[alsa-devel] When to start the capture device?

[Takashi Iwai]

At Thu, 20 Sep 2007 10:14:27 -0400,
J. Scott Merritt wrote:
> 
> On Wed, 19 Sep 2007 16:17:55 -0700
> "Sheng \(Sean\) Liu" <shengl at uci.edu> wrote:
> 
> 
> > 1. The control point here the start threshold, which defines the number of
> > frames of space/data necessary to start the device automatically. If set to
> > some value other than zero for playback, it is necessary to prefill the
> > playback buffer before the device will start. If set to zero, the first data
> > written to the device (or first attempt to read from a capture stream) will
> > start the device. 
> >  
> > My question: what does he means on " If set to zero"? Does anyone know which
> > parameter of which API should be set to zero?
> 
> I believe this would be snd_pcm_sw_params_set_start_threshold.
>  
> > 2. You can also start the device explicitly using snd_pcm_start, but this
> > requires buffer prefilling in the case of the playback stream. If you
> > attempt to start the stream without doing this, you will get -EPIPE as a
> > return code, indicating that there is no data waiting to deliver to the
> > playback hardware buffer.
> >  
> > My question: Is there anything to be done before calling snd_pcm_start in
> > the case of capturing stream? If there is, what are they?
> 
> I use:
>    snd_pcm_open
>    SetUpHwParams ()  ... /* lots of snd_pcm_hw_params_set ... calls */
>    SetUpSwParams () ...  /* lots of snd_pcm_sw_params_set ... calls */
>    snd_pcm_prepare
>    snd_pcm_start

snd_pcm_prepare() can be omitted in this case.  After
snd_pcm_hw_params(), the stream is set to PREPARED state.  This isn't
changed after snd_pcm_sw_params(), too.

snd_pcm_prepare() is required to recover the stream from XRUN or any
other errors.  But, for the recovery purpose, we have now
snd_pcm_recover() as a generic solution, too.


Takashi