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
sfar's avatar
sfar
1,768 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.
audioengr
3,282 posts
 
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
Manufacturer/Modder
audioengr
3,282 posts
 
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
Manufacturer/Modder
herman
2,277 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.
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