USB Audio Problems Late 2013 iMac - Clicking / Popping / Dropouts
I am having audio clicking and popping issues with a brand new late model 2013 iMac and a USB 2.0 multi-channel audio recording interface. Here is what I have learned so far:
Clicking and popping is occurring in both directions - playback and recording. I am not having any issues with internal audio, so this was easy to verify.
The same issue appears in many threads in Apple Support Communities and in various recording/pro audio forums, so this seems to be a common issue.
The issue is independent of the audio interface that is used. (I am using a brand new Focusrite Scarlett 18i20 that I purchased specifically to use with my new iMac, but the forums report the same issues for a variety of other interfaces and audio devices.)
Five potential root causes are mentioned in the various forums:
1) USB 3.0 compatibility issues - poor backward compatibility or other issues
2) Fusion Drive activity - artifacts caused by drive "swapping"
3) OS X version - newer versions may be worse than old versions
4) App Nap - ?
5) Driver issues
Potential short-term solutions include:
1) Change driver - The "Class Compliant Mode" app (that I downloaded from the link on the Focusrite website) improved the issue for me, but clicking and popping occurs every few seconds instead of constantly.
2) Exchange the Fusion Drive for a conventional 7200 RMP hard drive - Forums report that this does not fix the issue.
3) Install older version of OS - Forums report that this does not fix the issue.
4) Disable App Nap - Forums report that this does not fix the issue.
5) Belkin Thunderbolt express dock - Forums report that a Thunderbolt hub with USB gives excellent results, but the only one I am aware of is the Belkin one which is ~$300. (I will not pay $300 to fix thousands of dollars of new equipment that should not need to be fixed!!!)
6) Apple Care's advice was an SMC reset (essentially a reboot), which clearly did not work. The person that I talked to was not aware of this being a known issue.
I am at a point where I need to decide whether or not to get a refund for this iMac since my primary reason for getting this new computer was recording. So...can anyone help to answer these questions:
Has Apple acknowleged this issue? If so, has Apple committed to fixing this issue via a software upgrade or have they admitted that this is a hardware limitation that is not fixable?
Has anyone found a free or inexpensive workaround (i.e. not the $300 Belkin dock) that completely elimiates this issue?
iMac, OS X Mountain Lion (10.8.5)
