[alsa-devel] Opening a PCM device in shared mode

Sayem Ahmed sayem64 at gmail.com
Mon Nov 21 12:11:54 CET 2011

Hi Anders,

Thanks for your reply. I have created a /*.asoundrc*/ file in my home 
directory as you suggested and added the following configuration there 
(as you can see, these are basically the exact default configuration 
that the dmix plugin has) -

         type dmix
         ipc_key 1024
         ipc_key_add_uid true
         slave {
             pcm "hw:0,0"
             period_time 125000
             rate 48000
             format S32_LE
             channels 2

Now when I start my program while a media player is running, it doesn't 
show any /*Device or Resource busy */error. It successfully opens the 
device and starts to write the sine wave into it. But the problem is I 
can't hear its output. I should have heard two sounds simultaneously but 
I can only hear the song in the media player, not the output sound from 
the example program. Similar things happens when I start the example 
first and the start a media player, I can only hear sounds from the 
first one.

Also, when I try to run the example program on different rate (i.e., 
rate 8000, channel 1) it throws an error saying format doesn't match. I 
guess it's because of the configuration rate that I specified in the 
file. I tried to supply multiple values for rate using array notations ( 
rate [8000, 48000] ) but it threw an error telling that the /*.asoundrc 
*/file is corrupted or old.

Actually the whole point of my exploration is this - I want to display a 
list of available audio cards/devices in my system to my application 
user who will then select the input and output audio card to 
record/playback audio data accordingly.

Thank you.

Sayem Ahmed

On 11/21/2011 01:41 PM, Anders Gnistrup wrote:
> Hi Sayem
> It's because you open the device plughw:0,0.
> It's actually the device hw:0,0 with a additional "plug" pluggin. The plug pluggin takes care of resampling.
> Devices is not mixing device by design. This is handled by plugins (the dmix plugin)
> What you want is a device that can mix (and possible resample). For that you need a new virtual device.
> I have had the same problem myself, and the solution I have used is a .asoundrc configuration.
> pcm.mydmix
> {
>      type dmix
>      ipc_key 1024
>      slave {
>          pcm "hw:0,0"
>          period_time 0
>          period_size 1024
>          buffer_size 8192
>          rate 44100
> 	format "S32_LE"
>      }
> }
> Instead of the slave definition you could make your own. Now the device mydmix can be used as a mixing device.

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