[alsa-devel] USB hardware that supports implicit feedback?

Daniel Mack zonque at gmail.com
Sun Nov 18 11:10:29 CET 2012

On 16.11.2012 10:39, Daniel Griscom wrote:
> At 7:22 AM +0800 11/16/12, Daniel Mack wrote:
>> Hi Daniel,
>> Hi Elad,
>> On 16.11.2012 05:32, Daniel Griscom wrote:
>>>  At 10:02 PM +0100 11/15/12, Eldad Zack wrote:
>>>>  Hi Daniel,
>>>>  On Thu, 15 Nov 2012, Daniel Griscom wrote:
>>>>>   I'm having trouble getting implicit feedback working with my custom
>>>>>   capture/playback USB audio class device, even with the latest kernel. I
>>>>>   presume it's a problem with my device's configuration, but 
>>>>> can't figure out
>>>>>   exactly what the problem is (after a whole lotta investigation).
>>>>  What exactly doesn't work?
>>>  The audio output stream (from the computer to my device) runs at a
>>>  few frames per second higher or lower rate than that of the input
>>>  stream (from my device to the computer). The actual difference seems
>>>  to be stable on a specific machine, but varies greatly between
>>>  machines (I've seen differences from +7fps to -2fps; I presume this
>>>  is due to differences in CPU clock frequencies).
>> So you're saying that this is what you experience after you added code
>> to make the driver use the implicit feedback mechanism? Over which time
>> did you measure this and how exactly? Because frankly, I doubt that -
>> the data rate is purely derived from the number of incoming samples.
> This has been the case for a while; we started work with ALSA 1.0.23, 
> but upgrading to kernel 3.6.6 (with included ALSA code) didn't change 
> anything. We have yet to do any patching of ALSA for this matter.

Well, afaik, ALSA 1.0.23 is what ships with a kernel <= 3.4, and the
code for implicit feedback went in for 3.5.

>> Does your device demand the same number of *bytes* or samples/channel to
>> be sent?
> I'll have to check that.

Let me rephrase the question: does the number of input and output
channels match? Eldad had a patch in his series that should make it work
for both scenarios.

>> The code I wrote for implicit feedback is used by the M-Audio FTU
>> devices,
> So, with an FTU the implicit feedback code works?

Yes, it does for many others.

> What version of 
> kernel/ALSA should I use - the latest?

Yes, always use the latests from git, especially for such relatively new


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