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
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:

http://www.stereophile.com/accessoryreviews/505apple/index.html

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.