2. pc-audio
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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
audioengr
3,282 posts
 
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
Manufacturer/Modder
audioengr
3,282 posts
 
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
Manufacturer/Modder
audioengr
3,282 posts
 
Herman - here is a white-paper I wrote on jitter and computer-driven audio:
http://www.positive-feedback.com/Issue22/nugent.htm
sfar's avatar
sfar
1,769 posts
 
Steve
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.
pabelson
344 posts
 
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.
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