[alsa-devel] bandwidth issue with usb-audio in alsa

Kunal Singh kunal at singhkunal.com
Tue May 27 15:10:31 CEST 2008


	I have some update on the issue which I had reported earlier. In my last e-mail, I had not mentioned that iPod is connected to the target through a USB (HS) Hub. When I connect the
same hub to my Desktop and read its configuration using "usbview" - I see that "interval" for this hub is mentioned
as "256 msec". (I have two hubs and I get all the results same with both the hubs).

	With 36 URBs, the gap between two set of transactions is appx 220 msec (220+36 => 256). So, I am wondering
if the problem which I am seeing is because the external hub. I can not connect iPod directly to my target board,
because this board does not support enough current (as needed by iPod).

	Can some one suggest, where is the problem - is it with the host controller driver? 

Kunal Singh


Kunal Singh wrote:


  My Platform
  I am trying to stream audio data from a USB-device (an iPod) to my
arm-based embedded device (running kernel 2.6.23). The USB-device confirms
to usb digital audio (subclass - streaming) class. The endpoint buffer size
196 bytes and transfer period is 1 msec. So, I should be reading data every

  I am facing some bandwidth issues. The data is not being captured fast

  I notice that once one set of URBs are captured (one set equal to
MAX_URBS), there is a significant delay in capturing the next set of URBs. 

     I have set MAX_URBS equal to 32.
     When I connect a USB-Analyzer and analyze the timings, I can see that
32 URBs are captured at a interval of 1 msec each. After that there is a
break of around 250 msec. And then again 32 URBs captured in sequence.

      If I change the MAX_URBs to any other value N, N URBs get captured at
interval of 1 msec each, and then there is a break.

      Can some one suggest a way to improve this problem. Is there some
configuration or parameter which I can change, so that URBs get submitted
fast enough? May be as soon as the old one has retired.

Thanks and Regards,

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