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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
 
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?
blackstonejd
153 posts
 
I use a 3 meter USB cable with my Ayre QB-9 and the results are great.

As for volume control, you absolutely want your volume control at 100 or 0db. Otherwise, the computer is doing math that compromises the sound.
bbopman
53 posts
 
Steve N: Could you elaborate on the issues with Apple TV or direct me to another discussion or send a private email? I currently use Apple TV with a PS Audio DAC and have been pretty happy, but I would like to extract the best sound I can. My library (1600CD's) is ripped in ALAC.

Thanks
audioengr
3,282 posts
 
Bbopman: Two issues with AirPort Express and ATV:

1) ALAC is the only format transmitted
2) iTunes is the player

The sonic issues with ALAC cannot be avoided unless you use Transporter, Sonos, Squeezebox or Duet etc..

The sonic issues with iTunes can be avoided by using Amarra or Amarra Mini. See sonicstudio.com

If you have a truly resolving system, then you will hear the difference in these. If not, then you probably wont care.

Steve N.
Empirical Audio
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