Join play of pcm in multiple playback devices

Join play of pcm in multiple playback devices

[Irfan shaikh]

Hello All,

              I have written an application which supports join play of 
PCM Audio.
i.e Capture and write to playback device 1 and playback device 2 
simultaneously.

Below code is just to explain the above scenario :

/######################/

while(1)
{
     retval=snd_pcm_readi(pAlsaConfig->pcm_handle, buffer, 
pAlsaConfig->alsaBufferSize);
     snd_pcm_writei(pAlsaConfig->pcm_handle1, buffer,retval);
     snd_pcm_writei(pAlsaConfig->pcm_handle2, buffer,retval);
}

/######################/

In above case, renderer 2 lags behind the renderer 1 i.e there is delay 
between two audio devices when actually heard.


Please help me if some better implementation for join play.
I have seen some thing as snd_pcm_link(). But i was not clear with its 
usage, while doing snd_pcm_writei ???

Thanks and regards,
Irfan

Re: Join play of pcm in multiple playback devices

[Pierre-Louis Bossart]

> while(1)
> {
>       retval=snd_pcm_readi(pAlsaConfig->pcm_handle, buffer,
> pAlsaConfig->alsaBufferSize);
>       snd_pcm_writei(pAlsaConfig->pcm_handle1, buffer,retval);
>       snd_pcm_writei(pAlsaConfig->pcm_handle2, buffer,retval);
> }

This code doesn't work in the general case where the different devices 
don't share a common audio clock and have different physical 
capture/playback rates. Compensating for clock drift requires the use of 
asynchronous sample-rate conversion.
Assuming you have a perfectly synchronous system, you could make use of 
snd_pcm_delay() to compensate for delays incurred in the 
driver/firmware/hardware. This is not well supported by all drivers though.
-Pierre