[alsa-devel] USB audio devices on Audio 4DJ and similar

[Alan Horstmann]

Hi Daniel,

Thanks for your helpful replies; responses below.

On Sunday 08 April 2012 15:12, Daniel Mack wrote:
> On 06.04.2012 23:09, Alan Horstmann wrote:
> > At present I am involved with resolving some issues that have arisen from
> > people using the Audio 4DJ interface with Portaudio, and I hoped it would
> > be OK to check a few Alsa details with you?  Unfortunately I have limited
> > experience of USB audio.
>
> What are those issues? Can you elaborate?

These are essentially issues in the way Portaudio interfaces; I am not 
suggesting driver bugs etc, but in summary:

a) The Audio 4DJ uses fixed _BE format, almost uniquely AFAICT from grepping 
driver source tree.  Portaudio at present requests formats in the host 
platform endianness - so on a x86 PC an acceptable format is not available 
from the 'hw' device.  Users seem reluctant to use 'plughw'.

b) Subdevices are not enumerated by Portaudio at present, and so only 2 of the 
4 channels can be accessed.  If the unit had presented a single 4-channel 
device all the channels would have been available.

> The device itself implements 4 audio channels which are synced to each
> other. In other words: the hardware is not able to only enable one of
> them and leave the other ones paused or set sample rates individually.
>
> When I implemented the driver, it seem most logical to follow the way
> the device denotes it channels though. The documentation and silk screen
> presents them as pairs of stereo, and so the driver does the same thing.

I am just interested whether you think it would be possible for the driver to 
be modified to support a single 4-ch device, perhaps through a module option?

> > I don't see any USB audio units having files in /usr/share/alsa/cards; is
> > it not relevant, not normally necessary or something no-one has got round
> > to writing?
>
> For this particular device, there wasn't any need yet.

I couldn't see any USB cards there - is that probably correct?

Regards

Alan