Hello,
I am developing 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
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_stop_threshold(pcm_handle, swparams, 0);
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
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