Impact of SW on USB Audio?

Impact of SW on USB Audio?

[Pavel Hofman]

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.


Regards,

Pavel.

Re: 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

Re: 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.