[alsa-devel] running 32 chan AD/DA on Linux

Daniel Mack zonque at gmail.com
Mon Mar 11 09:48:21 CET 2013

Hi Igor,

please don't remove the alsa ML from Cc:.

On 11.03.2013 09:37, Igor Levin wrote:


> But now we got a bit of a breather and our developers are finally able
> to give Linux the attention it deserves
> to get it going. I have couple fundamental question which you guys can
> help us with:
> 1) Does Linux audio support the Implicit feedback mode where the Out
> pipe will source the same amount of
>      samples as are coming into related IN pipe.


>  This is the sync mode
> we use in Orion 32. If this is so, could
>     you kindly provide the file/line number in the source code where we
> can see how it makes the packetising
>      decision.

Please have a look at sound/usb/endpoint.c - this is where the packet
logic lives in. Please read the comments in this file - the packet
sending for this implicit feedback mode is implemented in

> 2) We are upgrading our Zodiac DAC to use asynchronous feedback mode.
> What Async feedback mode formats
>      does Linux support? Where can we see it in the source?

Linux supports feedback over dedicated isochronous endpoints as well.
The format is auto-detected in snd_usb_handle_sync_urb(), also in
sound/usb/endpoint.c, after each stream start. Internally, the driver
uses Q16.16.

Hope this helps. In case of more specific questions, please just let me


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