[gregorio]

Perhaps I'm reading it wrong. The highest spike in any of those charts appears to be -126dB. I can't imaging that being even close to audible.

True but the schiit Modi results indicate it's susceptible to noise through the USB cable, get a several meter USB cable and a very noisy source and you could be up into audible territory.

G

 

[bigshot]

He should try testing it with a longer cable and find out. If it comes out below the threshold, it probably doesn’t matter

 

[amirm]

He should try testing it with a longer cable and find out. If it comes out below the threshold, it probably doesn’t matter

I will put it on my TODO list.

 

[Arpiben]

The AES3 Jitter test signal used in the measurements (12kHz -3dBFS/ 1LSB 250Hz) is not the most appropriate for [DAP+USB cable + DAC+Audio Out cable] system noise characterization when dealing with USB Audio.

With USB (or HDMI) audio content is sent in packets and not as a synchronous continuous stream the J-Test signal can not induce jitter.
Because we can’t stimulate jitter at a particular frequency, unless there is a specific coherent interference source causing it, any jitter that is present may very likely be random and will just cause an increase in the noise floor with no visible spikes in the FFT.

At the end, by switching USB cables in the system you end up measuring differences among: Noise floor variations and spurious/spikes from coherent interference sources .

Nevertheless if we expand the actual frequency range of observation to 20Hz-20kHz ( not only fs/4 +/- 2kHz) or at the hundred MHz range we should be able to have a better view. The difficulty will always be how to interpret the results.

That said, I agree that the J-Test even if not used in its context allows to detect part of the design flaws.

 

[mandrake50]

@bigshot
Nor, perhaps more important, interfering with the transmission of the digital signal. Even if someone believes that somehow this is passed on through the DAC and into the audio stage (not likely if even possible) it is insignificant.

 

[amirm]

@bigshot
Nor, perhaps more important, interfering with the transmission of the digital signal. Even if someone believes that somehow this is passed on through the DAC and into the audio stage (not likely if even possible) it is insignificant.

Actually all of my measurements are through the DAC. I mentioned the DAC models in my post/links. I could care less what happens upstream on USB bus.

 

[amirm]

The AES3 Jitter test signal used in the measurements (12kHz -3dBFS/ 1LSB 250Hz) is not the most appropriate for [DAP+USB cable + DAC+Audio Out cable] system noise characterization when dealing with USB Audio.

With USB (or HDMI) audio content is sent in packets and not as a synchronous continuous stream the J-Test signal can not induce jitter.
Because we can’t stimulate jitter at a particular frequency, unless there is a specific coherent interference source causing it, any jitter that is present may very likely be random and will just cause an increase in the noise floor with no visible spikes in the FFT.

J-Test actually remains quite relevant in our tests of USB DACs. It is dither free, high-frequency and toggles all the data bits. All of this has a way of causing jitter components to become visible far, far more than they would with music. Imagine the last time you saw near 0 dBFS music content at 12 Khz! You would go deaf listening to that to say nothing of your tweeter cooking good. I plan to write a more detailed article on this in a future as there is a lot of confusion in this regard.

As to noise, we actually pay attention to noise floor rising and as a result, the amount of random jitter there. So by your own notion, it works there.

On packetization of USB, that actually shows up nice too at its packet timing if it bleeds into either Vref or Clock signal.

At the end, by switching USB cables in the system you end up measuring differences among: Noise floor variations and spurious/spikes from coherent interference sources .

Isn't that what the question was? That there is more than "1s and 0s" here?

Nevertheless if we expand the actual frequency range of observation to 20Hz-20kHz ( not only fs/4 +/- 2kHz) or at the hundred MHz range we should be able to have a better view. The difficulty will always be how to interpret the results.

That said, I agree that the J-Test even if not used in its context allows to detect part of the design flaws.

There is no difficulty here: we can't hear the ultrasonics so easy to interpret. I have done wide-bandwidth measurements at times. See example of my review of Uptone LPS-1 supercap power source: https://audiosciencereview.com/foru...ar-power-supply-review-and-measurements.1849/

As you see here, there was no reduction of noise as claimed by the manufacturer at higher frequencies higher.

Be careful of frequencies well above this as in Mhz you ask about. High-speed ADCs running at these speeds have much lower dynamic range than the 24-bit converters in my analyzer. And at any rate, things like probing matters as otherwise you pick up a bunch of common mode noise you can confuse as DUT output.

Anyway, the key here that J-Test signal was plenty to show differences here. Indeed this has caused the designers of a number of products to go back to the drawing board and rethink what they had assumed. In the case of UPtone ISO Regen for example, it showed mains AC leakage due to switchmode supply which has nothing to do with jitter:https://audiosciencereview.com/foru...so-regen-review-and-measurements.1829/page-18

 

[bigshot]

I really don’t see the point of discussing inaudible things. It just justifies the snake oil.

 

[Arpiben]

J-Test actually remains quite relevant in our tests of USB DACs. It is dither free, high-frequency and toggles all the data bits. All of this has a way of causing jitter components to become visible far, far more than they would with music. Imagine the last time you saw near 0 dBFS music content at 12 Khz! You would go deaf listening to that to say nothing of your tweeter cooking good. I plan to write a more detailed article on this in a future as there is a lot of confusion in this regard.

As to noise, we actually pay attention to noise floor rising and as a result, the amount of random jitter there. So by your own notion, it works there.

On packetization of USB, that actually shows up nice too at its packet timing if it bleeds into either Vref or Clock signal.


Isn't that what the question was? That there is more than "1s and 0s" here?


There is no difficulty here: we can't hear the ultrasonics so easy to interpret. I have done wide-bandwidth measurements at times. See example of my review of Uptone LPS-1 supercap power source: https://audiosciencereview.com/foru...ar-power-supply-review-and-measurements.1849/

As you see here, there was no reduction of noise as claimed by the manufacturer at higher frequencies higher.

Be careful of frequencies well above this as in Mhz you ask about. High-speed ADCs running at these speeds have much lower dynamic range than the 24-bit converters in my analyzer. And at any rate, things like probing matters as otherwise you pick up a bunch of common mode noise you can confuse as DUT output.

Anyway, the key here that J-Test signal was plenty to show differences here. Indeed this has caused the designers of a number of products to go back to the drawing board and rethink what they had assumed. In the case of UPtone ISO Regen for example, it showed mains AC leakage due to switchmode supply which has nothing to do with jitter:https://audiosciencereview.com/foru...so-regen-review-and-measurements.1829/page-18

Thanks for your detailed answer as well as the measurements provided.
Making it short, I am not arguing about Julian's Dunn J-Test being relevant for DAC' s charcaterization of various jitters: intrinsic/sampling/cable induced/etc...
Dealing with packets remove the 1 LSB/250Hz composant and keep only the fs/4 since it is not inducing any Packet Delay Variation or in other words any data jitter.

Now by swaping USB cables and comparing the different spectrum/FFT you need to assume that your Digital Audio Player used during testing is always sending the data with same PDV and same internal noise. You are also assuming that the EMI/RFI in your testbench area is constant or neglectable It is probably what you are already doing.

Now as an audio layman, I am also wondering like @bigshot and others if all this is audible?
In my short experience, I have been more bothered by AC components than anything else.
Rgds.

 

[bigshot]

I don't understand why someone would say that one DAC is better or worse than another because it measures higher or lower while still being well beyond the threshold of transparency. I judge equipment by its ability to do the job of playing music for human ears. I haven't run across any player or DAC or cable or amp in the past 20 years that doesn't fit the bill on that score. Sound Science people can sometimes be just as prone to focus on stuff that doesn't matter as audiophools. If they enjoy the mental exercise, that's fine. But I sure hope no one takes this stuff into account when they make buying decisions. It's a waste to spend money on things you can't hear. Better to focus on features and usability.

 

[reginalb]

...I haven't run across any player or DAC or cable or amp in the past 20 years that doesn't fit the bill on that score....

You're clearly knowledgeable, and have mentioned in the past that you're pretty careful in your selection of gear. I have definitely run across a couple items that don't fit the bill. I had a Hifiman DAC/amp that was very susceptible to noise, and it got really audible at times even with (quiet) music playing. I had a "Viper" (the company isn't around any more I don't think) amplifier for subwoofers in my car when I was teen (yeah, I was one of those kids) that produced so much distortion it was just awful to listen to. I had to fight with the local car audio store to take a return on the thing.

Those were the only audio devices that I've owned with clearly audible issues, but I'm just saying, they're out there for sure.

 

[bigshot]

They were probably defective from crappy manufacturing. Did you return them for your money back? I sure would. I had a laptop with a defective mother board, it wouldn't even boot up properly. And I bought an amp once that ran for fifteen minutes and made a popping sound and it was dead. Those both got packed back up and exchanged.

 

[reginalb]

They were probably defective from crappy manufacturing. Did you return them for your money back? I sure would. I had a laptop with a defective mother board, it wouldn't even boot up properly. And I bought an amp once that ran for fifteen minutes and made a popping sound and it was dead. Those both got packed back up and exchanged.

As I mentioned, the amp for the car I did for sure, but it took a fight with the store.

I didn't return the Hifiman, it was $20, I think, and I did use it. It was better at home where there were fewer sources of interference than my office. I eventually sold it.

But I think that at least in the case of the Hifiman, it was a design flaw. Couldn't tell you on the amp.

 

[bigshot]

I guess you can't expect much for $20.

 

[amirm]

I don't understand why someone would say that one DAC is better or worse than another because it measures higher or lower while still being well beyond the threshold of transparency. I judge equipment by its ability to do the job of playing music for human ears. I haven't run across any player or DAC or cable or amp in the past 20 years that doesn't fit the bill on that score. Sound Science people can sometimes be just as prone to focus on stuff that doesn't matter as audiophools. If they enjoy the mental exercise, that's fine. But I sure hope no one takes this stuff into account when they make buying decisions. It's a waste to spend money on things you can't hear. Better to focus on features and usability.

There are good number of reasons why we need to measure and get insight into DACs and make purchase decisions based on that:

1. Measurements are repeatable and reliable. This is far cry from someone saying this and DAC sound the same. How would I verify that?

2. Our goal should be for a DAC that is transparent for all people and all content. Again having someone test a few samples of music and their ears alone is not sufficient.

3. Where possible we need to reward best in class engineering as opposed to sloppy ones. Yes, this is a philosophy but there are thousands of DACs in the market these days. Many are put together by incompetent designers. We need to weed them out and give our money to those that know what they are doing. Why reward bad work? Would you buy a car that has some paint chips on it just because it drives the same as other that doesn't at the same price?

4. I have examples of DACs with audible problems: Schiit BiFrost.

5. For any technology, it is good to have a margin of safety. Well designed equipment goes beyond mere threshold of detection for distortion.

6. I have tested a ton of digital products in the last year in ASR Forum and there are some real dogs there that are only identified when we measure them. I like to see us avoid these products even if in casual listening we can't hear such problems. Why throw good money after bad?

7. We need to check our assumptions. Far too often as objectivists we throw our weight around that such as can't possibly be measureable or audible. Well, it is good to have data behind the former. My experience is that in many cases our dogma is incorrect. Take the example of USB cables making a measureable difference. As a group, we need to have data behind our conclusions than just saying, this and that is the same.

8. Another point is learning. By measuring and looking at the data, we learn more about our audio systems and can figure out how to better test them audibly. How would we know what type of music to use for audibility test if we lack such data?

So while in summary I agree that if two products are truly the same audibly, we should buy either one, that doesn't translate into real world where we can't really ascertain that in a reliable manner.