[alsa-devel] Impact of SW on USB Audio?

[Takashi Iwai]

At Thu, 05 Jun 2008 16:11:49 +0200,
Pavel Hofman wrote:
> 
> Hi,
> 
> Please excuse my non-devel question but the most knowledgeable people
> read this mailing list.
> 
> Can user-space software (regular players) directly influence quality of
> USB adaptive audio output? IOW, how sensitive is adaptive USB to the
> rhythm of the data infeed to the USB controller?
> 
> I understand for standard PCI soundcards it is all only about timely
> delivery of enough data to DMA memory,the card takes care of the rest
> itself. Do USB controllers shield applications from the rest in the same
> manner?
> 
> The adaptive mode is sensitive to jitter of the data stream from the USB 
> controller to the receiver. Can SW players influence the jitter, e.g. by 
> moving the wav files from HDD to RAM disk etc.?  I assume a lightly 
> loaded system with no major data delays.
> 
> Thanks a lot for any insight, I could not google-out any relevant
> information.

The usb-audio uses isochronous transport, i.e. you'll send packets of
data at a certain period.  But, it's a driver implementation issue and
from the user-space POV, there is no big difference.  The URB-complete
callback corresponds to the DMA IRQ of PCI devices.

(Of course, if you want a very short period in a real-time manner, the
transfer efficiency matters largely, though.  But it's a different
story than the load you described above.)


Takashi