[alsa-devel] Latency and timestamps

Takashi Iwai tiwai at suse.de
Wed Nov 21 11:50:24 CET 2007

At Mon, 19 Nov 2007 22:46:31 -0200,
Claudio Matsuoka wrote:
> Hi,
> I'm adding latency control to an application and didn't find much
> documentation about the pcm status functions aside from a very brief
> description and the latency.c example. What exactly are the "trigger
> timestamp" and "now timestamp" returned by
> snd_pcm_status_get_trigger_tstamp() and snd_pcm_status_get_tstamp()?

The trigger_tstamp is the time-stamp at the last time the PCM status
change occured.  For example, when the PCM is really triggered to
start, or stopped, or XRUN, etc.  It won't be changed as long as the 
PCM status is kept.

OTOH, the tstamp is the current timestamp (now).  But, this value has
a slightly different meaning when tstamp_mode is set to
SND_PCM_TSTAMP_MMAP.  Then it keeps the timestamp of the last period
update time instead of the now.


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