Hi,

please don't top-post.

On 22.03.2013 11:11, Zhang wei wrote:

But what is the mechanism to synchronize the data from different channel,and maybe different channel pairs use different DMA buffer.

You can try and see if syncing the individual, non-combinded PCM streams helps:

http://www.alsa-project.org/alsa-doc/alsa-lib/pcm.html#pcm_sync

So, I guess ,about the combine ,according to the alsa.conf,the alsa-lib opened the 4 devices(in my case) and got every channel paris’s data in turns in a monent. And then combine them together in order as one frame?

It will feed the input to all pcm streams. I don't really know if it uses the sync mechanism on that, but I'd guess so.

Because all the CODEC have the same clock timing,so the different DMA buffer stored the different channel data for the same moment. Alsa-lib obtains the same size data from each buffer in turns in once.repeat it again and again.

Right?

Yes, and so the streams won't drift.

Why don't you just go ahead and try it? You should play back some wav file with a test pattern which you can recognize on a DSO or IIS analyzer on the bus. Then see if they are in sync.

Daniel