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Airport Express performance: objective and subjective


usernaim250
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I'm moving a discussion started in another thread because it deserves its own. I inquired about two views of the technical performance of the Airport Express--one from Stereophile, and one from member Steve N. (of Empirical Audio). I'll recapitulate it here and solicit any opinions on that as well as listening opinions.

 

I asked:

Meanwhile, Steve, do you have any empirical evidence for your jitter claims about the Airport Express? The only published tests I know contradict your claims.

"The grayed-out trace in fig.7 shows a similar spectral analysis of the Musical Fidelity X-DACV3's analog output while it was driven by the AirPort Express via the Monster TosLink cable. The noise floor has dropped by 4–5dB, the word-clock jitter to a respectably low 258ps, which is actually better than the case with the standalone D/A processor driven directly by my PC's S/PDIF output (provided by an RME PCI card). ....The beauty of this unassuming component, however, is its S/PDIF data output, which allows the AirPort Express to assume a respectable role in a true high-end audio system. "

http://stereophile.com/digitalprocessors/505apple/

 

For comparison, measuring the same DAC:

"Musical Fidelity claims very low jitter for the X-DACV3. Using an older version of the Miller Audio Research Jitter Analyzer that Musical Fidelity uses, I got slightly higher figures than MF's figures: 300 picoseconds peak–peak with 16-bit data fed from my PC with a TosLink connection, 244ps p–p using a coaxial connection from a PS Lambda transport—both figures are still low in absolute terms."

http://stereophile.com/digitalprocessors/1204mf/index2.html

 

In other words, AE: 258ps, PS Audio Lambda, a highly rated expensive dedicated transport: 244ps. Either is likely to be lower than OP's cd player, despite the Toslink.

 

 

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""Meanwhile, Steve, do you have any empirical evidence for your jitter claims about the Airport Express?"

 

Sure, I have made measurements with my scope and done listening tests. These are both empirical. Measurements with equipment are not the only empirical measurements. My ears can measure too.

 

It is simply the worst jitter WiFi device I have heard, which validates the measurements.

 

Low jitter is:

http://www.positive-feedback.com/Issue43/jitter.htm

 

Steve N.

Empirical Audio"

 

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Hi Steve, could you say a bit more about your measurement technique? Do you have comments on why your results are so at variance with the published results?

 

I'm interested too in any specifics of your listening evaluation. I agree that listening is empirical evidence, but I'm not sure whether you can state unequivocally that it is direct evidence of jitter. As I recall, there is no consensus on the "sound" of jitter.

 

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"Hi Steve, could you say a bit more about your measurement technique?"

 

It's been years, so I dont remember exact numbers. Real-time, just comparing edge movement on a sampling oscilloscope. Nothing fancy. It was about 1nsec at least, depending on the computer.

 

You must understand that I had a products for several years based on both modded AE and Transit, the Original Off-Ramp and Off-Ramp WiFi. Both of these use the PCM270X series of parts from TI. Very jittery part, even with a Superclock driving it.

 

"Do you have comments on why your results are so at variance with the published results?"

 

The clock in the computer, the power over the USB cable, and the USB cable itself all play into this. If the computer has a jittery clock and you use a cheap cable 5m long and the power output on that particular USB jack is noisy, it could easily be 2-3 nsec even 10 nsec of jitter.

 

As for concensus, I dont give a rip about this. I know what I know, as do the other industry experts. As I stated in my paper, 99% of systems out there are too poor to do ANY jitter comparisons, even with 1000nsec of jitter versus ~0nsec. So when I say you need a resolving, noise-free system to hear this, I mean it. Look at how many industry experts consider jitter the #1 problem with Digital Audio:

 

http://www.positive-feedback.com/Issue41/ca_intro.htm

 

Steve N.

Empirical Audio

 

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"The clock in the computer, the power over the USB cable, and the USB cable itself all play into this. If the computer has a jittery clock and you use a cheap cable 5m long and the power output on that particular USB jack is noisy, it could easily be 2-3 nsec even 10 nsec of jitter."

 

a) We can forget about the computer for the moment. Atkinson also measured a PS Audio Lambda transport and got only very slightly less jitter than with the Airport Express over the Musical Fidelity DAC and pronounced it respectable. Yet you say it is awful. That is quite a big difference. Is the Musical Fidelity X-Dac that good at scrubbing jitter? And if so, do other DACs share this trait? Which ones?

 

b) You say a few years ago...were you working with the older -g model or the current -n (Stereophile measured the g). Do they use the same hardware relevant to jitter?

 

c) My dispute is not whether jitter is audible (I have no proof either way). What I am questioning is whether one can use the ear as a direct measure of jitter to the extent one could assess absolute levels of jitter very precisely or to say the difference between a and b is x amount of jitter.

 

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Usernaim - excellent topic and posting. I'm pleased you began this one. Reading this and other sources led me right back to my Airport Express. The comments made by stereophile are more than enough to convince me that coupled with the right DAC you can achieve high end performance from this cheap gadget. - if only I wasn't suffering from dropouts in iTunes 8 :-( ... if I knew buying a new router would solve my problem I would but what if it doesn't.... :-(

 

"slightly better than when driven by my PC's RME card" ... says enough for me ... RME - industry standard stuff. !?

 

I'll still use other media players and outputs of course as I like to tinker and always will.

 

 

HTPC: AMD Athlon 4850e, 4GB, Vista, BD/HD-DVD into -> ADM9.1

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I know you are not asking me, but I'll have a go on this anyway.

 

First off, we must assume that we hear jitter when we perceive a difference. And, I am ignorant enough to think that when a measurement tells there's hardly any jitter, there still is. Or, of course, it is something else we're hearing. But let's say that, so far, it can be about jitter only. In that case :

 

Jitter actually is about a "signature" it presses upon the music, which better can be translated to a "spectrum". Generally you could say that very high levels of jitter bears a signature that is all over the spectrum, and it would typically what AE (or WiFi in general) would incur for. However, I never heard it (and never tried to). A too long cable would IMO incur for the same, and this is what I sure heard. A smeared grey sound. Nothing to even begin with improving. Nowhere to start, nothing to grab.

 

Low(er) levels of jitter may have a signature towards worse bass. Or the other way around, towards better bass because it's more in the highs. Mind the "because", because my experience tells me it's almost mutual exclusive. A sort of : you can't have it all.

 

Now, I will certainly start to be confusing if I say that this was my "old school" perception of jitter, and actually this is/was about "better or worse";

There's also my perception of a thing which may be best explained by referring to wow of the turntable. This is about human voices being on key. But this is also about flanger. Both go along, and while the first is about frequency and the latter about amplitude, this looks more like jitter to me (interpreting the phenomenon to my best knowledge).

My own "new school" perception tells me that the stuff about "grey sound" is not about jitter, but about something else we don't know about yet. It may be jitter impeeded, but the effects go beyond it. It degrades more than jitter itself is capable of.

 

When Steve says this doesn't even allow itself to be measured, I sure believe that. If it's not jitter, it won't allow to be measured at such. But in the mean time things are wrong all over.

Keep in mind that jitter is measured by devices feed by software, and in the end the algorithms for it were designed by humans. That is, a long time ago. Today we slowly start to discover there's more. Or at least I am, btw driven by the sole fact of hearing huge differences where official measurement tells me I'm crazy.

 

So that's it. I am, until proven differently.

Peter

 

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Peter,

 

Are you suggesting that we can hear something that cannot be measured ? Any equipment used to measure jitter is going to be vastly more capable of doing so that our own ears, surely.

 

Matt.

 

HTPC: AMD Athlon 4850e, 4GB, Vista, BD/HD-DVD into -> ADM9.1

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Hi Matt,

 

You may have missed a few days, including my ever stating that what we hear on differences just cannot be measured ?

... and perhaps including what I just said : most probably this isn't about jitter at all.

 

You were the most asking for measurements. Here is a very small part of it : http://www.computeraudiophile.com/content/Its-Friday-here-so-here-BIG-one

But you must have seen that thread by now ...

 

Peter

 

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Here's the thing--I know what wow sounds like. But jitter as you describe it is both a moving target and indescribable.

 

Not only that, but you stipulated at the beginning that any audible difference is due to jitter, but that is a mighty big assumption. It may be true, but so far as I know, as tough as it is to say what jitter "sounds" like, there is likewise no demonstration that any differences a listener reports in a digital system are due to jitter.

 

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"Atkinson also measured a PS Audio Lambda transport and got only very slightly less jitter than with the Airport Express over the Musical Fidelity DAC and pronounced it respectable. Yet you say it is awful. "

 

It's my opinion. The AE and all Transports are unacceptable to me. I used to mod them you know, even the expensive ones. The modded ones were a bit better. This is why I dont mess with transports anymore.

 

"That is quite a big difference. Is the Musical Fidelity X-Dac that good at scrubbing jitter?"

 

Beats me.

 

"And if so, do other DACs share this trait? Which ones?"

 

Steve N.

 

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"You say a few years ago...were you working with the older -g model or the current -n (Stereophile measured the g). Do they use the same hardware relevant to jitter?"

 

They use the same output section and crystal. Same chips.

 

Steve N.

 

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"What I am questioning is whether one can use the ear as a direct measure of jitter to the extent one could assess absolute levels of jitter very precisely or to say the difference between a and b is x amount of jitter."

 

Well, I recently took the jitter test with 4 tracks, all different jitter levels.

 

 

My guess Actual

A A

B B

C D

D C

 

 

C and D differed by 10 nsec, so with this particular track, it was difficult to hear the difference through all of the other noise, and it was a noisy track.

 

Steve N.

 

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This is certainly interesting, but would you say there is a specific symptom of jitter? Because it appears from people's descriptions that it's not a single identifiable or set of identifiable qualities but just a general feeling of goodness or not across the entire spectrum of audio qualities. Tighter, clearer, smoother, better bass, less veiled, less fuzzy, etc. My point is that whereas for wow, for instance, or a particular bass resonance, there is a clearly delineated sound to it, the way jitter is described it is in the same category as many other general improvements. I'm not saying it's not audible or doesn't make a difference (for the moment I am agnostic on that point), but that the correlation from sound a to cause b is difficult to establish.

 

By the way, though I don't think it was out of bounds for HDD folks to point out that analysis tools would allow one to pick out the clips, I thought is was quite offensive that they instantly assumed you did not take the test by ear with no basis.

 

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Peter - I see where you're going with this - but any difference you hear can be measured. I don't think there is any argument one way or the other in this instance. If something sounds different, that difference is measurable because it is different. Otherwise it's your own ears playing tricks on you.

 

Usernaim - without be rude to Steve, I'd stick with what you have read regarding the Airport Express. Any decent DAC these days should reduce to jitter to an aboslute minimum and reading suggests it's not a big issue with the AE.

 

Steve - is there just a hint of a possibility that you measured the difference between those files ? I know you have measuring equipment so ... .. . ?

 

HTPC: AMD Athlon 4850e, 4GB, Vista, BD/HD-DVD into -> ADM9.1

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Beemb, indeed I will stick with published tests over anecdotal memories from a few years ago. I'm under no illusion the AE is any great shakes--but it is a router and a streamer for $79 refurbished! And is apparently at least not horrible in comparison to middle-rung equipment in this application.

 

That said, at the moment I am using mid-90s DACs, an Adcom GDA-700 and a Muse Model 2+, and I don't know if they have jitter rejection on par with modern units. I am shortly to replace the Muse with a Benchmark, but not on the system that uses the Airport.

 

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sure usernaim, it is .. but as a way to get a digital stream to a decent DAC, you don't need much more.

 

Thought you'd like that Steve ;-) .. It's worth asking though as you have mentioned your measuring equipment before.

As a manufacturer posting on here I think we need to ask such questions. The fact you respond politely is great.

 

HTPC: AMD Athlon 4850e, 4GB, Vista, BD/HD-DVD into -> ADM9.1

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It is not important at all, but why do you sound like Ashley ?

 

Why do you suddenly hammer the "what can be heard can be measured" thing and sound like Tim ?

 

Why do you urge Steve to use his measuring equipment while he once per week says he does NOT have that (it was even in this thread I think).

 

Wasn't it passed a 100 times that what we hear can NOT be measured by known means ?

 

Peter - I see where you're going with this - but any difference you hear can be measured.

 

Which is a response to my pointing to a thread which just shows that ? I don't get it.

I have measurement equipment, and at least what I'm talking about does not show. It will show jitter differences between using the AE and without though (I'm sure, but I don't have an AE).

 

So in the end you are right. What can be heard can be measured, but not by the common means but with special software I'm writing. You don't want pictures of common/official measurement means that don't show a difference to prove something, do you ?

 

?

Peter

 

Lush^3-e      Lush^2      Blaxius^2      Ethernet^2     HDMI^2     XLR^2

XXHighEnd (developer)

Phasure NOS1 24/768 Async USB DAC (manufacturer)

Phasure Mach III Audio PC with Linear PSU (manufacturer)

Orelino & Orelo MKII Speakers (designer/supplier)

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"Steve - is there just a hint of a possibility that you measured the difference between those files ? I know you have measuring equipment so ... .. . ?"

 

Dont even go there.

 

The second set of tracks, my wife and I both missed it by a mile. I consistently picked the worst track 100nsec as one of the best and the best track as one of the worst. It think it was a poor test case, thats all.

 

We are all, even me, trying to learn something here, and I appreciate what HDD audio is doing.

 

Steve N.

 

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Peter,

 

I guess in some ways my "measuring" comments may indeed sound like Ashley and Tim. However, what's wrong with some science and stats to back up claims.?

HTPC: AMD Athlon 4850e, 4GB, Vista, BD/HD-DVD into -> ADM9.1

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Hey Matt - I like your comment about the varied responses around here. I think real progress can be made with all sides of an issue being represented and accepted enough so people feel free to give an opinion. i don't think there is anything wrong with people who say "Measurements or it doesn't exist" or with people who choose the "ears only" route.

 

Did you read Michael Fremer's comments in the article mentioned on the front page today? It is interesting that he took a test at an AES convention where two amplifiers measured the exact and he was 5 for 5 in a blind test when asked to distinguish between the two. It's an interesting read.

 

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I'm not sure if this has ever been asked before, but isn't the question about what you are measuring rather than if the difference is meaurable.

 

To begin my hyphothesis here I'm going to make an assumption that two different devices truely have an audible difference. Now the two "devices" can be iTunes and Amarra; a Benchmark DAC and Lavey DAC; two different cables, etc.

 

So for my hyphothesis: two different speaker cables are put on a test rig one is a generic 186 strand OFC copper cable; the other a Nordost Valhalla. I think it's generally accepted as fact that both will measure exactly the same (given normal lengths and bandwidth). Certainly any difference will be marginal. Yet 3m length of the basic copper cable will cost you maybe £5-10 where as the Valhalla cable is in region of £2-3000 (hope this are reasonably accurate figures but I'm lying in bed with iPhone).

 

Now instead of using the cable between a signal generator and spectrum analyzer, we put them into a real world system of a high end audio system with suitably revealing components (all in anachroic chamber of course)

 

If we use a calibrated microphone and suitably accurate analogue to digital converter can we then measure any (repeatable) differences or are those differences just too minute to measure?

 

I don't know the answer. Just a thought. Maybe the differences just aren't measurable when measured in isolation. It's the interaction of the various parts of a system. Like the air flow around a butterfly's wings in the Amazon can't be measured, but the (theoretical) storm can definately be expeiences.

 

Just a few thoughts, I ward the van coming with the men in White coats now.

Eloise

 

Eloise

---

...in my opinion / experience...

While I agree "Everything may matter" working out what actually affects the sound is a trickier thing.

And I agree "Trust your ears" but equally don't allow them to fool you - trust them with a bit of skepticism.

keep your mind open... But mind your brain doesn't fall out.

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