[alsa-devel] RFC: PCM extra attributes

Takashi Iwai tiwai at suse.de
Wed Jun 24 23:05:05 CEST 2009

At Wed, 24 Jun 2009 18:18:48 +0100,
James Courtier-Dutton wrote:
> 2009/6/19 Takashi Iwai <tiwai at suse.de>:
> >> Another option could be to stream the gain control values with the PCM stream.
> >> I.e. Instead of attributes attached to the PCM, stream those
> >> attributes with the PCM stream.
> >
> > Hrm how can it be done?  Could you elaborate?
> >
> You said that you were adding a method to identify the channel maps.
> I.e. Is Front the first of second channel in the stream.
> Add extra channels in the stream that can be tagged as non-audio data
> and also tag the sort of non audio data they are. They could be gain
> values, time stamps, etc.
> So, basically just an extra way to capture metadata on a per sample basis.
> For example, when capturing this sample, the analog input gain was set to X.
> Obviously we don't have the functionality in the driver itself yet,
> but making sure the api could permit this would be useful.

I've discussed with V4L guys about a similar idea to sending
timestamps together with the PCM stream.  The concern was how to 
handle it, who multiplexes and who decodes.  Anyway, they weren't
impressed much by this.

This kind of thing is indeed technically interesting, but there aren't
so many real use cases.  It reminds me the fact that SPDIF raw format
is rarely used :)  But, timestamping would be a good candidate
requiring this kind of extension.



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