[alsa-devel] Capture from 3+ cards connected to a USB hub - distorted samples

[Pavel Hofman]

Dne 10.12.2011 00:23, Pavel Hofman napsal(a):
> Hi,
> 
> I have been informed about the following problem which I was able to 
> reproduce afterwards:
> 
> * Latest git.alsa-project.org drivers
> 
> * USB 2.0 hub, 3 or more USB soundcards (inexpensive USB sticks, USB 
> audio v.1 types, async capture, adaptive playback) hooked to the hub, 
> identified A, B, C, D.
> 
> * NONE of these soundcards is playing back
> 
> Now:
> 
> * Card A begins microphone capturing (mono 48/16), result OK
> 
> * Card B begins capturing (mono 48/16), both results still OK
> 
> * As soon as the third card C begins capturing, the signal captured from 
>   cards A and B gets distorted. Audacity shows some samples are 
> incorrect, distorting the expected sine waveform of the testing signal. 
> I can provide screenshots as well as recorded samples, if needed.
> 
> * The signal recorded from card C is OK.
> 
> BUT
> 
> * If any card hooked to the HUB is playing back (even a fourth one D), 
> the distortion does not occur. It happens only when there is no playback 
> stream going through the hub. Traffic on another USB port has no effect, 
> in fact the test signal for the three cards was provided by a 
> multichannel USB card hooked to another USB port.
> 
> It seems to me as if blocks of samples for each card in the USB frame 
> sometimes overlap by a few bits, corrupting the neighbouring card 
> samples, or are recovered incorrectly.
> 
> I understand I have provided no real data, I am ready to do so plus any 
> tests you would consider useful.
> 
> Thanks a lot for suggestions of the next debugging steps I should do.
> 

Hi, please may I ask for help? I think we could fix this likely bug.

Thanks a lot,

Pavel.