Re: Re: Audio and Video sync. Getting Timestamps

[James Courtier-Dutton <James@superbug.demon.co.uk>]

Juan Carlos Granda wrote:
> That's my app does:
> 
> 1.- Open the device for capture
> 2.- Set the access mode SND_PCM_ACCESS_RW_INTERLEAVED
> 3.- Set format 16 bits (SND_PCM_FORMAT_S16_LE)
> 4.- Set channels 2 (stereo)
> 5.- Set buffer time near 1 second
> 6.- Set period time near 0.1 seconds
> 7.- Copy the hardware params to alsa
> 8.- Set start threshold (0x7fffffffff -> Explicit start)
> 9.- Set transfer align 1
> 10.- Copy the software params to alsa
> 
> read loop:
>     - Read 'period size' frames (snd_pcm_readi)
> 
> How can i get the timestamp of the first frame read? If i use
> snd_pcm_status_get_trigger_tstamp i obtain the same timestamp all the times.
> If i use snd_pcm_status_get_tstamp i obtain the "now" time. Is there any way
> to get the timestamp of every period.
> 
> Thanks
> 
> 

I help develope xinehq.de.
We have needed to do audio, video sync, but we don't need any audio 
timestamps using the get_trigger_tstamp etc.
We just use the snd_pcm_delay() call.

Take an example.
Sample rate 10000 Samples a second (10000 just to make things easy to 
explain.)

We call snd_pcm_delay() and it returns 4000.
4000 samples at a sample rate of 10000 = 400ms.
If we now write some samples to the sound device, we know that they will 
arrive at the speakers in 400ms time.

If we have some audio that needs to be played in 500ms time, we just 
insert 100 samples(100ms of samples) before sending our audio. We just 
know that those samples will reach the speakers in 500ms time.

In xine, we do more compilcated things like resampling, so that samples 
are removed and added without the user noticing.

So, for your application, if you just call snd_pcm_delay() just before 
you write your samples, or just after you write them, you will get a 
measure of when they will play.

Cheers
James


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