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

[Pavel Hofman]

Takashi Iwai napsal(a):
> 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,

Thanks a lot for your reply which has narrowed down the area I will have 
to look at detail.

Regards,

Pavel.