AQ Jitterbug Measurements

Thanks for posting the graphs, Amir

Just a question - what do you think an eye pattern shows & how do you interpret it's acceptability "in this context"?

My understanding of these eye patterns - what you are seeing in these graphs is a repeated overlay of the scope trace of the waveform.
If there was no jitter each scope trace would exactly overlay the previous scope trace & the lines on the graph would be very thin
With timing differences (jitter) between the waveforms the overlays make lines appear wider in the graph.
The idea of an eye pattern is that USB compliance has a maximum allowable amount of jitter (jitter budget) which will not cause data errors
The eye pattern graph is a diagrammatic representation of this allowable jitter budget - the idea being that at a certain point of closure of the eye the compliance fails. There is a template to overlay on these eye graphs that can be calculated & allows one to judge if compliance is reached or not

But this is about compliance. Are there any guidelines about changes in this jitter pattern Vs audibility? Can changes in this jitter be written off so non-chalantly as irrelevant to audibility?

Remember also that the eye pattern is not showing the spectrum of the jitter just a feel for the overall level of it

Some further information about rise/fall time compliance in USB from here

"There has always been a problem accurately measuring rise and fall times, especially on high speed devices. The measurement of interest is the edge rate, or slew rate, during the state change time. To help improve accuracy of the measurement, the USB-IF is standardizing on one test fixture for high-speed signal quality.

Aside from the fixturing and probes used to take the measurements, major contributors to the inaccuracies in these measurements are the shape of the edge, noise on the signal and the method of calculating the 10% and 90% points as defined in Sections 7.1.2.1 and 7.1.2.2 of the USB 2.0 Specification.

A waveform with slow corners (see sample eye diagram below) will result in a measured rise time that is slower than the actual edge rate would indicate. Also a small change in the position of the 10% and 90% points due to noise on the signal, etc., can cause a relatively large change in the measured rise time. "



"The relaxed edge rate values of 300ps and 100ps apply for high-speed USB signaling"

As I said before - is eye pattern a good measurement for correlation with audibility? I think someone answered yes & used the JA measurements as evidence but I need to look into this further

dallasjustice said:

The jitter measurements look very much the same as JA's measurement. Did Paul Miller also measure with a better quality DAC? The higher quality DACs JA measured showed no difference at all.

It would interesting to see the result using your Berkeley alpha, which I think is Gordon's finest work.

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Can you clarify concerning "Gordon's finest work'? If understand correctly, BADA licensed the Streamlength code, as did Ayre and a bunch of others.
Am I wrong in saying he had nothing to do with designing the DAC, he just licensed the USB solution.

I believe that this Jitterbug device (although I haven't heard it) or the Regen device (which I have heard) should be good proving ground for the measurements Vs audibility debate & far more productive as differences can be heard but the few measurements produced are not conclusive.


The measurements promised for the Regen were with/without eye diagrams & possibly ground plane noise differences within a DAC

I'm just wondering what measurements are needed by objectivists to correlate with audibility?

Obviously the eye diagram is not a direct correlation - as I explained above, it is just a USB compliance test which shows a gross level jitter measure that ensures jitter will not be bad enough to cause drop outs or data errors

Do objectivists only want to see frequency & amplitude measurements of the analogue output of a DAC?

Why I think this will be a productive area for the debate of measurements Vs audibility is because the audible differences are undeniable & yet the current measurements done a very deniable. So I believe, that any true objectivists who searches for truth will chase down what measurements are needed to characterise these devices.

dallasjustice said:

Amir has the Berkeley USB to AES. Although I wasn't clear, he knows what I am talking about since I was talking to him. Just so you can keep your scorecard straight, Gordon did design the very fine Berkeley USB to AES converter.

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Ok, I am still confused. All the literature and reviews I have read of that product says he simply supplied the Streamlength code. Do you have specific
information about him designing the whole product? I am not saying you are wrong, having read a lot about that converter, I have never heard about
him being the designer.

dallasjustice said:

The primary claim being made with the Regen and Jitterbug is that jitter is reduced. I noticed that the Wyrd does claim to improve drop-outs but says nothing at all about lower jitter.

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actually, that is not correct. there is no primary claim about the REGEN. they say that it is just as effective not only reducing jitter, but in reducing packet noise, which actually causes jitter-and that the noise
can actually find its way into the output of the DAC chips.

fully explained here:
http://uptoneaudio.com/pages/j-swenson-tech-corner

Purite Audio said:

I can't seem to see any measuremtnts of the analogue output of the dac, showing the improvement?
Can you point me to them?
Thanks
Keith.

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Who said anything about measurements? You are talking to the wrong fellow Purite.

dallasjustice said:

The primary claim being made with the Regen and Jitterbug is that jitter is reduced. I noticed that the Wyrd does claim to improve drop-outs but says nothing at all about lower jitter.

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Nope, that's not the primary claim with the Regen - not sure about the Jitterbug.
The explained mechanism that the Regen is said to address is that signal integrity (SI) is important to the internal USB PHY (physical layer) of the USB receiver. Worse SI causes the PHY to work harder & as a result, the PHY itself generates noise (called self-noise or logic induced noise) which pollutes the ground plane which has an influence on the DAC's analogue output. This ground noise can cause issues on the ground of the audio clocks, the DAC's ground reference, etc. & thus effect the digital processing of the DAC.

Jitter is a much maligned term as it tends to be used as a catch all for the unexplained. Yes the SI can cause noise which correlates with the signal & some might consider this jitter but it would be a mistake to test for jitter - a totally different test should be used - this is one of the pitfalls of labelling or categorising issues without knowing or thinking about the mechanism of operation & deriving some measurements which best examine the proposed mechanism.

It's one of the reasons I asked Amr what measurements he proposed after he posted the comment along the lines that "it should take only minutes to measure the device"

The same issue arose a while back with Archimago suggesting that he would measure the 8KHz noise spike as a measure of the effectiveness of the Regen - totally missing the point of the Regen

Ah, in the writing of this post I see Andre has answered more succinctly

jkeny said:

Nope, that's not the primary claim with the Regen - not sure about the Jitterbug.
The explained mechanism that the Regen is said to address is that signal integrity (SI) is important to the internal USB PHY (physical layer) of the USB receiver. Worse SI causes the PHY to work harder & as a result, the PHY itself generates noise (called self-noise or logic induced noise) which pollutes the ground plane which has an influence on the DAC's analogue output. This ground noise can cause issues on the ground of the audio clocks, the DAC's ground reference, etc. & thus effect the digital processing of the DAC.

Jitter is a much maligned term as it tends to be used as a catch all for the unexplained. Yes the SI can cause noise which correlates with the signal & some might consider this jitter but it would be a mistake to test for jitter - a totally different test should be used - this is one of the pitfalls of labelling or categorising issues without knowing or thinking about the mechanism of operation & deriving some measurements which best examine the proposed mechanism.

It's one of the reasons I asked Amr what measurements he proposed after he posted the comment along the lines that "it should take only minutes to measure the device"

The same issue arose a while back with Archimago suggesting that he would measure the 8KHz noise spike as a measure of the effectiveness of the Regen - totally missing the point of the Regen

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Exactly. Well presented sir.

Purite Audio said:

You wrote that 'the noise can actually find its way to the analogue output of the dac'
So that noise would appear in any measurement of the analogue output?
Keith.

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Where did I say that. I said output of the DAC chips and I made it clear this was taken from the link.

Purite Audio said:

You wrote that 'the noise can actually find its way to the analogue output of the dac'
So that noise would appear in any measurement of the analogue output?
Keith.

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Really? Any measurement of the DAC's analogue output? Pray tell us more!

jkeny said:

Really? Any measurement of the DAC's analogue output? Pray tell us more!

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..LOL.

dallasjustice said:

Actually, it doesn't. If you know how a well engineered USB receiver works, you would know the rise time on the square wave ONLY matters to the extent it produces a data error.

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You mention data error as if the USB used for audio playback is the same thing as a file transfer using the USB protocol.

These aren't the same at all.

The assumption that these do not matter for real-time audio with USB doesn't hold.

There are a lot of things happening with USB and an audio stream system that have nothing to do at all with the bits as they are recovered (if recovered to the precise exact data sent), but everything to do with the electrical issues occurring all around the streaming and recovery of the data.

dallasjustice said:

Many have postulated logical explanations as to why these USB reclockers could help jitter. But nobody has ever proven their claim with a measurement in the analog domain. Maybe it's there. But there's no independent support for any of these hysterical claims.

So, I remain skeptical.

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Why don't you get a Regen and then do the measurements in the analogue domain as you would like to see them?

YashN said:

You mention data error as if the USB used for audio playback is the same thing as a file transfer using the USB protocol.

These aren't the same at all.

The assumption that these do not matter for real-time audio with USB doesn't hold.

There are a lot of things happening with USB and an audio stream system that have nothing to do at all with the bits as they are recovered (if recovered to the precise exact data sent), but everything to do with the electrical issues occurring all around the streaming and recovery of the data.

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There are a lot of things happening with USB and an audio stream system that have nothing to do at all with the bits as they are recovered (if recovered to the precise exact data sent), but everything to do with the electrical issues occurring all around the streaming and recovery of the data

I think the above statement is paramount.

Orb said:

IMO the key aspect here is scope-focus on what your active components are with the product, unfortunately it is not enough to look at just a limited few great engineered products and without considering other environments and say it shows the regen does nothing

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Exactly. I see that kind of fallacious thinking quite often these days, as if all equipment are perfectly engineered...

So the plot thickens . The results of HiFi news measurements runs counter to principles of electronic signals. As I explained in the 44 Khz thread in the science forum, a square wave has infinite bandwidth in theory. That is how it can get its sharp transitions on the edges. Any filtering as such, will reduce the waveform's fidelity, not improve it. Rise time for example should suffer because any filtering will impact the edge, reducing its slope, not increase as was reported in the review measurements.

So being the person that I am , I took the two measurements from the review, changed the color of the one that used the AQ Jitterbug to blue, and overlaid the two in photoshop. Here are the results:



Notice two things that AQ has done:

1. The flat portion of the waveform is now tilted down (left part of it is "tilted up" as I noted on the graph). What causes this? Boosting the low frequencies of a square wave. Here is a great web site on electronics in general, and this topic in the specific. This is the original square wave:http://sound.westhost.com/articles/squarewave.htm



And this is what happens when you boost the low frequencies by 6 db:



As you see the effect very much resembles what AQ has done to our USB pulse train.

2. The rise time is how fast the voltage rises from 10% value to 90%. This is a hard measurement to perform when you have jittery, noisy signals. You either perform a statistical analysis or eyeball it. Using the latter, to my eyes, both waveforms start at the same point at the bottom left, but by the time they get to the top, the AQ waveform on the average is actually lower than doing nothing (in red). The no AQ waveform in red rises to higher value on the average than the blue with AQ.

Seems to me then that the rise time was better without this device which is what the theory predicts.

It is also odd that Paul says that without AQ, the measured rise time was 22 nanoseconds. The limit for compliance with USB spec is 20 nanoseconds. I have a hard time imagining such a short cable, as generic as it may have been, to have failed compliance. With AQ, Paul says the rise time shortened to 14 nanoseconds. Per above, I don't see how that happened. Nor would the theory predict that this kind of improvement after filtering a square wave (unless there was some improved impedance matching).

My theory is that Paul may have swapped the two values. I expect the cable with no EQ to have the compliant rise time of 14 nanoseconds and the AQ worsened that to 22, not the other way around as reported. This would also agree with the previous review that we discussed where my read was that the AQ made his cable unreliable.

Of course all of this is based on some fuzzy pictures so accuracy is not there. But I wish Paul would have put the cursor markers on the waveform as is customary for such measurements to see what points he had used to compute the 10% and 90%. Paul's work is impeccable in quality so I don't expect him to have made such a mistake. But the data is hard to rationalize otherwise.

Anyway, as I said, all of this is immaterial in grand scheme of things.

dallasjustice said:

If you read the thread, you would know I ordered one.

If Amir would be willing to measure it with his gear, I would send it to him.

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I would love to measure it. But before I trouble you, how much was it?