between the tracks, even though the jitter ranged from 0 ns to 1000ns I believe.
    Another interesting thing about audibility of jitter is it's ability to mask other sibilance in a system. Sometimes, when the jitter is reduced in a system, other component sibilance is now obvious and even more objectionable than the original jitter was. Removing the jitter is the right thing to do however, and then replace the objectionable component. The end result will be much more enjoyable.
    Jitter can even be euphonic in nature if it has the right frequency content. Some audiophiles like the effect of even-order harmonics in tubes, and like tubes, jitter distortion can in some systems "smooth" vocals. Again, the right thing to do is reduce the jitter and replace the objectionable components. It is fairly easy to become convinced that reducing jitter is not necessarily a positive step, however this is definitely going down the garden path and will ultimately limit your achievement of audio nirvana.
    Sibilance in a system caused by preamp, amps and other components and cables can also be so high that changes in jitter are not very audible. This is why there is such contention on the web forums about jitter and its importance. What matters in the end is if you are happy with the sound of your system, and whether or not you can hear this distortion.
  2. robertsong
    The subject is a bit more complicated than most people seem to think. I wonder if there better listening tests on better systems yet. Anybody know?
  3. Theta Alpha 1
    It's been exemplified several times that differences in an audio signal will go completely unnoticed if the listener is not aware of exactly what to listen for.
    In audio threshold tests, the participants are often random or untrained, so the threshold result turns out much lower / higher than what the threshold is in real life settings with focused listeners and perhaps more resolving or transparent equipment, like you say.
    While it's nice they've made tracks containing synthetic jitter and some listeners may identify it, the tracks still have to pass through playback equipment containing jitter.
    Recording the sound of the sound of various components doesn't work very well since the sound of the playback system itself is the last link, which takes precedence over all links prior.  No one records DAC's or capacitors to showcase them partially for this reason, and partially since their statement is usually nothingness, perhaps it's difficult to showcase nothingness.  Zero jitter is also nothingness, impossible to showcase via somethingness.
  4. Theta Alpha 1
    "Denon AVR-3803A
    SPDIF: 560psec
    HDMI: 3700psec

    Onkyo TX-NR906
    SPDIF: 470psec
    HDMI: 3860psec

    Pioneer SC-LX81
    SPDIF: 37psec
    HDMI: 50psec

    Yamaha RX-V3900
    SPDIF: 183psec
    HDMI: 7660psec"
    Source: http://www.whatsbestforum.com/showthread.php?1151-Audible-Jitter-amirm-vs-Ethan-Winer&p=13207&viewfull=1#post13207
  5. mikeaj
    Those figures are well known (and often cited by some), but keep in mind they're theoretical and based on conservative calculations for what should be masked and inaudible based on other research on masking effects.
    Sometimes listening tests aren't as sensitive as they could be, sure. There could be more going on, but there could always be more going on. If you're going to throw out the listening tests run on the subject, then where are you going to start? Either the basis of concern is some conjectures or it's anecdotes. I don't dismiss it at all as an impossibility, but all the caveats and concerns quoted above have the same structure as the usual FUD arguments.
    Also, take a look at the output spectrum of most devices when tested. Those sidelobes and any peaks caused by jitter are frequently 110+ dB down from the original. Yeah, you can't consider it like typical harmonic or intermodulation distortion (it might be very much easier to detect than that), but consider how low it is relative to the signal. Also, how low it is relative to the noise floor of any typical music recording, not to mention the playback equipment. Some devices misbehave a lot more than that though.  Okay, if it's 7.6 ns and peaks of -80 dB, and I'd start thinking you could be on to something too—in the sense that I wouldn't be quick to dismiss it, not that it's important relative to other factors that people should care about more—but that's only on the basis of Benjamin and Gannon. Not like I know for myself for sure.
  6. bigshot
    There was an AES listening test that isn't mentioned there. I linked to it in the Ear Specs thread. (Thanks to MikeAJ for the link.)


    Just Detectable Threshold in Music 20ns
    http://www.aes.org/e-lib/browse.cfm?elib=8354 (needs subscription)
    http://www.nanophon.com/audio/1394_sampling_jitter.pdf (cited in section 2.2)

    Note that this figure is for 17kHz. As the frequencies go lower, jitter is even less of a problem. It's also useful to google time measurement and familiarize yourself with exactly how much a nanosecond or a picosecond is.

    The level of jitter as it occurs in most modern audio equipment is inaudible. Something has to be major league messed up for jitter to reach audible levels.
  7. Theta Alpha 1
    Normal jitter levels of HDMI audio can or will cause lossless to have performance like MP3.  That is a concern, especially if you want to try comparing lossless to MP3, or 16/44.1 to 24/96, or one D/A to the next and so forth.
    "This means the Yamaha creates reduces your signal to noise ratio from 96db for 16-bit audio to 80db or 13 bits of resolution. You decide if you want to pay to get 16 bits of quality or 13."  From http://www.whatsbestforum.com/showthread.php?1151-Audible-Jitter-amirm-vs-Ethan-Winer/page3
    If less than 1ns does that then 10 or 20ns must be in some way audible, or if not, perhaps simply render other aspects inaudible (such as lossless).
  8. bigshot
    How can jitter sound like lossy audio? Two entirely different things. Not even on the same planet.

    When jitter is down -80 or -100dB it flat out doesn't matter. You would have to turn your stereo up to ear splitting volumes to even get close to hearing it. That range isn't "low level detail". It's inaudible. You're reading the wrong side of the argument in that discussion.

    Redbook *is* overkill.
  9. Theta Alpha 1
    Lossy reduces bit depth.  Jitter reduces bit depth.
  10. mikeaj
    Isn't that spectrum for the 7.6 ns jitter over HDMI for that one receiver? That's not "less than 1 ns" doing that...? Most devices look more like that graph without the spikes at 7k and 13k.
    Xonar Essence STX sound card, output signal -6 dBFS
    Audioquest Dragonfly, output signal -6 dBFS
    HRT Music streamer HD, -6 dBFS output signal
    Okay, maybe 70 bux Chinese audio is still way too high end.
    Finally, here's a pretty low-end C-Media CM102 implementation. Might be onto something here (then again, it may have other more concerning issues too).
    I'm not so sure it makes sense to group together "HDMI audio", considering that the same thing you're citing lists one Pioneer receiver with 50 ps of jitter over HDMI (37 ps over S/PDIF). But actually, that's a good find because I wasn't aware that apparently it's not at all uncommon for receivers to have above 1 ns jitter on HDMI, that much worse than over S/PDIF... regardless of whether or not it matters. And also, comparison to mp3? Why? The mechanism, distortion produced, etc. are all way different. Also, mp3 with which encoder, which bitrate? None of these things are really like reducing bit depth in the usual sense.
  11. jaddie
    +1...totally different.  
  12. bigshot
    Jitter is a hoodoo.
  13. jaddie
    Hey Bigshot, you're familiar with an analog tape error called "scrape flutter", right?  High frequency longitudinal speed variations caused by tape moving over stationary guides with large unsupported areas of the tape path.  It caused frequency modulation of the audio with a modulation signal made up of a band of noise usually around 5KHz.  So, 5KHz FM of audio.  It places a kind of noise around audio signals, and would be come audible if it's bad enough around sounds that were close to pure tones.  The noise would sort of track the signal, would sound like it was amplitude modulated, kind of like bad dbx noise reduction,  but the cause was really FM.  That's a cousin of jitter, though to a much higher degree.  It's why the better reel to reel machines used a lot of roller guides.
    The difference was, tape noise and distortion covered a multitude of problems, so when we reduced scrape flutter to a level that was inaudible, it was still many times that of typical jitter.  We don't have noise at -55dB to cover anything anymore.  
    I'm not trying jitter say it's always audible or not, just drawing a parallel, though I admit the lines are miles apart.
  14. Theta Alpha 1
    Sorry, I thought it was 0.76ns, noted.
    Yes, it's for one receiver, but the flaw seems to relate to something within HDMI itself and the Yamaha costs $1,899 USD with 7.6ns jitter, so I'm inclined to think cheaper HDMI audio has weak SNR and bit depth due to jitter, so much for DVD-Audio then?
    The article told me so.  His analogy is convincing since you do need to train a lot to hear the differences in lossy versus lossless, such as MP3 320kbps versus FLAC which is mostly irrelevant to playback but has been ABX'd by those which care enough about the differences to prove it.
    He writes...
    "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."
    Personally I almost gave up trying to compare lossy versus lossess, but I know there's a niche crowd that practice listening to audio equipment or codecs night and day instead of listening to music.
    Thanks for the pictures of the DAC noise floor measurements, which are convincingly low, however as I understand it the low noise floor relates to random jitter, and not periodic or program related jitter which cause time errors?
    1. Random jitter. This raises the noise floor and reduces the dynamic range of the system. At high enough level, it reduces fidelity of the system but it is not as bothersome as other forms below. All of your testing and papers you have cited fall in this category.

    2. Periodic. This is one more more pure tones which change the signal timing. This could be the USB frame buffer timing, power supply noise, front panel high voltage oscillator, video clock related (e.g. as in HDMI that has video as master timing), etc. This can be more audible as each one of these tones modulates the music and creates sidebands that could fall within the music levels especially at high frequencies. I have shown examples of this type of jitter and consider it a potentially audible problem (depending on frequency).

    3. Program related. This is jitter that is self-dependent on the signal itself! A good example is cable induced jitter. A poor digital audio interconnect, changes the shape of the pulses, causing the time they are accepted by the receiver to change. This again from Julian's book:
  15. bigshot
    Read Ethan Winer's comments again

    Jitter is a timing error. Lossy compression is based on psycho acoustic principles and varies by the bitrate setting. Two totally different types of artifact, and two totally different reasons for the artifact to be occuring. Lossy artifacts are not subtle. You don't need to learn to hear them. They're either there or they aren't.

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