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