Re: When to start the capture device?

[Takashi Iwai <tiwai@suse.de>]

At Thu, 20 Sep 2007 15:10:12 -0700,
Sheng (Sean) Liu wrote:
> 
> Hi Mr. Iwai,
> 
> Thanks for your replies. I have another question for you.
> 
> After snd_pcm_start() is called, how long the first frame will be generated
> by the sound card? Does this duration vary in different nodes (support
> hardware+soundcard+OS are identical for all nodes)? If the duration is not a
> constant, do you have any idea what will be the largest jitter?
> 
> This jitter matters so much for my location detection application.  After
> the application starts, an user claps his/her hand. Then, the position of
> the user is calculated from the application. 
> 
> I have four VIA EPIA-M+Linux2.6 machines. A global time is setup among these
> four nodes. After the stream is started, the application detects a hand-clap
> sound and counts the number of frames from the stream start to the clap
> moment in each node. If I cannot have an accurate timestamp for the stream
> start, the application cannot caculate the clap sound timestamp accurately.

Then it's rather a question about RT-OS.  Once after entering the
ioctl syscall of kernel, and if the device is exclusive (no multiple
substreams), the latency is almost deterministic on preempt kernel.
Mostly the latency of the hardware is the issue.  But, the problem is
the latency before entring the kernel.  This can be (a kind of)
guaranteed only with RT-task running on RT-kernel.


Takashi