Close in jitter?

[oivavoi]

Jitter in digital audio is a complex issue. If you truly want to understand it, seek the advice of folks who build the very best DAC's on the market. Not self proclaimed experts on forums who have no whitepapers published, or even built any audio gear on their own.

Personally if I wanted to know more about how a BMW ECU functions, I would contact the BMW engineers who specialize in that department. I wouldn't contact some guy on the internet who reads Car and Driver on a regular basis, and attended a few marketing seminars.

I don't agree with that. People who sell stuff have a vested interest in touting the superiority of their solutions. If I want to know the merits a certain BMW model, I would much rather turn to independent testing. Probably wouldn't trust the manufacturer to give me the truth and nothing but the truth. And I would probably also search online for the experiences people have, if it's a model which seems to hold up well over time etc.

 

[Mivera]

I don't agree with that. People who sell stuff have a vested interest in touting the superiority of their solutions. If I want to know the merits a certain BMW model, I would much rather turn to independent testing. Probably wouldn't trust the manufacturer to give me the truth and nothing but the truth. And I would probably also search online for the experiences people have, if it's a model which seems to hold up well over time etc.

So just because people make money from their knowledge, this automatically debunks the fact that they know what they're doing? Using this logic we must seek advice from only the laymen of the world. Because the experts have a vested interest in their knowledge.

Guess what, there's a much simpler way to find out if it's real or BS than beating your head against the wall. Listen to the gear they make!! If you can't hear a difference Forget about it because it's of no concern to you!! Put on some music on what you own, and bask in the pleasure of knowing you didn't need the objectively superior gear to achieve your own personal nirvana from your system!

 

[DonH56]

Jitter in digital audio is a complex issue. If you truly want to understand it, seek the advice of folks who build the very best DAC's on the market. Not self proclaimed experts on forums who have no whitepapers published, or even built any audio gear on their own.

Personally if I wanted to know more about how a BMW ECU functions, I would contact the BMW engineers who specialize in that department. I wouldn't contact some guy on the internet who reads Car and Driver on a regular basis, and attended a few marketing seminars.

Well, most of my published papers are reports and papers at conferences other than audio, some classified so you couldn't read them anyway, and I have built audio gear but it's been a while. I don't read Car and Driver. I do understand jitter. In any event I am done with this.

Have a good day, unless you've made other plans.

 

[Mivera]

Well, most of my published papers are reports and papers at conferences other than audio, some classified so you couldn't read them anyway, and I have built audio gear but it's been a while. I don't read Car and Driver. I do understand jitter. In any event I am done with this.

Have a good day, unless you've made other plans.

I'm not questioning your engineering knowledge. However this is a very specialized field. What are some of the well regarded DAC's you designed?

What I like about Bruno Putzeys AES published whitepaper, is he actually has several well regarded DAC's, ADC's, and master clocks out in the field between Grimm and Mola Mola. And they have been confirmed to be both subjectively and objectively acclaimed. Widely acclaimed in both professional and audiophile circles.

So if you can provide some evidence that your accomplishments in the field match his, I'm all ears.

 

[amirm]

Jitter in digital audio is a complex issue.

On that you unfortunately have not taken the time to learn. Instead you are hero worshipping with "this and that said so." Well, we are here to examine their statements.

If you truly want to understand it, seek the advice of folks who build the very best DAC's on the market. Not self proclaimed experts on forums who have no whitepapers published, or even built any audio gear on their own.

I have written a number of papers on Jitter, which have been published in magazines and hugely discussed online. Indeed for a while if you googled for audio jitter my papers would show up.

I did that search right now and my article shows up on page 2 of google search:

It references the article I wrote for WSR magazine years ago (the link no longer works on WBF though). Out of half a million references on Google, mine came up in top 20 or so.

Besides me, you have Don here who actually designs ultra-high-speed DACs. He has authored a bunch of articles on WBF explaining jitter in detail, with simulations and such. You are completely mistaken that DAC designers understanding issues related to audibility.

Building a DAC does not at all require understanding of much of what we have talked about. There is a difference between electronic design and understanding audibility.

You like to challenge what we are saying, by all means do so. But this appeal to authority and then standing back does not work. Not here. Not in front of us.

 

[amirm]

So if you can provide some evidence that your accomplishments in the field match his, I'm all ears.

You are not all ears. You are not listening to anything that is being told to you. Don's credentials are beyond reproach on this topic. And certainly compared to paucity of knowledge you are demonstrating.

 

[Mivera]

On that you unfortunately have not taken the time to learn. Instead you are hero worshipping with "this and that said so." Well, we are here to examine their statements.


I have written a number of papers on Jitter, which have been published in magazines and hugely discussed online. Indeed for a while if you googled for audio jitter my papers would show up.

I did that search right now and my article shows up on page 2 of google search:

View attachment 6701

It references the article I wrote for WSR magazine years ago (the link no longer works on WBF though). Out of half a million references on Google, mine came up in top 20 or so.

Besides me, you have Don here who actually designs ultra-high-speed DACs. He has authored a bunch of articles on WBF explaining jitter in detail, with simulations and such.

Building a DAC does not at all require understanding of much of what we have talked about. There is a difference between electronic design and understanding audibility.

You like to challenge what we are saying, by all means do so. But this appeal to authority and then standing back does not work. Not here. Not in front of us.

Anyone can blabber away on the internet pretending to know what they're talking about. But the folks who do know what they're talking about, have actual results in the form of well regarded hardware to back it up. Holds much more weight for me than hot air.

 

[amirm]

Anyone can blabber away on the internet pretending to know what they're talking about.

That's you. My articles have been published in magazines and heavily discussed and examined. I have defended their content and explained what is in them. If you have issues with them, the floor is all yours. If you cannot, then you have lost the argument right then and there.

 

[Mivera]

You are not all ears. You are not listening to anything that is being told to you. Don's credentials are beyond reproach on this topic. And certainly compared to paucity of knowledge you are demonstrating.

I have no doubt he has great credentials. The family doctor I see also has great credentials. But If I ever run into special health problems, he refers me to a specialist. Some people know where to draw the line between what's known and unknown to them. Without the specialized knowledge, it's only speculation.

 

[amirm]

But the folks who do know what they're talking about, have actual results in the form of well regarded hardware to back it up. Holds much more weight for me than hot air.

You have not shown that they know anything about audibility of jitter. We know they are good design engineers. What you/they need to show is that they understand psychoacoustics, have performed reliable listening tests, etc. What holds weight for you doesn't amount to anything in this context. Happy to tell them the same thing to their face.

 

[amirm]

I have no doubt he has great credentials. The family doctor I see also has great credentials. But If I ever run into special health problems, he refers me to a specialist. Some people know where to draw the line between what's known and unknown to them. Without the specialized knowledge, it's only speculation.

Again, that is you. You are speculating about who knows what, about a topic you don't understand. Don's knowledge of this area is hugely superior to what you have said. And you have not shown any other references that counter what he or I have said.

 

[Thomas savage]

Mike ( @Mivera ) ,

There's nothing constructive/informative to be gained from discussing topics in this way( the thread started out well ) , if you can't challenge the information members post with actual first hand knowledge and expirence then please be respectful of those limits and indeed respectful of The contributions that do carry first hand knowledge.

As soon as you give up on the evidence, either because you don't have any or just lack the understanding to put forth a argument and start attacking the credibility of members ( members who have recognised expertise ) the arguments been lost and the value of the discussion undermined.

That's not what I want to see.

Keep this in mind before posting any additional contributions please. no quoting this message, it's a statement not a point for further discussion .

Thanks, I appreciate you challenging established ideas and starting debates but let's keep respectful and on the topic being addressed ( not pouring scorn on respected members and their credentials)

 

[Ken Newton]

I've always wondered about this as well. Such close-in jitter can have a period on the order of tens of seconds. Count out ten seconds aloud to yourself and then ponder how long a period that actually is for a single cycle of audio. It's more intuitive to think of this as clock wander rather than clock jitter. I suspect that it get's called out as a problem because it's typically due to oscillator 1/f noise, which increases dramatically as noise frequency drops below 10Hz. Measure low enough in frequency and the noise becomes relatively very high, and so, the jitter at deep infrasonic frequencies also becomes relatively very high.

That said, I learned long ago to keep an open mind mind about human ear-brain system perception with regard to multi-channel audio reproduction effects. Perhaps, such deeply low frequency jitter is intermodulating with with some DAC system processing mechanism (noise-shaping, quantizer element scrambling, quantizer glitch error energy, etc) in an audible way? I can't conceive of how else it could be audible, assuming, for the moment, that it is.

 

[fas42]

Sometimes you just need a good set of ears. If you don't have a good set of ears, don't concern yourself with it. I can assure you Stevie Wonder doesn't lose any sleep over the quality of his sunglasses tint.

Yes. Learn to be able to hear, easily distinguish the usual defects in audio sound, and that's all that's required to assess the quality of the playback. Whether something sounds 'better' or 'nicer' is 100% irrelevant to getting good playback - all that matters is whether the sound is faulty or not; "preference" is something a long way further down the track, like deciding whether to drive MB, or BMW.

 

[amirm]

The J-test was developed for "interface jitter" when using AES/EBU or SPDIF interfaces, not clock jitter. The testing isn't even accurate if you're using a USB interface, or other type of built in transport.

Now let's hit this claim.

It is true that J-test signal was produced to show jitter induced by serial digital cables used for S/PDIF and AES/EBU. The signal itself is a quarter sample rate tone that has a single bit that toggles back and forth. Due to encoding of PCM ("two's complicate") that one tiny toggle of the lowest order bit causes all the bits in the audio sample to change. The frequency of change for this square wave is Sampling rate/192.

Here is the waveform in digital domain:

The dots are the actual samples. The sinewave is an interpolation/simulation by the audio workstation program (Audition in this case), but is representative of what comes out due to filtering of the DAC. The tiny undulations explained above are too small to be visible.

The precise, quarter sampling rate ("FS/4") eliminates the need for quantizing/dithering them allowing the displayed noise to be that of the system under test, than the signal itself.

For a 48 Khz signal, the main tone will be at 12 Khz. For 44.1 Khz, it will be at 11.025 Khz. This signal is presented at full amplitude (or near it to avoid clipping) which is far higher than we have in music. That and its high frequency means that jitter effects will be amplified and captured in the output of the DAC.

This signal is played (as a wave file these days from a computer) and we then perform a spectrum analysis. The spectrum analysis is performed in software and can benefit from multiple runs/averaging and high resolution DFT (discrete fourier transform) to substantially reduce the measurement system's noise level. This then allows the jitter components of the device under test to show up even if they are at low levels.

Notice the extreme sensitivity of the test. At 8Khz we have a spike. That is 4 Khz below our main tone of 12 Khz. We add 4Khz to that and we land at 16 Khz and see another spike there. We would need to do an additional test to make sure that is not reference voltage modulation but for now, let's assume it is jitter. We are seeing a spike rising from amazingly low level of -130 db to -125 db (SPL). The system is that sensitive. This is despite my analyzer being a generation older than current ones on the market. Using the above signal processing technique we are able to still dig deep -- super deep -- into the output of the DAC.

Even tinier spikes are visible as you see in red. The two DACs are distinguished even though the difference is incredibly low.

For some of my testing I just use a simple high frequency tone, i.e. don't include the FS/192 toggling component. Here is an example of that:

So once again we see how revealing a simple, high frequency tone at high amplitude is in showing all that ails the DAC. For low frequency random jitter which triggered this thread, see the spreading of the main tone.

The simplicity of the test, i.e. a single tone, makes it easy to see all the distortion products as represented by the spikes, or changing of the noise floor. It is for this reason that J-test continues to get heavy use. It works and works simply to show incredibly small distortions including clock jitter.

While I have not seen it often, JA tests in Stereophile frequently show components of the FS/192 square wave embedded in J-test. Those are not due to interface jitte but crosstalk inside the DAC due to all the bits transitioning at once.

OK this got long. For a quick summary, J-test signal is highly revealing of any jitter induced in the signal on the output of the DAC. It is the ultimate test as it measures the analog, final, output of the DAC. Measuring clock jitter going into DAC is not representative of anything useful because we don't know how much of that jitter is filtered, attenuated or exaggerated by the DAC.

 

[RayDunzl]

Sometimes I'm pretty dense.

I don't understand why/how signal data - whatever it is - can become/simulate jitter.

Isn't the DAC just outputting what it was fed?

PS: Is "two's complicate" supposed to be "two's complement"?

 

[Mivera]

Now let's hit this claim.

It is true that J-test signal was produced to show jitter induced by serial digital cables used for S/PDIF and AES/EBU. The signal itself is a quarter sample rate tone that has a single bit that toggles back and forth. Due to encoding of PCM ("two's complicate") that one tiny toggle of the lowest order bit causes all the bits in the audio sample to change. The frequency of change for this square wave is Sampling rate/192.

Here is the waveform in digital domain:

View attachment 6726

The dots are the actual samples. The sinewave is an interpolation/simulation by the audio workstation program (Audition in this case), but is representative of what comes out due to filtering of the DAC. The tiny undulations explained above are too small to be visible.

The precise, quarter sampling rate ("FS/4") eliminates the need for quantizing/dithering them allowing the displayed noise to be that of the system under test, than the signal itself.

For a 48 Khz signal, the main tone will be at 12 Khz. For 44.1 Khz, it will be at 11.025 Khz. This signal is presented at full amplitude (or near it to avoid clipping) which is far higher than we have in music. That and its high frequency means that jitter effects will be amplified and captured in the output of the DAC.

This signal is played (as a wave file these days from a computer) and we then perform a spectrum analysis. The spectrum analysis is performed in software and can benefit from multiple runs/averaging and high resolution DFT (discrete fourier transform) to substantially reduce the measurement system's noise level. This then allows the jitter components of the device under test to show up even if they are at low levels.

View attachment 6727

Notice the extreme sensitivity of the test. At 8Khz we have a spike. That is 4 Khz below our main tone of 12 Khz. We add 4Khz to that and we land at 16 Khz and see another spike there. We would need to do an additional test to make sure that is not reference voltage modulation but for now, let's assume it is jitter. We are seeing a spike rising from amazingly low level of -130 db to -125 db (SPL). The system is that sensitive. This is despite my analyzer being a generation older than current ones on the market. Using the above signal processing technique we are able to still dig deep -- super deep -- into the output of the DAC.

Even tinier spikes are visible as you see in red. The two DACs are distinguished even though the difference is incredibly low.

For some of my testing I just use a simple high frequency tone, i.e. don't include the FS/192 toggling component. Here is an example of that:

View attachment 6728

So once again we see how revealing a simple, high frequency tone at high amplitude is in showing all that ails the DAC. For low frequency random jitter which triggered this thread, see the spreading of the main tone.

The simplicity of the test, i.e. a single tone, makes it easy to see all the distortion products as represented by the spikes, or changing of the noise floor. It is for this reason that J-test continues to get heavy use. It works and works simply to show incredibly small distortions including clock jitter.

While I have not seen it often, JA tests in Stereophile frequently show components of the FS/192 square wave embedded in J-test. Those are not due to interface jitte but crosstalk inside the DAC due to all the bits transitioning at once.

OK this got long. For a quick summary, J-test signal is highly revealing of any jitter induced in the signal on the output of the DAC. It is the ultimate test as it measures the analog, final, output of the DAC. Measuring clock jitter going into DAC is not representative of anything useful because we don't know how much of that jitter is filtered, attenuated or exaggerated by the DAC.

Yes if all you want to do is prove that a $15000 Mark Levinson DAC is no better than a $500 IFI, the J-test is great. But when you want to test the merits of a premium DAC, you need to pull out the big boy tools. You also need to pull off the cover so you can access the I2S inputs on the DAC chip. Digital measurements should be done in the digital domain. Analog measurements in the analog domain. That's the very best way to do things.

 

[Blumlein 88]

Here is an FFT of the 44/16 JTest file that JA uses. It is the 11,025 hz tone at high level and the toggled 229.7 hz square wave of the LSB. As the square wave has all the odd harmonics you get the tone and a decreasing series of low level tones at the odd harmonics of 229 hz or spaced every 460 hz roughly.

Here is Julian Dunn's write up about the signal and why this might stimulate data jitter related specifically to the SPDIF/AES interface. The switching of almost all the bits would occur at lowest signal levels meaning they would be more exposed due to lack of masking. This test signal was meant to be something of a worst case signal.

http://www.nanophon.com/audio/diagnose.pdf

 

[amirm]

Sometimes I'm pretty dense.

I don't understand why/how signal data - whatever it is - can become/simulate jitter.

It actually doesn't! The J-test signal is NOT a jitter test. It is simply a tone with a little bit of variation every few cycles. It is just that jitter in the DAC will modulate any signal -- that J-test signal or otherwise -- and we can then see that in spectrum analysis.

I am glad you asked this question because it is the biggest misconception about J-test.

Isn't the DAC just outputting what it was fed?

It tries to do that. And in an ideal DAC you would get what is fed. But we don't live in an ideal world. Let's look at this graph from my article on HDMI jitter: http://www.audiosciencereview.com/f...s/a-deep-dive-into-hdmi-audio-performance.56/

As we see, PCM is only one of the inputs to the DAC. There is a clock that provides regular samples to output. And a reference voltage that the DAC uses to divide down the output voltage based on PCM values. The PCM samples are constant but these other two are subject to their own variations. Those variations directly influence the analog output of the DAC.

Other parts of the DAC (system) can bleed into DAC (component) and its clock and reference voltage. This is how for example USB signals can couple and cause distortion in the output of the DAC.

 

[Mivera]

Here is an FFT of the 44/16 JTest file that JA uses. It is the 11,025 hz tone at high level and the toggled 229.7 hz square wave. As the square wave has all the odd harmonics you get the tone and a decreasing series of low level tones at the odd harmonics of 229 hz or spaced every 460 hz roughly.

View attachment 6729

JA isn't a DAC manufacturer. He doesn't have the tools that the best DAC manufacturers have. And he's not about to pull the covers off of all the DAC's to take digital measurements off the PCB test points. So he just does what's easy. Besides reviews are only for marketing purposes anyways. As long as the gear passes the industry standard testing, it did a good enough job to prove it's not flawed to the layman buyer. But unfortunately his measurements can't fully describe the sound of the gear like the subjective part of the review does. However with the right testing, the gap could easily close much tighter.