The reason I say this comes not just from measurements and my own listening to devices with different severities of sampling jitter, but also experimentation over the years in simulating the distortions introduced by jitter.
As I mentioned last time, with Yamamoto2002-san's WWAudioFilter, we can easily use DSP to introduce fixed amounts of sinusoidal periodic jitter to "bake in" the kinds of sideband anomalies often found with devices that suffer from jitter. For today's post, what I want to do is provide some test files you can use to actually hear what severe jitter distortions with sideband anomalies sound like. Think of this post as similar to one years ago when I demonstrated what poor USB cables sound like.
Furthermore, coming out of this article, I hope the audiophile reader can appreciate the magnitude of jitter that is necessary in order for the effect to become audible. In so doing, I hope it will help us appreciate the results from the J-Test FFT's I publish here and also when you look at measurements elsewhere (like on Stereophile).
First, remember that jitter is a complex phenomenon with many potential forms. A "primer" on the subject can be found from Silicon Labs describing jitter in devices of much higher operating frequency (MHz and GHz) than just the audible range. Consider for example the diagram at the start of the "primer" describing the "classes of jitter" we can think about and how they might manifest in our audio systems.
Remember that jitter is a timing anomaly and multiple elements can either cause or contribute to this abnormality as discussed in that PDF. Back in the old days, analogue timing anomalies in the form of "wow and flutter" were much easier to hear and describe because typically the variation is of low frequency and leads to obvious pitch fluctuation or "wobble" to the sound. The ease of hearing this is probably why there's less controversy; once you hear it and know what it is, we can all just talk about it without there needing to be any lack of clarity about the phenomenon. Digital sampling jitter however is different. Jitter effects obviously exist in the DAC's analogue output but the tiny timing inaccuracies from crystal oscillators affect higher frequencies more severely (where auditory acuity is not as sensitive) and the frequency anomalies show up in a much subtler way than slow wobbly frequency shifts off vinyl playback for example (we can easily measure this wow & flutter anomaly as previously discussed).
In the audiophile world, the J-Test we commonly see in measurements is based on Julian Dunn's work from the early 1990's created to measure the AES3 (adopted for S/PDIF coaxial or TosLink) digital interface jitter (see the linked paper for rationale around the test development). While not a S/PDIF-based device, as an illustration of what it typically could look like, here's a J-Test result from 2013 taken from an old 2008 15" MacBook Pro's headphone output:
The diagram above is a typical 16-bit Dunn J-Test. One of the give-aways is that with a low enough noise floor, the jitter-modulation tone which is a 225Hz square wave in the LSB (least significant bit) shows up as a harmonic sequence as you can see with the numerous peaks to the left used to stimulate jitter abnormalities. Also, 16-bit J-Tests have the primary signal sitting at 11kHz (whereas the 24-bit J-Test based on 48kHz sampling rate has the primary signal at 12kHz).
At the base of this J-Test primary signal is the "skirting" often talked about. This potentially represents some low-level timing imperfection showing up as noise. Perhaps with a higher resolution FFT, we can zoom into those frequencies and make out the difference between periodic jitter presenting as tiny sidebands as compared to random jitter with a typical Gaussian distribution. Remember also that the measurement system has some intrinsic jitter that can contribute to this (the FFT above was captured with my old E-MU 0404USB and the AKM AK5385a ADC chip). On either side of the primary signal for this MacBook Pro at +/- ~2.2kHz in blue, we see a pair of "jitter modulated sidebands" which are likely the result of periodic jitter which includes "sinusoidal" jitter owing to the undulating shape of the subtle timing irregularity.
Notice I also have a yellow question mark on the graph. We don't see a clearly elevated mirror spike on the other side of the primary signal to suggest the presence of a sideband pair. This could be "periodic spurious noise" or a "spur" likely correlates to something in the computer's circuitry. I'd need to go back to the MacBook Pro and examine when this noise shows up to understand whether this is some kind of constant noise related to the computer's components or perhaps jitter at a single offset frequency triggered during playback.
Having said this... Since most of the time, jitter (like for the MacBook above) presents itself strongest as sidebands, the question obviously is - "what do these jitter-modulation sidebands sound like"!? To answer that question will allow us as audiophiles to take a step forward in being able to perhaps identify the effect in our hi-fi systems when we hear them, and also consider whether descriptions by others such as equipment reviewers actually could be attributable to jitter effects. For example, I've heard audio reviewers claim that things like "three-dimensionality" deteriorates with jitter, or that jitter "reduces the air" around instruments, maybe "changes the timbre of sounds", or "blurs the transients". Are these descriptions really what jitter - specifically jitter-induced sidebands - do to the sound of actual music?
Let's cut to the chase. Using WWAudioFilter, I created files with simulated jitter sidebands to listen to. I used a number of samples to show off variations depending on the underlying signal / music. Here are 4 sets of audio files (archived as RARs - 13-115MB) for you to download, listen to, and consider.
1. 20-20kHz Sine Sweep Jitter Test - as the name implies, this is a simple sine sweep from 20Hz to 20kHz over 30 seconds.
2. Schubert String Jitter Test - 90 seconds excerpt downloaded from 2L Hi-Res Test Bench of "Schubert String Quartet No. 14 in Dm", performed by the Oslo String Quartet (from The Schubert Connection). I downsampled the 24/96 file to 24/44.1 prior to employing the DSP processing.
3. Piano Improvisations Jitter Test - 90 seconds excerpt downloaded from 2L Hi-Res Test Bench of Ola Gjeilo's "Piano Improvisation" (from Piano Improvisations). I also downsampled the 24/96 file to 24/44.1 prior to employing the DSP processing.
4. London Nights Jitter Test - how about non-acoustic studio productions? Here's a test set using a pop/disco recording from 1989 :-). This is 90 seconds of London Boys' "London Nights" (DR10, from The Twelve Commandments of Dance). This recording is from the 16/44.1 CD rip run through the DSP to 24-bit output prior to dithering.
As usual, I provide these short musical excerpts under the provision of "fair use" for the purpose of education, research, and discussion. If you enjoy the music, buy the recordings!
When you open each of the RAR files, you will see each set consists of 10 "versions", each with a certain total number of nanoseconds of sinusoidal jitter distortion applied. To manage file size, I dithered the 24-bit output down to 16-bits; trust me, you don't need 24-bits resolution to hear the magnitude of the effect being introduced. I have checked the files to make sure that the DSP did not overload and add distortions. To give you a visual sense of the amount of distortion I've applied to these test files, let me run through each setting used and show the severity of sideband distortion using the 16-bit J-Test FFT as graphed using Adobe Audition CS6 (65536 bins).
A. No simulated jitter added (0ns)
|DSP settings for WWAudioFilter.|
|16-bit J-Test (16/44) full spectrum FFT.|
B. 0.875ns (875ps)
Before talking about nanoseconds even, let's start with a few hundred picoseconds of cumulative sinusoidal jitter "baked in". While I'm taking some liberties and simplifying things, this level of sideband distortion from sampling jitter is around the order of magnitude of some relatively poor quality CD players and DACs. If we look through Stereophile reviews over the years, we see some machines with cumulative jitter sideband anomalies similar in value or a little higher - Esoteric SA-60 from 2007, Emotiva ERC-2 in 2012, Philips CDR880 CD-R/RW recorder in 1998, Burmester Reference Line 970 DAC & 969 CD Transport from 1999 . This level of jitter is in fact considered "moderate" already with digital players. Most CD players actually measure below a cumulative 400ps when Stereophile was still using the Miller Jitter Test to calculate a number (for example, here's the Squeezebox Transporter - estimated between 235-313 ps only).
For these simulations, I've chosen to create 3 sideband pairs for you to listen for. +/- 225Hz which is not uncommonly seen in many devices. A +/- 1125Hz pair. And then finally a pair +/- 2475Hz which is similar to what we saw with the MacBook Pro above. Notice that the strongest amplitude pair is the most proximal and it decreases going outwards. This is often what we see with actual equipment measurements. Of course, these frequencies are nonharmonic tones of the primary signal.
The most prominent sidebands are at an amplitude of about 92dB below the primary 11kHz peak. As you can imagine, this is very low amplitude and I'll leave you to mull over whether you believe your ears/brain would be able to hear this at normal volumes and of course listen to the music and sine sweep and decide for yourself. For completeness, also do not forget the importance of masking; those +/-225Hz sidebands would likely be much more difficult to hear than then +/-2475Hz pair.
Finallly, remember that at this sub-nanosecond level of simulated jitter, your playback equipment could hypothetically exacerbate the effect if your DAC introduces its own significant amount of timing inaccuracy.
Compared to essentially all DAC or CD players out there, this would be quite severe jitter sideband distortion.
The amplitudes of the sidebands are -80dB below the peak signal in the proximal pair and -90dB for the pair ~2.5kHz away. We have seen inexpensive Walmart-consumer-level devices like this "lo-fi" Sony boombox perform at around this level of distortion on J-Tests.
By the way, one of the worst performing devices over the years in Stereophile was the Zanden 5000 Mk. IV/Signature which measured a cumulative value of 4.4-4.9ns - just a bit over the amount here. Another device recently measured with sidebands of this magnitude appears to be the Aqua Formula xHD (NOS, R-2R DAC, remarkably poor technical performance for 2018!) with sidebands down around -80-90dB range using the 16-bit J-Test.
Getting more intense now with stronger sidebands. Again, if we look at the 16-bit J-Test results from actual DAC and CD players, this amount of sideband intensity is very strong! I would imagine only some of the very worst devices could even "hope" to approach this level of temporal inaccuracy.
And it gets worse! The sidebands are now intense enough to "spill over" to involve other frequencies beyond the 3 pairs (these are called "second order sidebands"). The strongest sidebands +/- 225Hz are now just about -40dB below the primary signal peak.
Okay, now we're getting really serious :-). Just shy of 1 microsecond (1000ns) of cumulative jitter among the sidebands. Only -32dB between the primary 11kHz peak and the strongest +/-225Hz sidebands with 500ns in that component alone. Looks really scary and bad...
Okay, another jump. Now we're at almost 2 µs of cumulative sinusoidal sampling jitter added by the 3 DSP filter actions! Seeing much more spill over and frequency effects. Hmmm, how bad does this sound?
Ugly folks. Almost 9 µs equivalent worth of sideband distortions across the audible spectrum. By this point the strongest sideband amplitude is only about -12dB down from the primary signal. I don't think anyone would be surprised that the effect on music fidelity will be strong.
Arrrrgggggghhhh... Anomalies present essentially across the audio spectrum now!
The worst audio system ever type of distortion by this stage! Simply a brutal, noisy mess with a total of 35µs sinusoidal jitter simulated by the DSP filter operations. Although the frequencies are still "centered" around the 11kHz primary signal, notice that the 11kHz tone amplitude in this test signal itself has been eclipsed by the sideband amplitude. I'm obviously pushing the distortion to extremes here for demonstration.
So... What does this sound like?
That is the obvious first question to ask yourself when you listen to the simulated jitter samples:
- How would I use words to characterize the effect?The second obvious question then is - "At what level of simulated sinusoidal jitter severity do I start hearing the abnormality?"
- Do the typical audiophile adjectives referring to things like "3D soundstage", "veil", "blurring", "dynamic range", "resolution", "air", "presence", "noise" apply?
- Do I notice a difference in quality depending on the type of music and instrumentation?
- Can I already hear it at the lowest 875ps level?For me, I cannot hear a difference with the 875ps or 3.5ns cumulative distortion tracks even though those first couple steps already incorporate more sideband distortions than the vast majority of devices I've come across! Sitting at my computer, with Audio Technica ATH-M50 closed headphones or 1MORE Quad Driver IEMs using my ASUS Xonar Essence One DAC, the lowest level where I could reliably hear the anomaly with the 20-20kHz synthetic sweep test is the 175ns sample. (Haven't tried with my Sennheiser HD800 yet, but will at some point.)
- It looks pretty bad already when I look at that 17.5ns J-Test FFT! How bad does it sound when listening to the music?
- Can I hear a difference between headphones and speakers?
- If I were to blind test or ABX these files (like using foobar ABX comparator), at what level would I confidently hear a statistically significant difference?
As for the real musical samples, for me, the effect is different with each of the tracks... For the "Schubert Strings" track, I notice a slight "nervousness" with the sound of the high frequencies and a "grittiness" starting at 875ns which progressively gets obvious. I found detecting the anomaly more difficult with the "Piano Improvisation" sample on my headphones (a bit more reverb to the track masks the distortion). What I did notice by 1750ns was a gradual deterioration of the piano decay with a "hardness" to the sound and this became very noticeable of course by 17500ns and 35000ns where the "dysphonic" fuzziness exerted themselves strongly!
For "London Nights", the first few notes (Big Ben quarter bells chime) provided an opportunity to hear the distortion. Remember that digital sampling jitter manifests more strongly in the higher frequencies. For me, those synthetic bells in the pop/disco song were subtly affected by the 875ns sample and became obvious by 8750ns with the harsh overtones. Once the song gets going with all kinds of sonic layers, the distortion became more difficult to tease out although it should be evident here and there throughout the song obviously once we hit the 17500ns sample amount.
The fact that the abnormalities can be heard easily at 175ns with the synthetic sweep while needing higher values like 875ns+ with real music is not surprising. Recordings have a higher noise floor, and when distortions fluctuate like these sidebands depending on the signal, we can appreciate them much easier with simple test signals where we can predict the sound (whether single tone or sweep) and "lock on" to anomalies.
What has always struck me, not just here with these samples but over the years as I have listened to other tests of jitter is simply how much is required before I can put my finger on the distortion. For me, it certainly isn't at the level of a cumulative tens or hundreds of picoseconds as typically found in decent digital gear. Instead even with synthetic tests at normal volumes (the 20-20kHz sweep test is very annoying at higher volumes), I need something like 100ns using the frequency distribution of the sidebands used here to appreciate the distortions! Needless to say, this level of jitter distortion is orders of magnitude worse than the vast majority of CD players and DACs out there.
ConclusionsWell, I hope you have fun with these test samples and take the opportunity to play around with WWAudioFilter yourself (excellent work Yamamoto-san). Try it out with your own music, maybe using other settings for the frequency of the sinusoidal jitter added. Remember that a larger frequency displacement (like +/- 5kHz) from the primary signal will make the distortion more audible but this is rather unusual for real equipment. Typically, I've seen sidebands show up less than +/-4kHz on either side of the J-Test primary signal.
A few summary points then:
1. You can now say that you have an idea of what periodic jitter that causes sideband distortions as shown on J-Test FFTs sound like. Remember that there are other jitter-related effects and the sonic anomaly will change depending on the sideband frequencies. Random jitter can elevate the noise floor potentially globally but also in a Gaussian distribution. Single spurs are also possible with certain DACs and equipment.
2. You've now also heard the effect not only with a test signal but with actual music. The effect varies depending on the audio signal and I think you can appreciate that the subjective nature of the jitter distortion is a bit different between the string orchestra, to piano, to pop music. Related to this is that if music is poorly recorded with low dynamic range and high noise floor, you're probably unlikely to hear jitter distortions at all even if high. This latter point is why I used the old pop/dance tune "London Nights" (DR10) from 1989 rather that typical DR6-level dynamic compressed modern masterings.
3. Speaking of sideband intensity, remember that for digital sampling jitter, higher frequencies are affected more. You can see this if you look at the FFT of the sine sweep sample as it moves from low to high frequency and the increasing level of the sideband "lobes" in relation to the steady primary signal amplitude:
This is probably another reason why it can be hard to "pin down" what jitter "sounds like". Also, because jitter affects higher frequencies more, as we get older, the high frequency sideband may be less noticeable as high-frequency hearing acuity decreases for us guys especially.
4. You've now also experienced the magnitude of effect that is required to make the distortion audible. This is perhaps the most important point in this whole exercise. For me, even with a synthetic sweep, it took >100ns of cumulative sinusoidal jitter in the frequencies used (+/-225Hz, +/-1125Hz, +/-2475Hz distribution with decreasing amounts for each as above) before I could hear distortion! This is obviously a level of sideband distortion significantly beyond what we see with CD/DAC measurements. As I said a couple weeks ago, finding jitter distortion is certainly not impossible out in the wild... But it will be like looking for the giant panda. You'll only rarely find it when you know where to look.
Over the years, I have discussed my suspicions about devices like "regenerating" USB hubs, "purifiers", passive filters, doubts about digital cables of normal length supposedly capable of reducing jitter, and I've read articles from folks who seem to find (and hear) jitter everywhere! The designers and manufacturers have had every opportunity to demonstrate that these devices are able to do something to improve sound quality. The fact that they don't show objective results for their products and in fact essentially ignores calls to produce evidence when confronted is telling (time for the snake oil detector to trigger!).
I trust that after listening to these test files and reviewing the graphs, it's not hard to understand why I have a hard time getting too excited (worried?) about jitter these days with decent digital audio gear. Obviously, I want equipment that reproduces the audio to have minimal distortion and ideally engineers should be striving for jitterless DACs. As we've seen recently with measurements of the Oppo UDP-205, we're already at a level that IMO no human being could complain about with good gear at reasonable prices! Beware of unnecessary fears, uncertainties, and doubts published in the "high end" consumer audiophile world to perpetuate the scary jitter boogeyman and attempts at selling equipment to ward off this apparition.
One last thing. Because humans are subjective creatures with idiosyncrasies, realize that it is possible that jitter might actually "add" to the sound quality and be subjectively preferred by some. For example, the limitations and distortions of LP playback are probably why some (many even) folks like the "analog" sound. As I mentioned above, there are a few digital devices like the Zanden DAC and certain NOS R-2R DACs that perform relatively poorly in regards to jitter. Another device like this would be the Audio Note NOS CD players. I think it's worth considering whether a higher but not extreme level of subtly audible jitter may add to why some audiophiles prefer the sound of these devices. While not technically ideal, nor "high fidelity", the distortions could be "euphonic" to some. Of course, I have never come across anyone agreeing to such a thing - after all, who wants to admit that they actually like the sound of jitter distortion!? :-)
BTW: If you want to read a good research paper on jitter audibility, have a look at this one from AES 1998: Benjamin & Gannon "Theoretical and Audible Effects of Jitter on Digital Audio Quality". Thresholds of audibility in the review are in the tens to hundreds of nanoseconds with more sensitive test material, better with trained listeners (no surprise).
Behold! In house now. Pillar hardware for the Archimago's Musings' "Generation 3" audio testing. :-)
We'll talk about this soon.
Have a great August, everyone... Hope you're enjoying the music!
Yeah something like the ADI-2 Pro FS is necessary to crush the Focusrite Forte according to the reviews I read.ReplyDelete
Don't know if it is relevant or not, maybe you knew this already...
About jitter... isn't Stereophile also agrees that a $25 PlayStation 1 can have pretty low jitter while a $6000 McIntosh can't?
Thanks Dtmer for the info.Delete
Certainly one of the things that impressed me about the RME ADI-2 Pro is the fact that it was designed with the folks using it to perform measurements in mind. I've checked to make sure I have the latest firmware updated so this should be all good :-).
Just starting to get it integrated into the sound system and listening to it as a DAC first... As usual, will put a few posts up on the device in good time :-).
Oh yeah, I forgot about the "case of the jitters" Stereophile post from awhile back!Delete
Man, that McIntosh media server is nasty - 14ns cumulative jitter but spread over quite a few sidebands with large skirting +/-1kHz. "Nasty" from an objective perspective of course.
The issue of course is why have audiophile magazines made jitter out to be such a big deal. Would that 14ns McIntosh measurement be audible in an A/B test? And if the McIntosh does sound inferior, would be necessarily point the finger to any significant degree on the jitter result?
Yup, the PS1 has low jitter which I found also!
One more thing Dtmer, from the "A Case of the Jitters" article you linked, last page conclusion:Delete
There is no consensus about what levels of jitter in a digital product's output are acceptable—the audibility will depend on both level and spectrum. Some authorities also insist that the ear will tolerate relatively high levels of jitter, up to a few nanoseconds, though that has not been the experience of this magazine's writers.
Wow, the "magazine's writers" (notice plural) have as a collective group rather impressive golden ears! I don't believe I would be about to hear a difference with less than "a few nanoseconds" of equivalent jitter, especially with music.
Quite the remarkable claim with no evidence. Let's just say I don't believe 'em :-).
Sure. I listened to your test files and my hearing is no better than yours. Also, I wonder why some self-proclaimed objectivists (e.g. NwAvGuy) have such a strict requirement for jitter:Delete
The last graph just above the comment section is his claimed threshold of audibility (the green line). While such a target is easy to achieve even for budget devices, it is definitely not my threshold of audibility.
I dunno about that green-line limit. Can't say I've ever seen that threshold mentioned anywhere else in the literature. As you noted, not terribly difficult to achieve these days with decent (even budget devices) for the last decade plus...
Hey Archimago, I just discovered a very interesting thing:Delete
Look at AP's Apple Pi (blue trace), apart from the skirting, I can almost exactly duplicate PCM1794's distortion pattern from 14-20kHz with a completely different device, it looks like a miracle to me! So it means while the Apple Pi's skirting suggests some real jitter, things show up in J-Tests are not necessarily jitter and no matter how good the "clock" is, you can't remove them at all!
you probably know this article alreadyReplyDelete
I remember looking at that fantastic piece of work a few years back... Certainly the kind of technical paper essential for all the audio designers out there! I miss the Audio Critic.
Hey, are you "the" David R. Moran of the Meyer & Moran Boston Audio Society listening test paper from 2007?
Very revealing tests again, thank you Archimago! So now we know that jitter simply adds what is perceived as distortion and also some parasitic sweeps with the sweep test. Nothing to do with "presence" or "air"...ReplyDelete
I have more difficulty hearing the effect in the sweep test than in the Schubert or piano (the fourth one I can't listen to for more than a second...sorry!)
Maybe because it's difficult to listen to the sweep test at the same volume as the other ones, I begin to hear the parasitic sweeps only at the 875ns level. For the other two, I can hear the effect at 175ns, a grittier sound in the soft notes for the string quartet, and some superposed distortion on the decay of the first soft note for the piano. It's harder to hear for louder notes because the sustain pedal causes a lot of sympathetic resonances between the notes that are hard to separate from the effect being added.
Anyway, as you write, current DACs have no such problems. I wonder though about older ADCs. I have an old set of CDs (1984) of the Melos quartet playing early Beethoven quartets, and I always found the sound very hard and gritty, even though this is a first rate quartet, in comparison with your very smooth sounding Schubert. I wonder if ADCs of that period had jitter problems or if it simply due to recording too close from the strings (normal bowing can be gritty). Your Schubert is recorded further away. What do you think?
As usual Gilles, great ears ;-).Delete
Yeah, turn down the volume a bit on the sweeps and have a listen. For that one I can hear the parasitic distortion at 175ns. Yeah, as for the others, I can appreciate that added "grit" as well certainly by 875ns but harder for me to put my finger on at 175ns.
Good question about the old CDs like the ones from 1984. I dunno... Maybe some of the technical folks who are versed with stuff like the old Sony PCM-1630 can tell us. Was the Melos quartet pure DDD back in 1984?
Even if pure DDD I suppose one could also wonder what kind of processing the sound went through and the transparency of the editing and mastering technology back in those days.
Yes its pure DDD. Could be the microphones also. They have improved a lot since that time.Delete
Congratulations to that fine new device! I use the previous non-FS-named version [the only differences lie in SteadyClock FS and AutoDark] and it is by far the finest audio interface I ever got.ReplyDelete
Exemplary about RME is also that they continuously update its firmware, so you might want to check on their Forum occasionally. Regarding the latest release 177/87, there is one bug you might want to keep in mind, until fixed:
Thanks Daniel for the info...Delete
I'll probably keep the initial measurements at 192kHz like I have over the last few years before venturing into the 352+kHz territory until the new firmware.
Nice to see companies update firmware regularly instead of saving features that could be done on existing equipment only to be implemented in the next iteration of hardware.
You are welcome, Archimago.Delete
Indeed: since the device is on the market (2016), there are regular firmware improvements (+ updated manuals); some even of major impact. That is great, as you said! Just for the 'SteadyClock FS' and 'AutoDark' [introduced with the later ADI-2 DAC] they needed to upgrade the PCB; hence the new ADI-2 Pro FS.
Btw, headphone EQ with an optional miniDSP EARS works quite well - no more need for Sonarworks. :-P
All the best!
I really like the incorporation of that EQ into the device. Makes sense to have features like this "all in one" on the DAC side for those who want a more advanced device!
Started to do some tests on the ADC... Wow. ;-)
Thank you for using WWAudioFilter.
This jitter adding function is implemented using time domain Sinc interpolation resampler and it may generate inter-sample peak distortion if output sample value magnitude exceeds 1.0 ! (WWAudioFilter FLAC writing stage maps 64bit float to 24bit integer with the scaling of 1.0 ⇒ 8388608. 24bit integer can express -8388608 to +8388607 and out of range value is saturated and it causes distortion) Therefore, in some cases it is better to reduce gain about -6dB before adding jitter to prevent undesirable artifact :)
Again, great work man, and I hope you stay cool in Japan this hot summer (looks like it has cooled over the last few weeks)!
Thanks for the reminder about potential overloading. I checked the samples I used and it looks good (or at least if there is overloading, very minimal) since each of the tracks had a bit of overhead for the DSP operations.
Keep up the great work ;-).
Your numbers have good correlation with the 2005 research publication by Ashihara et al in Acoustical Science & Technology.ReplyDelete
In which the audible threshold of jitter turned out to be 300,000 picoseconds. Yes, not a typo. And taking into account other experiments, the researchers are happy to say with confidence that one tenth of that number is surely inaudible. This is very distressing for the high-end audio industry, because it means the 1983 Sony CDP-101 and every CD player and mainstream DAC since then has inaudible jitter. This also distresses high-end audio industry consumers (aka audiophiles), so between them these two stakeholders have invented the usual stream of excuses why they can ignore the research -- starting with "I trust my ears" (no comment) and ending with "the researchers didn't take into account which types of jitter they were producing, and we don't have measurements for each type of jitter at present, so they might not have been producing the most audible jitters" (plausible but utterly unproven, and, by the very wording of the statement, unprovable at present).
Note that Julian Dunn in the late 90's wrote a paper on jitter, where he said the higher the audio frequency, the lower the jitter needs to be to avoid audibility, starting at 900,000 psec at 20-250 Hz and finishing at 10 psec at 20,000 Hz. Now, before we get excited, we need to understand that his criterion for audibility was insane: when the music signal plays at 120 dB, he assumes you can hear the jitter at 0 dB. Good luck with that. Good luck with playing 20 kHz at 120 dB and even staying in the room. What Dunn was doing was setting a standard that couldn't reasonably be disputed. He never conducted listening tests to validate it: he just wanted to set a standard where he could say "meet this with your hardware and no one can doubt you". But, more importantly, if you want the findings of placebo-free listening tests, see above paragraph where I discussed Ashihara et al.
I think reality lies somewhere between these two. Dunn's numbers are extreme and clearly involve a margin of safety that might turn out to be very generous. The research finding of 300,000 picoseconds audible threshold might never be bettered, but there doesn't seem to be enough validation going on -- at least not in locations I can find. Nevertheless, applying a 10x margin of safety, we still get 30,000 picoseconds, which is way more than any commercial player, from the very first.
So your validation is very interesting. Thanks for doing it.
Thanks for the detailed note tnargs...Delete
That's 300ns threshold and inaudible at 30ns "with confidence" :-). I can totally go with that.
Yeah, the implications of that data is scary for Industry "stakeholders". It takes away one of the frequently trotted out explanations, mechanisms, excuses with which the advertising department can use and certain goldenears hold on to as faith.
There's also a mystique about jitter which is harder to explain than most things in audio and can therefore be used to describe almost any dysphonic sound that an audiophile claims to hear... "I heard a harshness/flatness/veil/lack of pacing/poor transient response/loss of air which likely is the result of jitter."
Obviously not saying that reviewers and writers didn't hear something bad. But I think the audiophile world needs to start realizing that jitter was never the scary thing some made it out to be!
>There's also a mystique about jitter which is harder to explain than most things in audio and can therefore be used to describe almost any dysphonic sound that an audiophile claims to hear...Delete
Because jitter instead of wow and flutter, similar to digital filters, are related to digital systems. So you know what those hardcore analog enthusiasts are thinking about.
I am happy that mansr also posts here since I am not going to join CA. See his digital filter listening test results:
The most funny comment I read is someone said cubic interpolation resembles Mojo. Don't know how Rob Watts thinks after reading that. Basically reading all those comments gave me headache, caused some "ringing" or "aliasing" in my brainwave.
Amazing work from mansr!Delete
Massive almost 3GB test samples there, wow... Grabbing them myself to have a listen. This is gonna need a pretty modern DAC capable of 705kHz to listen to the full signal!
Yeah, the quick cubic interpolation comments are clearly the most interesting.
Clearly over the years, Chord likes to play the numbers game with # of taps for their digital filters. From what I can tell, the sequence goes something like this:
DAC64 (2001) - 1080
QBD76 (2008) - 14,832
QuteHD (2013) - 10,240
2Qute, Hugo TT, Mojo (~2015) - 26,368
DAVE (~2016) - 164,000
Hugo 2 (2017) - 49,152
Blu Mk.2 (2017) - 1,015,808
Hugo M Scaler (2018) - 1,015,808
Hugo TT 2 (coming 2018) - 98,304
Of course, oversampling amounts varied over the years with these devices. Surely there must be some point of diminishing returns in there. Maybe Chord and MQA can get together somewhere in the UK and help us decide whether very short (MQA) or very long (Chord) tap lengths are "needed" and what does or does not make a "significant" difference.
In the meantime, we can just enjoy something like the higher quality SoX filters with <5000 taps for free :-).
As an old fart who today bought two new CDs (David Byrne and Beck's most recent), this is all above my pay grade. I confess I haunt these precincts in dread of the day someone reviews a new format and says "Now THIS smokes 16/44 all to hell!" I may have to resort to bank robbery then.ReplyDelete
Still, I'm not a total ninny, and I appreciate the work you do, if only because I'm a disabled veteran of the Loudness Wars. I understand fully, having spent the equivalent of a Third World country's national GDP on equipment and cables in the '90s until I picked up a copy of The Audio Critic at a magazine stand (Remember physical magazines? You may be a bit young for that...), I did witness the revelation of the Jitter Problem, which drove some of us to frankly embarrassing amounts of investment. I remember Kimber Cable sold a digital cable (The Orchid(?) - designed by Chris Somebody) for about $300 US/gullible meter that Stereophile swore rendered any other digital cable as useless as an Orgone box for reproducing 1s and 0s at the right time. And yeah, I bought one. And I could SWEAR that it had more "air," "PRAT," and "presence" than, well, anything I'd ever heard.
Bollocks, of course. I talked to one guy who knew Chris Something (and no offense to Chris Something - I have no doubt he was sincere, and heard all those improvements), and Chris Something told him he used a minimum of 25' of this cable himself to ensure he was getting the cleanest sound. Even I wsn't that stupid.
Jesus (no offense to Jesus, while we're at it), this is what I was being told was the gospel. And as a naive, babe-in-woods of only 47 or so, I bought it and, worse, HEARD it!
Okay, end of old fart reminiscing. I will close by saying that jitter was up there with Orson Welles' UFOlogy for scientific validity as an audible phenomenon. Obviously, extreme amounts of jitter are audible and bad, just like radiation. In other words, if you expose yourself to massive doses, bad stuff will happen, but for the everyday dosages, meh.
Otherwise, thanks for another outstanding article. I don't pretend to keep up with computer audio and home recording, but I can still smell bullshit when it wafts near my presence, and I have never, NEVER detected it in your posts. And even to an old idiot such as myself, a lot of what you write penetrates and informs.
I suspect you are just as conscientious and honest in your day job as you are here, and that many people are better off for having been touched by your work. Keep it comin', Bro!
I have strong impression that jitter audibility is much better in case of properly made speaker configuration due to phase troubles. This can be really hard to hear in headphones.ReplyDelete
I'm not sure if that about jitter, but i've played with PLL settings of my Pioneer DAC and if i set it much narrower than factory default foolproof (just one step wider than setting inducing dropouts) all 3d and spatiality really differs. In headphones it's almost the same, but with phase-aligned nearfield monitors difference is obvious. Maybe like adjusting analog PC monitor, connected by VGA...
It would be useful, if you could somehow evaluate audibility of spatial information if it present in record.
Jitter Speed TestReplyDelete