[alsa-devel] first and last audio sample timestamp

[Takashi Iwai]

On Mon, 25 Jun 2018 16:50:20 +0200,
Subhashini Rao Beerisetty wrote:
> 
> On Mon, Jun 25, 2018 at 8:04 PM, Takashi Iwai <tiwai at suse.de> wrote:
> 
> > On Fri, 22 Jun 2018 16:00:10 +0200,
> > Subhashini Rao Beerisetty wrote:
> > >
> > > [ Please keep me in CC as I'm not subscribed to the list]
> > >
> > > Hi All,
> > >
> > >
> > >
> > > Hello all,
> > >
> > >
> > > I’m using aplay and arecord utilities for playback and capture. I need to
> > > capture the timestamps for the first and last audio samples that arrive
> > at
> > > the driver level.
> > >
> > > For this I’m capturing the CLOCK_MONOTONIC_RAW timestamp at the
> > > .trigger(for playback & capture) callback in SNDRV_PCM_TRIGGER_START
> > case
> > > – is it the correct timestamp for the first audio sample arrived at the
> > > driver level?
> > >
> > > Similarly does .trigger callbacks SNDRV_PCM_TRIGGER_STOP gives the last
> > > audio sample timestamp?
> >
> > No, it's the timestamp upon calling the trigger callback.
> > It has nothing to do directly with the first or the last sample.
> > (But usually the start trigger is the time of the first sample for
> >  capture, though.)
> >
> 
> Okay then if start trigger is the time of the first sample for capture,
> then is stop trigger is for last sample?

No, it's not necessarily the last sample.  It's the just time you stop
the stream.  The stop might happen some time after the last sample got
received.  The accuracy in that regard pretty much depends on the
hardware and the driver implementation.


Takashi

> For example, i'm printing the
> timestamp as given below. Similarly in which call back I can use the
> getrawmonotonic()
> for playback case.
> 
> static int snd_mychip_capture_trigger(struct snd_pcm_substream *substream,
> int cmd)
> 
> {
> 
>     struct timespec ts;
> 
> 
> 
>     switch(cmd) {
> 
>         case SNDRV_PCM_TRIGGER_START:
> 
>             getrawmonotonic(&ts);
> 
>             printk("1st audio sample received time:
> [%.6lu:%.9lu]\n",ts.tv_sec, ts.tv_nsec);
> 
>             break;
> 
>         case SNDRV_PCM_TRIGGER_STOP:
> 
>             getrawmonotonic(&ts);
> 
>             printk("Last audio sample received time:
> [%.6lu:%.9lu]\n",ts.tv_sec, ts.tv_nsec);
> 
>             break;
> 
>     }
> 
> 
> 
> }
> 
> 
> >
> >
> > Takashi
> >