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Topic: (Not a) good explanation of jitter in TAS (Read 90458 times) previous topic - next topic
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(Not a) good explanation of jitter in TAS

Reply #125 –
I'm wondering - we have had two prominent engineers of a well-known, 'non-esoteric' high end DAC manufacture join this thread. Both have shown considerable insight into the matter and claimed hands-on experience with ABX switching equipment. Still they could not yet present anything verifiable regarding audibility.

How about some test samples or protocols?

(Not a) good explanation of jitter in TAS

Reply #126 –
Quote from: hlloyge on
But if they published ABX tests, they would be marked in the HiFi community as people who doesn't hold some sort of "true values". I've never seen ABX test results from HiFi companies that make DACs, amplifiers, CD/DVD players, and so on. Maybe I am wrong about this, but I have the feeling that publishing ABX results would actually undermine their efforts in making (and eventually selling) good DAC.

We are not afraid to publish specs and/or ABX test results.  Our manuals typically contain about 15 pages of graphs and 2 pages of specifications.  This is unique in the HiFi community (and often frowned upon), but HiFi is not where our roots are.  We began 26 year ago as a manufacturer of Audio for broadcast TV and radio.  In the broadcast market buyers live by specifications and test results, and they conduct extensive tests of their own before buying.  We sell the same exact products to broadcast, recording studios, and home HiFi.  As you can imagine this is an interesting mix!  Your forum is refreshing relief from the pseudo-science that is pervasive in the HiFi industry.  We attempt to bring this same sort of relief to the more down-to-earth customers in the HiFi industry.  We don't play the pseudo-science games.

As for publishing, time is always the issue.  Any time spent on publishing takes away from our product development resources.  Our research is conducted for the purpose of developing better products.  We are always willing to have others pick up where we left off and conduct well-controlled tests that are suitable for publishing.

The goal of our tests is to confirm that a problem may exist and then confirm that a proposed solution is more than adequate to remove audible artifacts.  We usually do not attempt to determine audibility thresholds as this is much more difficult, requires many more trials, and more human subjects.

We are considering the creation of an ABX jitter test that could be posted on this forum.  The test would allow comparisons between a jitter-contaminated track and an original track.  We will keep this forum posted on our progress.

John Siau
Vice President
Benchmark Media Systems, Inc.

(Not a) good explanation of jitter in TAS

Reply #127 –
Quote from: John_Siau on
We are considering the creation of an ABX jitter test that could be posted on this forum.  The test would allow comparisons between a jitter-contaminated track and an original track.  We will keep this forum posted on our progress.


That seems like a coincidence, since I don't expect you have written those four paragraphs within 3 minutes. I'm looking forward to your proposal.

(Not a) good explanation of jitter in TAS

Reply #128 –
Quote from: Pio2001 on
Quote from: John_Siau on
It was fairly easy to score perfectly on the ABX tests.  The CD players with modulation problems sounded like they had more midrange when played through their internal D/A converters than when played through the external DAC.


Interesting.

How did you match the playback levels of the DACs ?

The output of our DAC prototype was equipped with precision 10-turn trimmers that allowed us to match gains to better than 0.05 dB.  Left was matched to left on the CDP, right to right.  We used an audio precision System 2 to normalize the gains at 1 kHz prior to testing.  We also used only one input channel on the AP to insure that all channels were fed to the same analyzer channel on the AP.  We also shut off the auto ranging on the AP to minimize the gain errors that can occur when the unit auto ranges.  We used a CD with a TPDF dithered 1kHz -20 dB FS test tone for calibration.
John Siau
Vice President
Benchmark Media Systems, Inc.

(Not a) good explanation of jitter in TAS

Reply #129 –
Quote from: rpp3po on
Quote from: John_Siau on
We are considering the creation of an ABX jitter test that could be posted on this forum.  The test would allow comparisons between a jitter-contaminated track and an original track.  We will keep this forum posted on our progress.


That seems like a coincidence, since I don't expect you have written those four paragraphs within 3 minutes. I'm looking forward to your proposal.

Yes indeed, I read your post after I posted.

Here is what we could do with the equipment we have (comments and suggestions welcome):

Jitter ABX:

We have low-noise 96 kHz 24-bit recordings that are known to have been made with very low-jitter A/D converters.  We can use these as test material.  We will also apply the same procedure to a 10 kHz sine wave test tone (to show jitter sideband amplitudes on an FFT analysis).

We have D/A converters in-house that will reproduce these tracks without any significant jitter artifacts (sidebands at least 135 dB below the music signal or test tone).

We have an A/D converter in house that allows us to switch jitter attenuation on and off.  When jitter attenuation is on, jitter-induced sidebands will be at least 135 dB below the music signal. 

We will use the A/D to apply jitter modulation to the clean analog audio reproduced by the "jitter free" D/A converter.  Two versions will be created; one "jitter free" and one with added jitter.  The result will be two 96 kHz 24-bit digital files (one with encoded jitter artifacts and one without).

We will use one of our Audio Precision 2722 Test Systems to generate the jitter signal.  We can generate sinusoidal, square and/or random jitter functions of varying amplitude (but must limit the choices if we want to get anything done).

I suggest the use of the foobar ABX test plug-in, but careful attention must be paid to the following:
1) Jitter performance of the playback DAC
2) Distortion performance of the playback DAC
3) Data path from foobar to the DAC
4) Playback levels should be documented.
5) Playback equipment should be documented




CD player ABX:

Capture the output of a CD player using a "jitter free" A/D with precisely normalized gains.  Capture the output of the same CD player through a "jitter free" D/A feeding the same A/D.  Publish original track and captured CD player track, and the captured "clean track" for download and ABX comparison.  All three tracks would be 44.1 kHz 16-bit.

Play a 10 kHz TPDF -20 dB FS tone through both chains and capture the results for FFT analysis (to confirm presence of sidebands).

Measure frequency response of both chains.



The CD player test is much simpler and we may want to start with this.

Comments and suggestions welcome please!
John Siau
Vice President
Benchmark Media Systems, Inc.

(Not a) good explanation of jitter in TAS

Reply #130 –
Quote from: John_Siau on
We will use one of our Audio Precision 2722 Test Systems to generate the jitter signal.


I think I saw one of these at work
TAPE LOADING ERROR

(Not a) good explanation of jitter in TAS

Reply #131 –
Quote from: John_Siau on
Jitter ABX:

We have low-noise 96 kHz 24-bit recordings that are known to have been made with very low-jitter A/D converters.  We can use these as test material. 

We will also apply the same procedure to a 10 kHz sine wave test tone (to show jitter sideband amplitudes on an FFT analysis).

We have D/A converters in-house that will reproduce these tracks without any significant jitter artifacts (sidebands at least 135 dB below the music signal or test tone).

We have an A/D converter in house that allows us to switch jitter attenuation on and off.  When jitter attenuation is on, jitter-induced sidebands will be at least 135 dB below the music signal. 

We will use the A/D to apply jitter modulation to the clean analog audio reproduced by the "jitter free" D/A converter.  Two versions will be created; one "jitter free" and one with added jitter.  The result will be two 96 kHz 24-bit digital files (one with encoded jitter artifacts and one without).

We will use one of our Audio Precision 2722 Test Systems to generate the jitter signal.  We can generate sinusoidal, square and/or random jitter functions of varying amplitude (but must limit the choices if we want to get anything done).

I suggest the use of the foobar ABX test plug-in, but careful attention must be paid to the following:

1) Jitter performance of the playback DAC
2) Distortion performance of the playback DAC
3) Data path from foobar to the DAC
4) Playback levels should be documented.
5) Playback equipment should be documented

CD player ABX:

Capture the output of a CD player using a "jitter free" A/D with precisely normalized gains.  Capture the output of the same CD player through a "jitter free" D/A feeding the same A/D.  Publish original track and captured CD player track, and the captured "clean track" for download and ABX comparison.  All three tracks would be 44.1 kHz 16-bit.

Play a 10 kHz TPDF -20 dB FS tone through both chains and capture the results for FFT analysis (to confirm presence of sidebands).

Measure frequency response of both chains.

The CD player test is much simpler and we may want to start with this.

Comments and suggestions welcome please!


These are IMO the current "rules" for blind testing:

(1) Program material must include critical passages that enable audible differences to be most easily heard.

(2) Listeners must be sensitized to a audible differences, so that if an  audible difference is generated by the equipment, the listener will notice it and have a useful reaction to it.

(3) Listeners must be trained to listen systematically so that audible problems are heard.

(4) Procedures should be "open" to detecting problems that aren't necessarily technically well-understood or even expected, at this time. A classic problem with measurements and some listening tests is that each one focuses on one or only a few problems, allowing others to escape notice. 

(5) We must have confidence that the Unit Under  Test (UUT) is representative of the kind of equipment it represents. In other words  the UUT must not be broken, it must not be appreciably modified in some secret way, and must not be the wrong make or model,  among other things.

(6) A suitable listening environment must be provided. It can't be too dull, too bright, too noisy, too reverberant, or too harsh.  The speakers and other components have to be sufficiently free from distortion, the room must be noise-free, etc..

(7) Listeners need to be in a good mood for listening, in good physical condition (no blocked-up ears!), and be well-trained for hearing deficiencies in the reproduced sound.

(8) Sample volume levels need to be matched to each other or else the listeners will perceive differences that are simply due to volume differences.

(9) Non-audible influences need to be controlled so that the listener reaches his conclusions due to "Just listening". 

(10) Listeners should control as many of the aspects of the listening test as possible. Self-controlled tests usually facilitate this. Most importantly, they should be able to switch among the alternatives at times of their choosing. The switchover should be as instantaneous and non-disruptive as possible.

Many of these requirements (1) Program material must include critical passages that enable audible differences to be most easily heard.

(2) Listeners must be sensitized to a audible differences, so that if an  audible difference is generated by the equipment, the listener will notice it and have a useful reaction to it.

(3) Listeners must be trained to listen systematically so that audible problems are heard.

(4) Procedures should be "open" to detecting problems that aren't necessarily technically well-understood or even expected, at this time. A classic problem with measurements and some listening tests is that each one focuses on one or only a few problems, allowing others to escape notice. 

(5) We must have confidence that the Unit Under  Test (UUT) is representative of the kind of equipment it represents. In other words  the UUT must not be broken, it must not be appreciably modified in some secret way, and must not be the wrong make or model,  among other things.

(6) A suitable listening environment must be provided. It can't be too dull, too bright, too noisy, too reverberant, or too harsh.  The speakers and other components have to be sufficiently free from distortion, the room must be noise-free, etc..

(7) Listeners need to be in a good mood for listening, in good physical condition (no blocked-up ears!), and be well-trained for hearing deficiencies in the reproduced sound.

(8) Sample volume levels need to be matched to each other or else the listeners will perceive differences that are simply due to volume differences.

(9) Non-audible influences need to be controlled so that the listener reaches his conclusions due to "Just listening". 

(10) Listeners should control as many of the aspects of the listening test as possible. Self-controlled tests usually facilitate this. Most importantly, they should be able to switch among the alternatives at times of their choosing. The switchover should be as instantaneous and non-disruptive as possible.

Many of these requirements relate to ensuring the listener's sensitivity, so leaving them up to the listener makes sense.

There are two requirements that require actions by the people who prepare the data - requirements 3 and 5:

(3) Listeners must be trained to listen systematically so that audible problems are heard.

This means that files for listener training should be provided. The best form of listener training is the same music with the most audible relevant distortion generated by the UUT augmented in such a way that there are samples with unmistakably audible distortion, working in logical steps of less distortion, to the point where the actual distortion of the UUT is the object of the test.

(5) We must have confidence that the Unit Under  Test (UUT) is representative of the kind of equipment it represents. In other words  the UUT must not be broken, it must not be appreciably modified in some secret way, and must not be the wrong make or model,  among other things.

This means that the supplier of the .wav files for testing also needs to provide the results of technical tests of the test environment and the UUT.  The standard set of tests for audio gear that is generally availble are the Audio Rightmark tests. The Audio Rightmark tests may be criticized as being limited when it comes to evaluating jitter, so whatever additional tests that the developer of the test materials wishes to apply should be OK as long as it is applied uniformly to the test environment and the UUT.





(Not a) good explanation of jitter in TAS

Reply #132 –
Oh boy, jitter audibility test! I am looking forward to it. Does all this, Arnold, means that ordinary people can't do ABX test?
And other thing, if the samples aren't 48 kHz or 96 kHz, I wouldn't be able to do the test, since I have Audigy 2 ZX Platinum card, and I would have to use foobar's upsampling. If the jitter would be induced into the audio signal, would it be "interpolated", and therefore possibly less audible when upsampled?
TAPE LOADING ERROR

(Not a) good explanation of jitter in TAS

Reply #133 –
Quote from: hlloyge on
Does all this, Arnold, means that ordinary people can't do ABX test?


It does sound much more complicated than it really is. All points Arnold mentions can improve the significance of ABX results. I would rather call them recommendations than rules, though. It's not that your results are necessarily worthless if you don't stricly follow this 3 page ruleset.

Much is, for example, already automatically taken care of when you just use Foobar's ABX component. Should that show with a very high probability that you can hear a difference, there are good reasons to assume that you really do, without any necessary additional effort. When you can't hear a difference, though, it get's more complicated. To demonstrate that differences are inaudible you must rule as many external factors as possible.

(Not a) good explanation of jitter in TAS

Reply #134 –
Quote from: John_Siau on
Comments and suggestions welcome please!

Jitter ABX:
...


This would certainly be interesting, but I'm not sure though what it would actually demonstrate. Can jitter be made audible? Sure it can, we don't need ABX tests to answer that question. It all depends on the amount of jitter you add to a signal.

Showing that jitter still is an audible real world phenomenon with the current (low-cost) state of the art would be the much more interesting demonstration!

That's were you are heading here:

Quote from: John_Siau on
CD player ABX:

Capture the output of a CD player using a "jitter free" A/D with precisely normalized gains.  Capture the output of the same CD player through a "jitter free" D/A feeding the same A/D.  Publish original track and captured CD player track, and the captured "clean track" for download and ABX comparison.  All three tracks would be 44.1 kHz 16-bit.

Play a 10 kHz TPDF -20 dB FS tone through both chains and capture the results for FFT analysis (to confirm presence of sidebands).


I would love to see those tests conducted. I would also prefer employing not some single CD player that most of us don't own, but common audio IC's found on several million PCs like the Realtek ALC line from inside a HF noisy PC case. You could also choose popular devices as a Mac mini (analog out vs. the integrated S/PDIF into a jitter tolerant DAC) or Creative's sound cards.

(Not a) good explanation of jitter in TAS

Reply #135 –
Quote from: hlloyge on
Oh boy, jitter audibility test! I am looking forward to it. Does all this, Arnold, means that ordinary people can't do ABX test?
And other thing, if the samples aren't 48 kHz or 96 kHz, I wouldn't be able to do the test, since I have Audigy 2 ZX Platinum card, and I would have to use Foobars upsampling. If the jitter would be induced into the audio signal, would it be "interpolated", and therefore possibly less audible when upsampled?

Foobar's upasmpling should not be a problem.  We have measured it here and I personally have good confidence in its performance.  I cannot say the same for the Audigy 2 ZX Platinum card.

You should be able to hear the differences with a training track (where jitter is added at very high levels), but your sound card may limit your ability to to hear the differences between the output of a CD player and the output of on outboard D/A.  To hear the differences reliably, your sound card (and playback chain) will need to be significantly better than the CD player.  In general, sound cards are one step below most CD players.
John Siau
Vice President
Benchmark Media Systems, Inc.

(Not a) good explanation of jitter in TAS

Reply #136 –
Quote from: rpp3po on
Can jitter be made audible? Sure it can, we don't need ABX tests to answer that question. It all depends on the amount of jitter you add to a signal.

Showing that jitter still is an audible real world phenomenon with the current (low-cost) state of the art would be the much more interesting demonstration!

That's were you are heading here:

Quote from: John_Siau on
CD player ABX:

Capture the output of a CD player using a "jitter free" A/D with precisely normalized gains.  Capture the output of the same CD player through a "jitter free" D/A feeding the same A/D.  Publish original track and captured CD player track, and the captured "clean track" for download and ABX comparison.  All three tracks would be 44.1 kHz 16-bit.

Play a 10 kHz TPDF -20 dB FS tone through both chains and capture the results for FFT analysis (to confirm presence of sidebands).


Yes, but ...

The CD player will have many defects other than jitter that are at audible levels.  The D/A converters and output stages in most CD players are surprisingly bad.  Inexcusably bad!  It is not at all uncommon to see non-harmonic distortion artifacts that are only 40 or 50 dB below the level of a test tone.

The differences are easy to hear but the differences are only partially due to jitter.  IMD is a major issue, hum is an issue, microprocessor crosstalk is an issue, and random noise is an issue, and yes, jitter is an issue.  The player may boast "96 kHz, 24-bit D/A" but deliver 12 to 14 bits of performance.  I like to call the other 8 to 10 bits "marketing bits".


Having a "jitter-only" ABX test will allow us to hear jitter in isolation.  I think both tests are valuable.
John Siau
Vice President
Benchmark Media Systems, Inc.

(Not a) good explanation of jitter in TAS

Reply #137 –
Quote from: rpp3po on
Quote from: hlloyge on
Does all this, Arnold, means that ordinary people can't do ABX test?


It does sound much more complicated than it really is. All points Arnold mentions can improve the significance of ABX results. I would rather call them recommendations than rules, though. It's not that your results are necessarily worthless if you don't stricly follow this 3 page ruleset.


They are 'rules' for maximizing the discriminative power of the test. 

If you are a scientist trying to determine JNDs for jitter, they apply.

If you are testing some particular audiophile's claim to ALREADY HAVE HEARD A DIFFERENCE UNDER MUNDANE CONDITIONS, I would say they can be relaxed; all you need to is 'blind' the listener and let them try to repeat their 'sighted' performance with the same gear, music, etc.

What audiophiles tend to do, disingenously IMO, is react to the often-negative results of the latter kind of ABX by claiming that the full rigor of a scientific ABX was not applied. THey become conveniently evangelical for scientific rigor when it suits them.

(Not a) good explanation of jitter in TAS

Reply #138 –
Quote from: John_Siau on
The CD player will have many defects other than jitter that are at audible levels.  The D/A converters and output stages in most CD players are surprisingly bad.  Inexcusably bad!  It is not at all uncommon to see non-harmonic distortion artifacts that are only 40 or 50 dB below the level of a test tone.

The differences are easy to hear but the differences are only partially due to jitter.


Then why have the 'easy to hear' differences not been demonstrated easily with extant publicized DBTs?

Really, please, settle this longstanding matter.  Give us data on exactly which CDPs display this easy-to-hear behavior, under what conditions, so it could be replicated.


(Not a) good explanation of jitter in TAS

Reply #139 –
Quote from: krabapple on
Quote from: John_Siau on
The CD player will have many defects other than jitter that are at audible levels.  The D/A converters and output stages in most CD players are surprisingly bad.  Inexcusably bad!  It is not at all uncommon to see non-harmonic distortion artifacts that are only 40 or 50 dB below the level of a test tone.

The differences are easy to hear but the differences are only partially due to jitter.


Then why have the 'easy to hear' differences not been demonstrated easily with extant publicized DBTs?

Really, please, settle this longstanding matter.  Give us data on exactly which CDPs display this easy-to-hear behavior, under what conditions, so it could be replicated.



Try this explanation of jitter and its audibility:

Jitter White Paper

Steve N.

(Not a) good explanation of jitter in TAS

Reply #140 –
Quote from: krabapple on
Give us data on exactly which CDPs display this easy-to-hear behavior, under what conditions, so it could be replicated.


1) Noise Floor:
The most easy-to-hear behavior is the elevated noise floor of consumer CD and DVD players.  Very few of these devices deliver the SNR that is possible with a 16-bit PCM TPDF dithered system.  Many have SNR numbers around 80 dB.  Furthermore, the noise floors of these devices are not white, but include servo noise, CPU noise, and AC line noise.  To her the differences, the playback system must be adjusted to a level high enough to make the noise floor of the CDP audible.  The SNR of the playback system must exceed that of the CDP.  Under these conditions the noise issues are very obvious.

Other issues include:

2) IMD and THD+N:
Some CDPs measure up to 1% THD+N (including significant IMD components).  This energy is only 40 dB below peak audio levels and may be audible under the right conditions.

3) Jitter-induced sidebands:
Jitter sidebands 40 to 60 dB below audio levels.  Again these sidebands are high-enough that they may not fall under masking curves, and may be audible under the right conditions.

It is hard to verify the audibility of these other issues (items 2 and 3) when the differences in the noise floor of the D/A converters is so noticeable.  The elevated noise floors of the CDPs were a dead-giveaway in most of the informal ABX tests we have conducted.  The anecdotal evidence that I have for (2) and (3) is an apparent difference in frequency response (when no differences in frequency response could be measured).  In attempting to test for (2) and (3) we had to reduce the playback levels to make the noise floor differences inaudible.  Obviously this reduced playback level will make it harder to hear (2) and (3). This is why I am proposing the creation of a "jitter-only" ABX test.

Obviously we need to take a step beyond anecdotal accounts of differences, and measurements that suggest audible differences, and verify audibility (or lack thereof) with ABX tests.

I have some FFT plots of the outputs of CDPs that I can post if there is interest, but I do not want to imply that these plots verify audibility.  They simply indicate that there are defects that may (or may not) be audible.
John Siau
Vice President
Benchmark Media Systems, Inc.

(Not a) good explanation of jitter in TAS

Reply #141 –
John, how well can this issue be modelled as straight-up sampling jitter?

(Not a) good explanation of jitter in TAS

Reply #142 –
Quote from: audioengr on
Quote from: krabapple on
Quote from: John_Siau on
The CD player will have many defects other than jitter that are at audible levels.  The D/A converters and output stages in most CD players are surprisingly bad.  Inexcusably bad!  It is not at all uncommon to see non-harmonic distortion artifacts that are only 40 or 50 dB below the level of a test tone.

The differences are easy to hear but the differences are only partially due to jitter.


Then why have the 'easy to hear' differences not been demonstrated easily with extant publicized DBTs?

Really, please, settle this longstanding matter.  Give us data on exactly which CDPs display this easy-to-hear behavior, under what conditions, so it could be replicated.



Try this explanation of jitter and its audibility:

Jitter White Paper

Steve N.


Interesting but by your own admission, still quite inconclusive.  You 'believe' that the human sensitivity to jitter is grossly underestimated, butstill haven't got the robust listening test evidence -- just a lot of systems and test signals that are apparently 'not resolving enough'.

Once again, I would say from such work that the differences COMMONLY REPORTED by audiophiles whenever they compare two digital sources -- they almost always claim to hear  a difference --are UNLIKELY to be due to jitter.

This does not rule out the possibility that some people have heard bad jitter.  But audiophiles probably don't even know when they're hearing it.  "White hats' need to start telling audiophiles (and some professinals) that 'jitter' can't just be handwaved as an explanation whenever they try to explain why they 'know' two CDPs or DACs sound different from a sighted comparison.

(Not a) good explanation of jitter in TAS

Reply #143 –
Quote from: Axon on
John, how well can this issue be modelled as straight-up sampling jitter?

Separating artifacts into different categories (noise, jitter, distortion, etc.) can give us the ability to determine audibility of individual defects (in this case jitter).  The problem is that the poor SNR performance of CD players makes identification easy in an ABX test. If SNR differences make identification easy, then it is impossible to determine if the other artifacts are also audible.
John Siau
Vice President
Benchmark Media Systems, Inc.

(Not a) good explanation of jitter in TAS

Reply #144 –
Quote from: John_Siau on
Quote from: krabapple on
Give us data on exactly which CDPs display this easy-to-hear behavior, under what conditions, so it could be replicated.


1) Noise Floor:
The most easy-to-hear behavior is the elevated noise floor of consumer CD and DVD players.  Very few of these devices deliver the SNR that is possible with a 16-bit PCM TPDF dithered system.  Many have SNR numbers around 80 dB.  Furthermore, the noise floors of these devices are not white, but include servo noise, CPU noise, and AC line noise.  To her the differences, the playback system must be adjusted to a level high enough to make the noise floor of the CDP audible.  The SNR of the playback system must exceed that of the CDP.  Under these conditions the noise issues are very obvious.


Let me explain where I am coming from:  familiarity with an audiophile world where every digital player is CLAIMED to have its own sound, by the editors and writers of the print and internet high-end press.  These are , for better or worse (mostly the latter), the mainstream 'authorities' on 'high end' sound -- the people the popular press will go to, for example , when they need a quote or sound bite about sound quality.

Differences revealed under conditions one would not encounter normally while listening to music,  do not strike me as convincing sources of such reports.  Meyer and Moran found that the only audible difference in their blind tests between DSD and Redbook audio was encountered using abnormally high output level while playing 'silence' -- a predicted noise floor difference.  Listening to music at that level for a lenght of time would be uncomfortable and possibly dangerous.


Quote
Other issues include:

2) IMD and THD+N:
Some CDPs measure up to 1% THD+N (including significant IMD components).  This energy is only 40 dB below peak audio levels and may be audible under the right conditions.

3) Jitter-induced sidebands:
Jitter sidebands 40 to 60 dB below audio levels.  Again these sidebands are high-enough that they may not fall under masking curves, and may be audible under the right conditions.

It is hard to verify the audibility of these other issues (items 2 and 3) when the differences in the noise floor of the D/A converters is so noticeable.  The elevated noise floors of the CDPs were a dead-giveaway in most of the informal ABX tests we have conducted.  The anecdotal evidence that I have for (2) and (3) is an apparent difference in frequency response (when no differences in frequency response could be measured).  In attempting to test for (2) and (3) we had to reduce the playback levels to make the noise floor differences inaudible.  Obviously this reduced playback level will make it harder to hear (2) and (3). This is why I am proposing the creation of a "jitter-only" ABX test.



Again, it is not unusual to be able to hear or see a difference *easily* when it is *purposely magnified*.  But that doesn't mean it's perceptible at '1X' magnification.

Were the playback levels used in these ABX tests, levels that the subjects would use to listen to music for enjoyment?

(Not a) good explanation of jitter in TAS

Reply #145 –
Quote from: krabapple on
Were the playback levels used in these ABX tests, levels that the subjects would use to listen to music for enjoyment?


That's what I asked myself, too. You can always show that bit depth n is inferior to n-1 when you just raise the volume level high enough. It never stops.

(Not a) good explanation of jitter in TAS

Reply #146 –
Quote from: krabapple on
Were the playback levels used in these ABX tests, levels that the subjects would use to listen to music for enjoyment?

Yes, at normal playback levels of 90 to 100 dB SPL A-weighted slow.

At normal playback levels, the noise of a 16-bit TPDF dithered 44.1 kHz PCM system can exceed audibility thresholds.  Many CDPs are 10 to 20 dB noisier than CD quality.  Yes, this noise is audible at normal playback levels, especially when playing music that has little amplitude compression applied.

Here is a link to a list of CD players that measured "Less than "CD quality"":
"A Case for the Jitters" - stereophile.com

The first player on the list has a noise floor that is elevated by 12 dB relative to a perfect CD system.
John Siau
Vice President
Benchmark Media Systems, Inc.

(Not a) good explanation of jitter in TAS

Reply #147 –
Quote from: John_Siau on
Quote from: krabapple on
Were the playback levels used in these ABX tests, levels that the subjects would use to listen to music for enjoyment?

Yes, at normal playback levels of 90 to 100 dB SPL A-weighted slow.

At normal playback levels, the noise of a 16-bit TPDF dithered 44.1 kHz PCM system can exceed audibility thresholds.  Many CDPs are 10 to 20 dB noisier than CD quality.  Yes, this noise is audible at normal playback levels, especially when playing music that has little amplitude compression applied.
That would seem to suggest a concrete counterexample to Meyer/Moran, except for the fact that 100dB A-weighted slow seems extremely loud to me. You could only play 2 CDs at that loudness a day before exceeding OSHA occupational noise limits.

If used with "uncompressed" music, suggesting a crest factor of 20db, you're looking at peak SPLs in the 120-130dB range. I'm not really sure that is a normal playback level to begin with.

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Here is a link to a list of CD players that measured "Less than "CD quality"":
"A Case for the Jitters" - stereophile.com

The first player on the list has a noise floor that is elevated by 12 dB relative to a perfect CD system.
... Right, but a freaking iPod tested 6db better than that. I think it's fair to suggest that the McIntosh tested represents a pathologically bad (and perhaps defective) design, that is not representative of what exists in even low-end consumer audio today. And that's not even getting into audibility considerations.

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Separating artifacts into different categories (noise, jitter, distortion, etc.) can give us the ability to determine audibility of individual defects (in this case jitter). The problem is that the poor SNR performance of CD players makes identification easy in an ABX test. If SNR differences make identification easy, then it is impossible to determine if the other artifacts are also audible.
I ask this because, among the items in my bag of tricks, is a program that can simulate sampling jitter. Given a numeric description of a real-world sampling jitter spectrum, and a test music signal, I might be able to provide some simulated jitter files to facilitate ABX testing of that specifically - assuming that the obvious issues regarding interaction of playback device jitter interaction with simulated jitter are taken care of, of course.

(Not a) good explanation of jitter in TAS

Reply #148 –
Quote from: Axon on
Right, but a freaking iPod tested 6db better than that. I think it's fair to suggest that the McIntosh tested represents a pathologically bad (and perhaps defective) design, that is not representative of what exists in even low-end consumer audio today. And that's not even getting into audibility considerations.

I have several DVD players here that don't look much different than the McIntosh.  Players this bad do exist and they are being cranked out in great quantities.  The problem is that nobody has bothered to measure them.

Quote from: Axon on
I might be able to provide some simulated jitter files to facilitate ABX testing of that specifically - assuming that the obvious issues regarding interaction of playback device jitter interaction with simulated jitter are taken care of, of course.

Interesting.  This could eliminate the D/A and A/D process that I would use to apply the jitter modulation using jitter signals supplied by an Audio Precision test station.  I have good confidence in the transparency of the D/A and A/D that we would use, but my jitter test signals are somewhat limited.  The AP is limited to sinusoidal, square wave, or wide band random jitter signals.  I can apply jitter over a frequency range of 10 Hz to 5 kHz with this test setup.

I suspect you may be able to do more than this if you can phase modulate the audio in a DSP.  Tell us more!

Regarding the playback D/A:
I have playback D/A converters here with jitter-induced sidebands that are below -135 dB FS under all input conditions that we can use for testing here.  Obviously these tests cannot be replicated if played back from a computer sound card that has jitter levels reaching or exceeding the test levels.  However, files with exaggerated jitter content could be instructional for those who do not have low-jitter playback equipment.  It would also be useful for training to listen for specific jitter-induced defects.
John Siau
Vice President
Benchmark Media Systems, Inc.

(Not a) good explanation of jitter in TAS

Reply #149 –
Quote from: John_Siau on
I have several DVD players here that don't look much different than the McIntosh.  Players this bad do exist and they are being cranked out in great quantities.  The problem is that nobody has bothered to measure them.
OK, that's news to me. And I wholeheartedly agree with the notion that the fact that nobody measures such devices is a problem - and I suppose that not having decent sideband measurements with tools like RMAA contributes to the problem. Still... aren't there significant shades of gray here? You clearly show that the McIntosh is in fact representative of low-end audio, but is it representative of mid-end?
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Interesting.  This could eliminate the D/A and A/D process that I would use to apply the jitter modulation using jitter signals supplied by an Audio Precision test station.  I have good confidence in the transparency of the D/A and A/D that we would use, but my jitter test signals are somewhat limited.  The AP is limited to sinusoidal, square wave, or wide band random jitter signals.  I can apply jitter over a frequency range of 10 Hz to 5 kHz with this test setup. I suspect you may be able to do more than this if you can phase modulate the audio in a DSP.  Tell us more!
It's nothing particularly magical - as far as this is concerned, it's an implementation of a jitter simulator according to eg Hawksford, except without the optimization assuming modulation amplitudes much smaller than the sampling period. It's actually using a variable-rate resampler under the hood, with pre/post up/downsampling of arbitrary factor. So as long as the resampling time offsets don't go backwards, I should be able to simulate modulation at any amplitude and from DC to, well, light.

The simulation is about automodulation right now *cough*, but it could very easily be supplied from an external file or generated from a formula or spectrum. I wrote it up in conjunction with something I'm presenting at the October AES convention. (I'm a noob, wish me luck).

In terms of accuracy, on one null test I've determined the peak error of the simulation with truly ludicrous amounts of modulation to be ~~ -111dbFS vs theoretical results. I haven't tested this particular configuration yet, though.

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Regarding the playback D/A: I have playback D/A converters here with jitter-induced sidebands that are below -135 dB FS under all input conditions that we can use for testing here.  Obviously these tests cannot be replicated if played back from a computer sound card that has jitter levels reaching or exceeding the test levels.  However, files with exaggerated jitter content could be instructional for those who do not have low-jitter playback equipment.  It would also be useful for training to listen for specific jitter-induced defects.
True - but before we get into trying to find the best converters, I think it's important here to delineate what the meaning of a given transparency threshold is, relative to a known quantity of playback jitter. I mean, without that knowledge, in theory, one could dismiss all ABX tests by claiming "but you could have used a better converter to get a better result!"

More specifically: is it safe to argue inaudibility purely on the basis of masking?