2. pc-audio
  3. 1147036039

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.

Thanks,

Matt
descartes

Showing 8 responses by herman

herman
2,216 posts
 
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.
herman
2,216 posts
 
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.

c)
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.
herman
2,216 posts
 
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?
herman
2,216 posts
 
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.

?????
herman
2,216 posts
 
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 doesn’t make a whole lot of difference what format you choose.
herman
2,216 posts
 
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 http://www.wavelengthaudio.com/usbdac.html 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.
herman
2,216 posts
 
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.
herman
2,216 posts
 
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