Article: Measurements: First and Second Generation Apple AirPort Express

[The Computer Audiophile]

You can view the page at http://www.computeraudiophile.com/content.php?r=466-Measurements-First-and-Second-Generation-Apple-AirPort-Express

[jhwalker]

So am I reading this properly: the new (second?) generation Apple Express has lower jitter over the digital (Toslink) out, but very bad jitter if you're using analog out? So if I'm using the AE purely as a streamer into a remote DAC, the jitter (though still high) will not be nearly so bad as if I were using the analog out?

 

Just want to make sure I'm reading this properly.

[Old Listener]

Chris,

 

Thanks for posting this article. I'm not in the market for an old or new Airport Express but I like to see measurements for audio gear and learn from what I see.

 

Bill

[mayhem13]

Very Nice Chris!....From an avid Apple streamer....thank you!

[MikeJazz]

This is an opportunity for an after-market service....an "Audiophile" Airport Express, working as a pure streamer to a external DAC would be very nice...

Altough it would have to be done underground...I guess...

[TimU]

So am I reading this properly: the new (second?) generation Apple Express has lower jitter over the digital (Toslink) out, but very bad jitter if you're using analog out? So if I'm using the AE purely as a streamer into a remote DAC, the jitter (though still high) will not be nearly so bad as if I were using the analog out?

 

Just want to make sure I'm reading this properly.

 

I am also confused. It would be helpful if the conclusions were stated more clearly.

[JR_Audio]

Fingerprint

 

The analog output, is time wise build by the same master clock, that drives the bit cells at the digital out. So the jitter performance of the analog out has some good correlations to the audio band part of the jitter at the digital out.

 

Taking this into account, even your outboard DAC has better jitter suppression than the internal DAC of the AE, the sonic fingerprint of the jitter will find also its way to the analog out of the outboard DAC.

 

To make a long story short, yes, the AE First Gen will have a lower jitter and better sound with your outboard DAC, than the AE Second Gen. An outboard DAC can only suppress incoming jitter, but can never remove.

 

Juergen

[Akapod]

I think I get it.

 

"The second generation AirPort Express shows a reduction of 41% (average) and 18% (peak) jitter on the digital optical output. Comparing jitter

and

THD on the analog output of the first and second generation AirPorts,

it's clear there has been a major change between the units. Jitter and THD from the second generation AE has increased dramatically. This increase has the

largest impact

on over all performance,

even for audiophiles using the digital output only

. "

(Emphasis and a little punctuation added.)

 

So when looking only at jitter, you'd think the AE2 is the superior model. But when looking at the (more sonically important combination of) jitter and THD, the AE1 is the winner.

[HiFiInsider]

It's too deep for me to consume.

[JR_Audio]

To make my explanation a bit more clearer, I made a FFT of the FM Modulated Bi-Phase Signal (measuring the audio band related part of the jitter on the bi-phase digital output, similar what Ed Meitner has done in the past with his LIM detector on the master clock). Here you can see the impact of jitter into each audio frequency and you see, that the Airport Express 02 has in real, (real means what is left in the audio band), especially in the bass region, a much higher jitter than the latest version of the First Generation.

 

Forget all the numbers that are given about the total amount, or peak peak, or average jitter of the digital out signal. This is only one aspect, but not the most important aspect. The most important question is, what will be the impact onto the audio signal, that we are listening to.

 

Airport Express First Generation (latest production)

 

 

Airport Express Second Genertation (actuall production)

 

 

Juergen

[hearingaide]

I don't know if the charts were competently created or not, but assuming they were, the jitter results from either generation would likely be inaudible. The noise floor of even awesome systems is usually well above the jitter dB levels shown in the charts. It would be interesting to see a valid double blind ABX test between the two generations with actual music to see the overall difference of each generation compared to the other. I also wonder if there are any gain difference between the two generations, which would make volume leveling for a valid comparison very important.

[roscoeiii]

Chris,

 

I believe that there were more generations of the Airport Express before the one released in 6/2012. I think that the 2004 802.11g ones that Sterophile measured were replaced by a model often referred to as Gen 2 that came out in 2008, which upgraded to 802.11n.

 

Details here:

AirPort Express - Wikipedia, the free encyclopedia

 

But regardless, thanks so much for these measurements.

[The Computer Audiophile]

Hi roscoeiii - Thanks for the info. Maybe I should have called it gen 1 and gen 2 of the 802.11n version :~)

 

From Wikipedia:

 

• June 2004: AirPort Express released
  
• March 2008: AirPort Express 802.11n (1st Generation) released
  
• June 2012: AirPort Express 802.11n (2nd Generation) released
  

[hearingaide]

As the writer notes, jitter is just one piece of the puzzle in audio quality, but comparing the chart below to the two charts in a few posts before this, I suspect the jitter is inaudible in both generations:

 

[myco]

Nice work on the measurements, Chris. Very interesting... Juergen, also nice amplification of the above. - Rob

[JR_Audio]

I respect the work of Julian Dunn a lot. I have lot of my knowledge about jitter from his work. I have known him from his early works at Nonophon and then with Prism Sound and finally with Audio Precision. His book „Measurement Technique for Digital Audio“ is one of my standard books, when it comes to understand all differnt jitter terms.

 

But when his graph about the threshold of hearing jitter would be right, for experienced listeners, then we could stop all discussions and measurements about jitter and can stop looking for better oscizaltors, .... For me, the new Airport Express Second Generation is, via digital out, by far too bad, even for background listening.

 

I do not want to open a discussion, what amound of jitter is audible and what amount not, but if this threshold of hearing jitter graph would be right, then every digital transport would sound the same and every software player would sound the same. Just take the measurend graphs for your information. What you are doing with this, is up to you.

 

Juergen

[designmule]

I believe that there were more generations of the Airport Express before the one released in 6/2012. I think that the 2004 802.11g ones that Sterophile measured were replaced by a model often referred to as Gen 2 that came out in 2008, which upgraded to 802.11n.

 

I have both a wireless G model & a wireless N model. My DacMagic frequently lost lock when used with the N but is rock solid with the G. At the time I was struggling with this issue, forums and such indicated that the jitter in the N model was much higher than the G. I recall at least one DAC manufacturer stating that they were trying to work with Apple to get it under control. FWIW, my N model is now connected to an Onkyo receiver and it has had no trouble with losing lock.

[mayhem13]

It was mentioned earlier the possible 'need' for a HiFi grade AirPlay compatible network audio player. I've often thought about this myself as a self contained network receiver, much like the currecnt Airport, but including a well implemented DAC with analog outs as well as digital pass through. AirPlay/AirTunes protocol has been 'cracked' so to speak and google results would lead you to the info. I would suspect that for a manufactured device, a license would be required from Apple at a substantial cost. Don't see why an open source project would be a trade infringement though. Food for thought if there's any developers out there. XBMC started out as a small DIY project on a forum and wound up being a huge part of the streaming/HTPC movement with commercial developers moving towards dedicated ARM processors to run it natively. Might be a great DIY/CA project that the whole audio streaming community could enjoy. Surprised one of the DAC mfgrs hasn't moved in the direction of an AirPlay/DNLA capable network DAC. Cambridge has a network player, but it's not AirPlay enabled. Denon, Pioneer and Marantz have AirPlay enabled players but I wonder if they focus enough on DAC Implementation or more on functionality?

[ringenesherre]

I do not want to open a discussion, what amound of jitter is audible and what amount not, but if this threshold of hearing jitter graph would be right, then every digital transport would sound the same and every software player would sound the same.

 

That sounds like an entirely plausible conclusion. Of course, given that the transport is reasonably well engineered (as the Airport Express seems to be).

 

Cheers,

Peter

[1audio]

Jurgen:

Please post more details on your settings for the jitter measurements so others can duplicate them. I believe they are standard for the AP but may not match other systems.

 

Jitter is one of many aspects that can impact the audio quality of an audio chain. While it may be below the threshold of audibility the chart published is old work and newer research may show different levels. However, getting it right is no harder than getting it wrong in this case. It requires some effort at the original design stage but these levels do not require heroic levels of engineering and expensive components. They do require attention at the design and acceptance stages, something Apple is usually very good about. I don't believe Apple devotes much attention to audio quality as we define it. It has not hurt their bottom line.

[JR_Audio]

FM Demodulation

 

Basically, this is a FM Demodulation and analysing the result with an FFT, calibratet do time.

 

Ed Meitner was the first (to my knowledge), doing something similar. He was looking at the master clock, direct at the master clock pin of DA chips. Here you have to know what oversampling rate the unit is running, then you tune a fixed, low jitter oszilator to that master clock frequency, and use a FM Tuner as demodulator to get the FM part (= Jitter) out of the master clock.

 

When Julian Dunn was working for / with Audio Precision (his above mentioned book is a very, very good source about Jitter and more), they devoloped a mention method even one step further, to FM demodulate the Bi-Phase Signal. The exact method for measureing the Bi-Phase signal is maybe a step too far at this stage, but …

 

You can search for some old literature about Ed Meitners LIM detector and also the highly recommended book from Julian Dunn (even when is limits about hearing of jitter is, at least in my opinion, by far too high), but he has a very clear writing about jitter. You will find also some of his earlier work with Nanophone on the web. Good luck.

 

Juergen

[1audio]

I do understand the principle. I have worked out a way to use an FM tuner to demodulate the jitter of a master clock. It works pretty well for all but the really high performance clocks. Calibrating in standard terms has eluded me so far. Following the transforms and effects of frequency multiplication became too many steps for now. However its a great diagnostic tool and the design and troubleshooting stage and way cheaper than a Timepod or a Symmetricon.

 

Looking at the output of the DAC ultimately matters more because the effects of layout and other modulations all appear. I'm trying to duplicate the typical measurements from an AP without the AP. There are a few issues that relate to sample rate, window function, FFT resolution etc. that affect the measurements a lot. JA has provided some very useful info and I wanted to see if your measurements are comparable to his collection at Stereophile.

 

Demodulating the bi-phase signal has the issue of the short pulse, creating a harmonic. Also I believe the current best practice is to use only the header file to extract clock information since its less affected by the bandlimiting of the signals.

[JR_Audio]

Limits of the FM demodulators

 

As it was with the Ed Meitner LIM detector, so it is with the Audio Precision “detector”. They are both limited to “worser” Jitter cases (as for example this Airport Express Second Generation). They both will fail, when having real good devices. So the measurement limit of both FM Demodulators is above, what High End Ears, are hearing.

 

As you said in the second paragraph, the most important point to look for jitter is the analog output. But for the above mentioned AE Sec Gen I made an exception and included the FM demodulated results in order to leave out any second DAC and just to show, that the Jitter at the internal DAC of the AE will leave his fingerprint also to external DACs.

 

And my main point, what I have tried to explain is, that just looking at one number of the jitter on the Bi-Phase signal does tell you nothing, really nothing about the jitter at the end and it is a waste of time and energy to discuss about that. They are only those parts, that will be demodulted into the audio frequency, that matter.

 

To your last point. My experience, using only the frame sync pattern to create a low jitter clock or to measure the Bi-Phase jitter, I have had not really success. The PLLs are normaly using the bit cells to create the output clock, and I have more success using them for ectract the bit clock and to measure the FM demodulated jitter, that with the frame sync cells.

 

Juergen

[wgb113]

I'm confused a bit as well. So each iteration of the Airport Express has produced worse jitter numbers? I thought that the 802.11g model that Stereophile tested measured better than the 802.11n model and Chris you're saying that the newest AE (that looks like a white AppleTV) measures worse still?

 

Bill

[puncher]

Is there a post somewhere that explains the equipment used and signals levels etc. in generating these test results?

I'm a little confused that you're measuring noise floors down at -130 ... -140dB dBFS from a 16 bit system?