It seems that Alan's asynchronous DAC is not the only one that worked
fine. My friend has an Arce MDAC2 which however is labeled
"asynchronous", not affected by this bug. That DAC supports only two
sample rates: 44100Hz and 48000Hz at 16bit. I happen to know it use a BB
PCM1792 as its DAC chip, but not sure about the USB controller chip. It
might be BB PCM2707. Arce MDAC2 has a BB SRC4392 that asynchronously
upsamples input audio to 192Khz to reduce jitter, which is the reason
it's labeled "asynchronous".

I searched google and find this article: 
http://www.pstracks.com/pauls-posts/myth-asynchronous-dac/6291/ 
"Now, many DACS have asynchronous USB inputs, but that doesn’t mean they are an 
asynchronous DAC. To qualify as an asynchronous DAC, every input of the DAC 
(not just the USB) would be asynchronous – where the designer would throw away 
the source clock and rely only on the DAC’S internal clock.  There are scarce 
few of these around."

According to this article, since Arce MDAC2 is still using the source
clock, it can not qualify as a "real" asynchronous DAC.

As for the CONFIG_USB_DEBUG enabled kernel, I haven't built a linux
kernel before, but I'll try to figure it out later.

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https://bugs.launchpad.net/bugs/1136110

Title:
  USB Audio Codec choppy playback

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