[alsa-devel] Playback - Overwrite buffer with silence

[Nilhcraiv]

Hello,

I am working an application and I have problems playing the audio with 
ALSA. In my application, I receive the sound data, this data must be 
decoded to be reproduce. In this process, always obtain an 
under-runerror. To fix this problem, I followed the documentation i.e. I 
use snd_pcm_sw_params_set_silence_size() and 
snd_pcm_sw_params_set_silence_threshold() to overwrite the buffer with 
silence. --> 
http://www.alsa-project.org/alsa-doc/alsa-lib/group___p_c_m___s_w___params.html#gaeb4a335a16981b5ea3fa671946fbdca3 
but this does not work. Does the doc is wrong?

If I obtain the boundary for ring pointers in frames at the start 
(snd_pcm_sw_params_get_boundary), the boundary is always zero. If I 
obtain the boundary each time (in the received_data() function) it is 
more than the buffer size and I get error when I put it in 
snd_pcm_sw_params_set_silence_size(), ¿Why?, Is it correct to use the 
boundary value?, ¿How I can avoid under-run by putting silence if I have 
not data at time?

Here you can see a light version of my code (without error checking):

     #define FRAMES                 240
     #define PERIOD                  1*FRAMES
     #define BUFFER_SIZE         PERIOD*2
     #define CHANNELS             1
     #define SAMPLE_RATE        48000

     snd_pcm_hw_params_t *hwparams;
     snd_pcm_sw_params_t *swparams;
     int exact_rate,dir;
     snd_pcm_uframes_t boundary=0;

     snd_pcm_stream_t stream = SND_PCM_STREAM_PLAYBACK;
     snd_pcm_hw_params_alloca(&hwparams);
     snd_pcm_sw_params_alloca(&swparams);


     /*PlayBack hw */
     snd_pcm_open(&pcm_handle, "default", stream, 0);
     snd_pcm_hw_params_any(pcm_handle, hwparams);
     snd_pcm_hw_params_set_access(pcm_handle, hwparams, 
SND_PCM_ACCESS_RW_INTERLEAVED);
     snd_pcm_hw_params_set_format(pcm_handle, hwparams, 
SND_PCM_FORMAT_S16_LE);
     exact_rate = SAMPLE_RATE;
     snd_pcm_hw_params_set_rate_near(pcm_handle, hwparams, &exact_rate, 0);
     snd_pcm_hw_params_set_channels(pcm_handle, hwparams, CHANNELS);
     snd_pcm_hw_params_set_buffer_size(pcm_handle, hwparams, BUFFER_SIZE);
     snd_pcm_hw_params(pcm_handle, hwparams);

     /*Avoid under-run  sw*/
     snd_pcm_sw_params_current(pcm_handle, swparams);
     snd_pcm_sw_params_get_boundary(swparams, &boundary); /*Here, 
boundary is allways ZERO*/
     snd_pcm_sw_params_set_silence_threshold(pcm_handle, swparams, 0); 
/*According to the doc.*/
     snd_pcm_sw_params_set_silence_size(pcm_handle, swparams, boundary); 
/*According to the doc.*/
     snd_pcm_sw_params(pcm_handle,swparams);
     /*END Avoid under-run*/

     snd_pcm_prepare(pcm_handle);

     void received_data(void *t_data)/*short version of the function*/
     {
         short *decoded_data = NULL;
         int len=0;

         decoded_data = (short *)malloc(PERIOD * FRAME_SIZE);
         len = decoder(&decoded_data,t_data);

         snd_pcm_sw_params_get_boundary(swparams, &boundary); /*Here, 
boundary is always a large number*/
         snd_pcm_sw_params_set_silence_size(pcm_handle, swparams, 
boundary);/*Here, boundary in frames is always a large number, and there 
are an error...*/
         snd_pcm_sw_params(pcm_handle,swparams);

         pcmreturn = snd_pcm_writei(pcm_handle, decoded_data, len);/*In 
most cases there are UNDERRUN...*/
         xrun_recovery(&pcm_handle, pcmreturn);/*Recover in case of an 
error.*/
     }

Thanks in advance!
Vicente