Exemple of source code for capture without using callback

Exemple of source code for capture without using callback

[brunal2496 gmail]

Hi all,

it's my first time on this list and my knowledge about using poll()  
mutex and thread creation is a bit far away from now, so please, if  
this list is the wrong place for such a question, could you point me  
in the right direction?

Does anyone can point me a place to find a good exemple of source code  
for capturing sound with alsa without using callbacks?

I'm trying to interface a voice recognition software with the alsa  
drivers.

I have to get audio from alsa convert it and then send to the  
recognition engine.
I've read everywhere that using callback is a bad method to use the  
alsa drivers.

As the recognition software is a server, it needs to be synchronized  
with my application. And so my main() cannot loop on a endless loop  
with the snd_pcm_readi() function inside.

My first idea was to create a new thread and to let this thread  
running with a endless loop that do the snd_pcm_readi() and send the  
buffer to the recognition software.
But I 've read the existence of smd_pcm_poll_descriptors() function.  
For now I not shure was it is for and how to use it properly.

What do you think?

I'm sorry if my question is a real newbie question, but exemple online  
are only for playback not capture.

Many thanks for all your answers.

Bruno.

Re: Exemple of source code for capture without using callback

[James Courtier-Dutton]

2009/10/20 brunal2496 gmail <brunal2496@gmail.com>:
>
> I'm trying to interface a voice recognition software with the alsa
> drivers.
>
> I have to get audio from alsa convert it and then send to the
> recognition engine.
> I've read everywhere that using callback is a bad method to use the
> alsa drivers.
>

This is wrong. callback is the best method.
With current distributions all doing pulseaudio, it would probably be
better interfacing with pulse.
If you need a more professional real time interface, use jackd interface.
For what you need, jackd is probably the simplest api to use.

Re: Exemple of source code for capture without using callback

[Clemens Ladisch]

James Courtier-Dutton wrote:
> 2009/10/20 brunal2496 gmail <brunal2496@gmail.com>:
> > I've read everywhere that using callback is a bad method to use the
> > alsa drivers.
> 
> This is wrong. callback is the best method.

But _not_ the snd_async_add_pcm_handler function, which is evil.
(This is the only ALSA PCM function that mentions "callback" in its
documentation, so these are often confused.)

The 'good' callback method means that the audio driver informs the
application when its wants to have new audio data, not the other way.


Best regards,
Clemens

Re: Exemple of source code for capture without using callback

[James Courtier-Dutton]

2009/10/22 Clemens Ladisch <clemens@ladisch.de>:
> James Courtier-Dutton wrote:
>> 2009/10/20 brunal2496 gmail <brunal2496@gmail.com>:
>> > I've read everywhere that using callback is a bad method to use the
>> > alsa drivers.
>>
>> This is wrong. callback is the best method.
>
> But _not_ the snd_async_add_pcm_handler function, which is evil.
> (This is the only ALSA PCM function that mentions "callback" in its
> documentation, so these are often confused.)
>
> The 'good' callback method means that the audio driver informs the
> application when its wants to have new audio data, not the other way.
>
Yes, to clarify, not that call.
A poll() that then does the callback is the preferred method.

Re: Exemple of source code for capture without using callback

[brunal2496 gmail]

Tanks for your answer.

>> But _not_ the snd_async_add_pcm_handler function, which is evil.
>> (This is the only ALSA PCM function that mentions "callback" in its
>> documentation, so these are often confused.)
>>
>> The 'good' callback method means that the audio driver informs the
>> application when its wants to have new audio data, not the other way.

If I'm not wrong you're talking about playback in that case, isn't it?  
I'm considering capture so you mean that the good callback method is  
when the audio driver tells the application that it have new audio  
data (the buffer contains at least a "period_size" of audio samples)  
to give to it. Am I right?

> Yes, to clarify, not that call.
> A poll() that then does the callback is the preferred method.

Ok. But I cannot find a source code using poll() for capture on  
internet. Would you please gimme a pointer or at least a pseudo-code  
for an application using poll() i.e. just what is the sequence of the  
function?
Many thanks.