Comment 78 for bug 1136110

I took a look in the information of the commit that caused the regression:
https://patchwork.kernel.org/patch/2164101/
And I think there are some details to report:

1) So far as I know, the affected Asynchronous DACs are all USB 1.1 devices, and therefore low-speed (despite being labled as full speed).

2) A Synchronous USB connection uses a one way digital connection for music playback, while Asynchronous mode has a feedback loop so that the amount of data in the frame can be controlled (because an Asynchronous DAC has its own clock which is not synchronized with the host clock). Since there is extra bandwidth needed by the control here, I wonder if the two modes have different true bandwidth for payload data.

3) The crack noise sounds very similar to what caused by a wrong buffer-size setting. I can make the same noise with an unaffected kernel if the ALSA/PulseAudio buffer setting is too small.

Combining 2) and 3) and the patch description, is it possible that the choppy playback is caused by data overflow or partially dropping package content because Asynchronous mode provides less bandwidth than a normal (synchronous) connection?

I'm not a hardware professional, so I can only make some guesses here.