Hello everyone,

If I stream audio via WLAN to an Apple Airport Express base station (Source: either PC or Mac with iTunes and Apple Lossless encoded files) and use the *digital* output of that base station, will the data that gets to the DAC of my amp be the same as if it came directly from CD (with digital out)?

I always assumed that the answer should be yes. It doesn't seem to make sense that there is any loss in quality, as long as there is no DA conversion... But recently I read a lot about how difficult it is to get bit-perfect digital audio output on PC soundcards -- that's why I am not so sure anymore.

I apologize if my question has been asked before -- if so, I was not able to find the answer. Maybe someone can confirm that AirTunes playback is bit-perfect and relief me of my paranoia?! TIA, any comments appreciated.

Andreas

Yes it is.

Airport Express uses ALAC so the music data that reaches it is bit-perfect. I don't know about the sound card that it hosts though, but I would expect it to give good results (for one thing the analog output is certainly line level).

You always have the option to connect using the optical digital output then you are guaranted a bit perfect result.

Thanks for your replies.



According to an article in the highly regarded German computer magazine c't (issue 4/2005), the *analog* output of the Airport Express base station is of inferior quality. I don't know if this is true, but maybe one cannot expect to much for that price (after all, the audio function is just a bonus to the Airport Express, which serves as a WLAN access point primarily...). This isn't really an issue, as I am using the *digital* output, as I said in my original post



I hope so. But on the other hand, if the audio stream reaches the Airport Express bit-perfect, this doesn't necessarily mean that it comes out that way, too...
After reading this thread: http://www.hydrogenaudio.org/forums/index....f=1&t=5755&st=0 , I wasn't so sure anymore... I always thought that the digital output of a soundcard must give bit-perfect results, too. Apparently this is not the case most of the time if you use Windows2000 or XP (BTW: How about Mac-OS???). I really hope that the Airport Express base station doesn't do resampling... Has anyone actually tested that it leaves the audio stream untouched?

Thanks again, regards,

Andreas

Why would it do that?

Why don't PC soundcards leave the audio stream untouched? I admit that this whole tech talk is way over my head, so I can't really explain. But it seems to be a fact that digital output doesn't necessarily mean bit-perfect 1:1 output (shown so in the thread I cited in my last post).

So after finding out that I get resampled, quality-reduced digital audio output from my PC soundcard, it seems to be not totaly off-hand that this might apply to an Airport Express, too.

Andreas

I know nothing about the Apple Airport (never even heard of it before), but I can explain why some PC soundcards resample everything....

These resampling soundcards have built-in mixer capabilities, so that you can feed in several streams and mix them into one output. That mixing is done in the digital domain, and as you probably know, any digital audio stream needs a clock. Now, if you've got several streams coming in, there's no guarantee that their clocks will be synchronised. If you assume they are and start trying to mix them, all sorts of unpleasant glitches will result due to the unsynchronised clocks. The only way you can successfully mix multiple digital audio streams together is to have a single clock applied to them all. So the soundcard resamples all the incoming streams to its own internal rate. You might rightly argue that if you're just playing a single stream which isn't going to be mixed with anything else, then it doesn't need to be resampled, but unfortunately the driver implementations are often lazy and just resample everything that comes in.

If you want a bit-perfect digital output stream, then you'll have to seek out a non-resampling card. I'm a bit out of touch with current models, but everything in the M-Audio range from the AP2496 upwards provides bit-perfect digital I/O. Don't know whether the same is true of their Revolution range. I'm pretty sure that any semi-pro card (eg. Echo, Terratec, Lynx, DAL, etc) will get this right and provide bit-perfect digital I/O.

Sorry for broaching the subject again. Still, I am not 100% sure that I get bit-perfect digital audio from the Airport Express...

@Fuchal: Thanks for your reply and sorry for bothering you again. Was your statement based on an actual test you did? How do you know that the audio stream is untouched?

Andreas

@cliveb: Thanks for your very infomative reply. For me (as a non-expert) it made things much clearer.

As to the Airport Express: It is basically a WLAN access point with the bonus of serving as an audio bridge via WLAN, too. Data is transferred from iTunes in Apple lossless format and gets decoded by the Airport Express station. You can connect it to your amp via digital or analog output.

http://www.apple.com/airportexpress/

Andreas

The digital out portion of the Express would have no need to do resampling. Unlike a pc soundcard, it does not need to mix multiple streams. But it may have to resample if your source is something like 24/192, its digital out chip is probably not good enough for that. But in this case the resampling is probably done by the computer that is feeding the audio out. I actually wouldn't be surprised if the playing computer converts everything to 16/44.1 in advance to save on network transport bandwidth.

If you are worried about bit-perfect digital audio, I don't think a $130 device is what you want to be looking at.

This sounds plausible. In reality all my audio is CD quality, so I wouldn't need anything other than 16/44.1.



From the previous posts it seems conceivable, that the Airport delivers an untouched 16/44.1 digital audio stream. That's all I need. As long as it is digital, it's just a serial stream of 0 and 1, isn't it? I know that very good DAC's and amps can be very, very expensive. But as far as a digital audio *source* is concerned it should be possible to get bit-perfect result quite cheap nowadays.

Please tell me if I am wrong here (I am here to learn and appreciate every help I can get), but why shouldn't I get bit perfect results from a 130€ device if I can (just an example) make a bit-perfect rip of an audio CD with a 30€ drive? I personally use the best drive for DAE that I know of: a Plextor Premium. It costs about 90€. I would be willing to spend 500€ on a CD drive if it would improve quality -- but it's impossible to make better rips than perfect ones...

Andreas

The digital out does require a chip that can maintain a synched clock, which isn't as easy as it sounds. Optical needs high quality leds and photodiodes (not relevant to the Airport). So it isn't free, but you're right that it's a lot cheaper than a high end DAC or op-amp.

Yes, that all sounds perfectly right, in theory. I was just sayin' that it isn't unusual for a cheap device to have little quirks. But if it did, I think that forced 16/44.1 resample would be the most likely, which as you note makes no difference for cd source. Anyways, even if is wasn't 100% bit-identical I don't think the difference would be ABXable, so it might as well be called perfect.

I know what you mean. It's one of the biggest problems in the whole computer business (hard- and software alike) that quality is so frequently sacrificed for a slightly lower price. But that's the only thing most customers want, obviously.


I will never be able to afford the best HiFi equipment that exists. But within my limits, I want to be it as good as possible. On the *digital* side, "as good as possible" might even be *perfect*. I would hate to loose quality there if it is not necessary and I don't think it is. Even if I wouldn't be able to ABX the difference. (Restriction: always talking about CD quality (16/44.1) here, which is practically the format that all my music is in...).

Andreas

edit: typo