Hello!
I am trying to use alsa to play a sine wave in async mode. Everything seems to work fine, except what I am hearing is _not_ a sine wave. I am checking for XRUNs, but none occur, so I guess the problem is that I am supplying the wrong sample data to alsa.
I want the tone only on one channel, that seems to work fine the way i do it, so i guess my assumptions on how the interleaved mode works are not completely wrong.
Can anyone give me a hint on what I am doing wrong in the example program below? I already checked the example in tests/pcm.c but it didn't really help me.
Regards, Timo
Example Program (compile with cc test.c -o test -lasound):
#include <math.h> #include <stdlib.h> #include <inttypes.h> #include <alsa/asoundlib.h>
struct sound_settings { // alsa parameters snd_pcm_t *pcm_handle; snd_pcm_hw_params_t *hw_params; snd_pcm_sw_params_t *sw_params; snd_async_handler_t *pcm_callback; snd_pcm_uframes_t buffer_size; snd_pcm_uframes_t period_size; unsigned int sample_rate; char *sample_buffer; size_t sample_buffer_size;
// user settings made in gui char *cardname; char *modename; int startfreq; int endfreq; int duration; int channels;
// keep track of gnerated samples long int actsample; long int lastsample;
};
struct sound_settings sdata;
void generate_samples(struct sound_settings *sdata) { int i = 0; if (sdata->channels == 1) i=0; if (sdata->channels == 2) i=1; if (sdata->channels == 3) i=0;
if (strcmp(sdata->modename, "Sine")==0) { while (i<(sdata->sample_buffer_size / 8)) { int samplerate = 44100; int freq = sdata->startfreq; double sample = sin(sdata->actsample * (double) 2*M_PI / ((double) samplerate / (double)freq)); int16_t rsamp = sample * 65535; sdata->actsample++; *(((int16_t*) (sdata->sample_buffer))+i) = rsamp; i += 2; }
} }
void MyCallback(snd_async_handler_t *pcm_callback) {
snd_pcm_t *pcm_handle = snd_async_handler_get_pcm(pcm_callback); snd_pcm_sframes_t avail; struct sound_settings *sdata = (struct sound_settings*) snd_async_handler_get_callback_private(pcm_callback);
avail = snd_pcm_avail_update(pcm_handle); if (avail == -EPIPE) { snd_pcm_prepare (pcm_handle); printf("Underrun occured\n"); } while (avail >= sdata->period_size) { generate_samples(sdata); snd_pcm_writei(pcm_handle, sdata->sample_buffer, sdata->period_size); avail = snd_pcm_avail_update(pcm_handle); }
}
void init_alsa(struct sound_settings *sdata) {
int rc;
// set some sound_settings data (i.e., sample_rate) to sensible defaults sdata->buffer_size = 1024; sdata->period_size = 64; sdata->sample_rate = 44100;
// Open PCM device for playback rc = snd_pcm_open(&(sdata->pcm_handle), "default", SND_PCM_STREAM_PLAYBACK, 0); if (rc < 0) fprintf(stderr, "unable to open pcm device: %s\n", snd_strerror(rc));
// Allocate a hardware parameters object snd_pcm_hw_params_malloc (&(sdata->hw_params)); snd_pcm_hw_params_any(sdata->pcm_handle, sdata->hw_params); snd_pcm_hw_params_set_access(sdata->pcm_handle, sdata->hw_params, SND_PCM_ACCESS_RW_INTERLEAVED); snd_pcm_hw_params_set_format(sdata->pcm_handle, sdata->hw_params, SND_PCM_FORMAT_S16_LE); snd_pcm_hw_params_set_channels(sdata->pcm_handle, sdata->hw_params, 2); snd_pcm_hw_params_set_rate_near(sdata->pcm_handle, sdata->hw_params, &(sdata->sample_rate), NULL); snd_pcm_hw_params_set_buffer_size_near(sdata->pcm_handle, sdata->hw_params, &(sdata->buffer_size)); snd_pcm_hw_params_set_period_size_near (sdata->pcm_handle, sdata->hw_params, &(sdata->period_size), NULL); rc = snd_pcm_hw_params(sdata->pcm_handle, sdata->hw_params); if (rc < 0) fprintf(stderr, "unable to set hw parameters: %s\n", snd_strerror(rc));
// tell alsa we want to work asynchronously snd_pcm_sw_params_malloc(&(sdata->sw_params)); snd_pcm_sw_params_current(sdata->pcm_handle, sdata->sw_params); snd_pcm_sw_params_set_start_threshold(sdata->pcm_handle, sdata->sw_params, sdata->buffer_size - sdata->period_size); snd_pcm_sw_params_set_avail_min(sdata->pcm_handle, sdata->sw_params, sdata->period_size); rc = snd_pcm_sw_params(sdata->pcm_handle, sdata->sw_params); if (rc < 0) fprintf(stderr, "unable to set sw parameters: %s\n", snd_strerror(rc));
// Use a buffer large enough to hold two periods snd_pcm_hw_params_get_period_size(sdata->hw_params, &(sdata->period_size), NULL); sdata->sample_buffer_size = sdata->period_size * 8; // 2 bytes/sample, 2 channels, two periods sdata->sample_buffer = (char *) malloc(sdata->sample_buffer_size); generate_samples(sdata);
// prepare dev so that we can write snd_pcm_prepare(sdata->pcm_handle); /* fill buffer with some data */ generate_samples(sdata);
/* write first chunk of data */ rc = snd_pcm_writei(sdata->pcm_handle, sdata->sample_buffer, sdata->period_size); if (rc == -EPIPE) snd_pcm_prepare(sdata->pcm_handle);
/* register callback */ rc = snd_async_add_pcm_handler(&(sdata->pcm_callback), sdata->pcm_handle, MyCallback, sdata); snd_pcm_start(sdata->pcm_handle);
}
int main(int argc, char **argv) {
sdata.channels = 1; sdata.modename = "Sine"; sdata.startfreq = 220; sdata.actsample = 0; init_alsa(&sdata);
static int i=0; while (i<1000000) { i++; sleep(1); } return 0; }