The basics of PC audio

Some questions for you:

Assuming you have the PCU (in my case, a Mac Mini) near the stereo, with a USB DAC coming out of that into the preamp (or do you have it configured some other way?):

a) What does one do with the monitor--perhaps run an extra long cord and put it away from the system to keep the RF from feeding into the audio?

b) Can the CPU be placed far enough from the preamp to where RF from it won't affect the sound of the system?

c) What about using Apple AirPort Express and AirTunes and running a USB DAC out of that into the preamp; are there any advantages/disadvantages to that sonically vs. having the CPU feeding directly into the DAC?

d) Suppose you buy music from the iTunes store in MP3 form. Can it be converted to aiff or some other "lossless" format such that you'll wind up with a high quality file? Or does the fact that it was already converted to MP3 doom it to sonic mediocrity?

e) How quickly are USB DACs improving in quality? I don't want to buy a DAC and have it be obselete the next year.

I appreciate your answers.


a) Interference is a very local phenomenon. There's no way to know what will be necessary in your situation. But interference is also far less of a problem than you might imagine. (Think how many TV monitors are adjacent to preamps in home theaters.)

b) Ditto. Probably won't be a problem, but you never know till you try.

c) A wireless network is inherently less stable than a wired one. Cordless phones and microwaves can interfere with the connection. If you go the Express route, I'd connect a standard audio DAC to the audio out, rather than using a USB DAC. But sonically, it's probably a horse apiece in general, though specific set-up issues might be a factor.

d) You can convert an iTunes file (which is AAC, not MP3) to an AIFF file, but it won't be any better than the AAC file. Once a file is compressed with a lossy codec, there is no getting back the information that's lost. OTOH, mediocrity is in the ear of the beholder. For critical listening, you'd want full-resolution digital files (AIFF, WAV, or a lossless codec). But for casual or background use, you aren't going to notice that it's "only an MP3."

e) I don't know the USB DAC market well, but the only thing I'd worry about is noise. Assuming you can get adequate S/N from today's models, future improvements will be marginal.
a) and b) I don't think RF interference is a concern with most systems

c) I'm almost positive that the USB port from the AirPort Express is not compatible with a USB DAC

d) once a file is compressed with a lossy format such as mp3 it can never be converted back

e) things constantly improve but the USB DAC is a mature format. It is basically the same DAC that has been around for 20 years with a slightly different interface to accept the digital data via USB rather than spdif.
I appreciate the responses. I hadn't realized that there's no going back from an MP3; though now that I think about it, it's very reasonable. Given that all I have are MP3s right now (and I have lots of them), the DAC on Airport Express should be more than adequate, I should think? Or, perhaps even the DAC in my laptop via the headphone jack would be adequate?

Thanks again,

d) Pabelson is corect, when I said it could not be converted back what I meant to say was that the information lost could not be recovered, but it can be converted to other formats.

I'd connect a standard audio DAC to the audio out, rather than using a USB DAC.

??? If you use the audio outs from the express you don't need a DAC. The express USB port is for a printer and won't work with a DAC anyway.
The one weakness of the Express is jitter. (Jitter gets far more attention than it deserves, but in this case it's not just a theoretical concern.) Feeding the audio out digitally to a DAC would be better than using its analog output. Whether that improvement is worthwhile, given that you're already dealing with compressed files, is something you'll have to judge for yourself. I use my Express mostly to stream Internet radio, which is REALLY compressed, so a little jitter hardly matters. YMMV.
I'm using an Airport Express, connected to a Benchmark Audio DAC-1 by the digital/Toslink output from the Express. In theory, at least, the Benchmark eliminates any jitter coming from the Express, though I don't have any way to verify that.

At any rate, the sound of Apple Lossless files is pretty extraordinary, indistinguishable to me from the same files off the CD, and even mp3 files (most of mine are at a 192 kbps sample rate) sound remarkably good. The Benchmark is a moderately expensive DAC but it's incredibly versatile and one of those audio buys I've been happier with the longer I've used it.
I'd agree that at 192kbps, compressed files start to put up a real fight. But Matt was talking about iTunes Store downloads, which are 128kbps. They aren't quite so clean, though perfectly adequate to my ear for casual listening.
I'm not disputing it but I've never heard the concerns about jitter from these type of devices (Express, Squeezebox, etc)

Do you guys have more info or links to some?
John Atkinson measured very high levels of jitter in the analog output of the AE:

See Figure 7, and the text surounding it.
I don't understand. He says the jitter on the digital output is due to the fact it has to reconstruct the clock from the asynchronous data it is being fed. OK, but why does he say the point is moot when using the digital out? Doesn't the digital data stream also get its clock from the same reconstruction. I understand if the data is reclocked by the DAC, the DAC he mentions upsamples so I assume it has to have its own clock, but not all DACs do that.

Reclocking is part of the DAC's job. If you have a DAC that does not reclock, you have a defective DAC. The DAC in the Express just doesn't do it very well, which is why he recommends using an outboard DAC instead. I suspect the DAC in even a low-end A/V receiver would be sufficient to do this properly, though I can't claim any personal experience on this.

I should say that I don't use an external DAC with mine, but then I use it mostly to listen to Internet radio streams, which are highly compressed, so I'm not looking for perfect reproduction. Even so, I don't find it at all unlistenable.
Once again, I am no expert on DACs, but I am pretty sure that relocking the data is not the standard. All DACs do not reclock the data??? I am open to any opposing view.

I do agree that with lossy formats it really doesnÂ’t make a whole lot of difference what format you choose.
I think it's true that there have been a few DACs made that did not reclock the data. This is what we call "defective by design." Reclocking data is how you eliminate jitter; it's a necessary step. And as I said, even the DACs in AV receivers do it.
Pabelson wrote:
"Reclocking is part of the DAC's job. If you have a DAC that does not reclock, you have a defective DAC."

Sorry, not true. Only upsampling DAC's actually need any sort of local clock. The clock is usually recovered from the S/PDIF signal by a receiver chip or comes free if it is an I2S interface.

Steve N.
Empirical Audio
Sfar - If you heard the same AirPort Express with a Coaxial S/PDIF interface and a Superclock3 in the AE you would be shocked at how much better it is than the stock AE with Toslink interface. Not only as good as your transport, but better.

The Same AE with Superclock3 and I2S interface (possible) would be incredibly good.

BTW - Apple Lossless, like most lossless compression algorithms is identical sounding to .wav or uncompressed.

Steve N.
Empirical Audio
Herman - here is a white-paper I wrote on jitter and computer-driven audio:
Thanks very much for the detailed information.

I got the Airport Express sort of as an afterthought. I already had the Benchmark DAC and a transport and was very happy with that combination but when I started having trouble with a Netgear wireless access point after a firmware update I decided to switch to the AE because I knew it had a digital output that I could try with the Benchmark.

The only thing I've tried to do to improve the output of the AE is to get a glass Toslink cable to replace the inexpensive plastic fiber one I first bought. I don't have any doubt about the potential superiority of a hard-drive based system and will be anxious to see what happens with modifications to devices like the Airport Express.
Not being an engineer, I may have expressed myself badly (and thanks to Steve for setting me straight), but I was responding to a question about how adding a DAC could reduce jitter coming from an Airport Express. Implicit in that question is the idea that the bytes are supposed to come streaming down the digital cable exactly 1/44100 of a second apart. They don't, and yet DACs manage to produce an analog signal without audible jitter. That's part of their job.
Glass-fiber is certainly better. I have tried this with a stock AE as well. I have sold some AE upgrades, with Superclock3, new power supply and S/PDIF coax interface, and the customers seem to love them. I have designed a board to add I2S interface to the AE as well, but I have not fabricated any yet, too busy. I already have an I2S interface for the Benchmark DAC-1. The combo of AE with I2S and DAC-1 with I2S would be killer. I guess I'll have to get this done...

Steve N.
Empirical Audio
Pabelson - You are correct, some DAC's, such as the Benchmark DAC-1 will reduce the jitter from the Toslink of an AE, but most of them do a fairly poor job of this. Even the DAC-1 sounds different with different digital sources. The theory behind the Lavry DAC's is the best I have seen. This will actually work if implemented well. The best solution currently is to make the jitter very low in the AE, which is what I try to do. An I2S interface and a Superclock3 accomplishes this.

Steve N.
Empirical Audio
Good link Steve, perhaps you could clear up a few points.

Did the Creative Audigy 2 NX turn out to be asynchronous? I am guessing not as you have chosen others to modify.

Basically the DAC has a single digital USB input. USB unlike SPDIF is bidirectional and therefore has error correction and buffering on both sides. This happens automatically so the data on the disk is identical to what is going out all the time. Also since this interface is asynchronous the clocking problems associated with SPDIF go away.

This is a quote from the Wavelength Audio website and is in direct contradiction to the link from John Swenson in your white paper, both in terms of USB audio being bi-directional with error correction and being asynchronous. Perhaps things have changed since your paper was published but I don't think so. Since Gordon is active on other forums I have alerted him to this thread and hope to hear from both of you on this matter.

I have no axe to grind with any of this. I am just a curious DIY guy that isn't using either of your guy's stuff. I'm presently using a Scott Nixon USB DAC that I've modified a bit (coupling and filter caps) and running from batteries.

I will say I'm not as enamored with upsampling as you are and find the non-oversampled to be more organic and musical.

I'm also considering the DDDAC or at least experimenting with running parallel 1543 DACS.
"Did the Creative Audigy 2 NX turn out to be asynchronous? I am guessing not as you have chosen others to modify."

No. I believe it is. It was just more difficult to mod and make into a product, besides it was discontinued as all of the other decent solutions were (Edirol UA-5, Waveterminal etc..) The transit lives on and they have no plans to discontinue it. I wil not be modding them for long anyway, as I have my own design ready to go. It will still use the M-Audio drivers and ASIO as they sound the best for PC IMO. The only upsampler that I use is Secret Rabbit Code, which I have licensed. It is better than 44.1 every time I try it.

As for the quote above, I suggest that John Swenson and Gordon resolve this. They both know more than I do about USB interfaces.

I was impressed with the Lavry DAC technology. HAve not heard one yet though...

Steve N.
Thanks Steve. I sent a note to Gordon and I'll try to contact Swenson, but everything I've read points to Swenson being correct.

I think the best solution is a wireless USB device server feeding a USB DAC. That way you aren't tied down to any single device like Squeezeboz or Airport and it's associated software so you can use any USB DAC you choose. The problem is that even though
wireless USB device servers exist, at this time there are none that can handle the isosynchronous data stream. Silex tells me that they hope to have one later this year so I am waiting for that.
For those that want wireless USB (the best), Belkin has announced the CableFree USB hub and dongle, which replaces wired USB with Ultrawideband, high-bandwith wireless USB. The dongle plugs into your computer or laptop and the hub goes next you your USB converter or USB DAC and uses a short USB cable. Replaces USB 2.0. Demonstrated at 2006 CES in Jan.. Latest is that they are shipping in July. Pricing expected to be under $150.00. The advantage is that you can use ANY player software with this, unlike iTunes for AirPort Express or SlimServer for Squeezebox, and it will pass 24/96!!!
Matt, there is no RFI unless you have really susceptable front end. It is best to connect the dac directly to the MAC. You cannot recover information that is in ACC so why convert it. Upsamplers and such suck the life out of music. Most AAC sound pretty good, not as good as Lossless but who cares. Sometimes you just need to listen and have fun and not get so tied up in the details.

As I see it the dacs at 44/16 will be here for a long time. The next logical conclusion will be lossless downloads. We are pretty far away from high definition PC audio as loading those files would be at least 4x the space.

FYI I doudt I will be back here, email me if you have questions. I have the answers.
Question for Steve:

Do you have any idea about the range of the new Belkin wireless USB hub?

Can't find any info on the range.

Steve N.
Audioengr I have been reading all posts I can find on usb dac an such to get informed.I see the Belkin wireless usb mentioned but I can not locate them on the net? Please tell me how to contact them thanks Gary
Cable-free from Belkin is still not available. Last I read it was November 2006.

steve N.
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