Detect when sound card is in use

Detect when sound card is in use

[Claus Ilginnis]

Hi everyone,

I have a problem and I dont't know where to ask my questions anymore, so
I hope this mailing list can help me make progress with my problem....

I have a speaker set connected to an USB-Power-Plug (by which I can turn
the speakers on and off )

Now I want to detect, when the soundcard is in use and when not.
(to turn the speakers on when needed...)
(I am a C++ programmer so I can mess some things up)

So my question is:
Which way is best to detect wheather my sound card is in use or not ?
Is there a general overview/documentation of the sound kernel modules
which describes the general relationship between them ?
What would you do ?

PLEASE help !


-  

Mit freundlichen Grüßen / Best regards
Claus Ilginnis



_______________________________________________
Alsa-devel mailing list
Alsa-devel@alsa-project.org
http://mailman.alsa-project.org/mailman/listinfo/alsa-devel

Re: Detect when sound card is in use

[Takashi Iwai]

At Tue, 28 Apr 2009 21:16:26 +0200,
Claus Ilginnis wrote:
> 
> 
> Hi everyone,
> 
> I have a problem and I dont't know where to ask my questions anymore, so
> I hope this mailing list can help me make progress with my problem....
> 
> I have a speaker set connected to an USB-Power-Plug (by which I can turn
> the speakers on and off )
> 
> Now I want to detect, when the soundcard is in use and when not.
> (to turn the speakers on when needed...)
> (I am a C++ programmer so I can mess some things up)
> 
> So my question is:
> Which way is best to detect wheather my sound card is in use or not ?
> Is there a general overview/documentation of the sound kernel modules
> which describes the general relationship between them ?
> What would you do ?

The easiest way would be to check the status of device files, such as
/dev/snd/*, whether any process opens it, e.g. via lsof command.
But, this doesn't cover the case like an app such as a mixer applet
which keeps opening the device but access only occasionally.


Takashi

Re: Detect when sound card is in use

[Clemens Ladisch]

Takashi Iwai wrote:
> Claus Ilginnis wrote:
> > I have a speaker set connected to an USB-Power-Plug (by which I can turn
> > the speakers on and off )
> > 
> > Now I want to detect, when the soundcard is in use and when not.
> > (to turn the speakers on when needed...)
> > 
> > Which way is best to detect wheather my sound card is in use or not ?
> > Is there a general overview/documentation of the sound kernel modules
> > which describes the general relationship between them ?
> > What would you do ?
> 
> The easiest way would be to check the status of device files, such as
> /dev/snd/*, whether any process opens it, e.g. via lsof command.

This would require polling.

> But, this doesn't cover the case like an app such as a mixer applet
> which keeps opening the device but access only occasionally.

I guess the mixer doesn't matter as long as nothing is playing, so it
would be enough to monitor /dev/snd/pcm*.


The easiest way would be to wrap your playback application in a script,
but this works only if that is the only application that plays sounds.

If all your applications use the ALSA API (and not OSS), you could
write a filter plugin that just passes through all requests to the
actual device and switches the power plug in the open/close callbacks.


Best regards,
Clemens