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USB Length, jitter and Volume Control Questions

I've been working on a PC audio system for a few months now and I have a few questions that I cannot find the answers to.

I have a new iMac running USB>DacMagic DAC>Onix Sp3>Merlin TSM speakers. I hav had a Valab NOS DAC for a bit and have a Stello DA100 on the way. My questions is how does the volume work in these setups? With the DacMagic I can control the volume through itunes but not through the system menu bar. With the Valab I could control both iTunes and system volume. Is there a difference between the two? Does using the volume control in iTunes or system conpromise the sound? If I'm getting gain from the computer (regardless of system volume or iTuens volume) am I not essentially using the computer as a lowgrade "preamp".

Also, how long can a rung of USB cable be before the signal begins to degrade? Does USB transfer introduce jitter?

Thanks,
mailman199
audioengr
3,282 posts
 
If you want the best results for jitter, I would go with a 1.5m cable for USB. Same with S/PDIF cable for the following reasons:

http://www.positive-feedback.com/Issue14/spdif.htm
mailman199 OP
34 posts
 
Unfortunatly I don't have the ability to use a short cable unless I go wireless (airport express to toslink). My library is 30 feet away.
pdreher's avatar
pdreher
1,208 posts
 
I tried running wireless via I-Mac to Airport Express to Empirical Audio Pacecar to my DAC. Was not happy with the results and have since gotten out of computer audio.

Next time I try computer audio, I'll buy a Mac Mini direct to a DAC (MHDT Paradisea or comparable) via USB. I don't think you'll be happy with a long (>15 ft) cable or wireless to an airport express) unless sound quality is not important. Suggest you invest in a Mac Mini, a good, short USB cable and a USB capable DAC.
audioengr
3,282 posts
 
The problem with AirPort Express and AppleTV is that they both muck-up the data doing the conversions to ALAC and back as well as the mucking that Core Audio does.

I understand that much of this can be circumvented now using Amarra and a tool called "Airfoil". If you use Airfoil and select the internal speakers, the Amarra feed evidently goes directly to AirPort Express or Apple TV.

Steve N.
mailman199 OP
34 posts
 
If the files you are pushing across the airport are already in mp4 (ALAC) does it still go through the conversion?

Also, would using Airfoil and selecting "internal speaker" use the internal DAC in the Mac?
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