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
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:

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

See Figure 7, and the text surounding it.