Linux ALSA音频PCM播放编程

使用ALSA播放两个频率的单音，并使用GNU Radio中的Audio Source和FFT来观测声音的频谱。

 #include <alsa/asoundlib.h>
 #include <math.h>
 #include <inttypes.h>

 int main(int argc, char **argv)
 {
     long loops;
     snd_pcm_t *handle;
     snd_pcm_hw_params_t *params;
     snd_pcm_uframes_t frames;
     unsigned int val;
     int rc;
     int size;
     int dir;
     char *buffer;

     /* Open PCM device for playback. */
     rc = snd_pcm_open(&handle, "default", SND_PCM_STREAM_PLAYBACK, );
     if (rc < ) {
         fprintf(stderr, "unable to open pcm device: %s\n", snd_strerror(rc));
         exit();
     }

     /* Allocate a hardware parameters object. */
     snd_pcm_hw_params_alloca(&params);

     /* Fill it in with default values. */
     snd_pcm_hw_params_any(handle, params);

     /* Interleaved mode */
     snd_pcm_hw_params_set_access(handle, params, SND_PCM_ACCESS_RW_INTERLEAVED);
     /* Signed 16-bit format */
     snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_S16);
     /* Two channels (stereo) */
     snd_pcm_hw_params_set_channels(handle, params, );
     /* 16000 samples/second sampling rate */
     val = ;
     snd_pcm_hw_params_set_rate_near(handle, params, &val, &dir);
     /* Set period size to 32 frames. */
     frames = ;
     snd_pcm_hw_params_set_period_size_near(handle, params, &frames, &dir);

     /* Write the parameters to the driver */
     rc = snd_pcm_hw_params(handle, params);
     if (rc < ) {
         fprintf(stderr, "unable to set hw parameters: %s\n", snd_strerror(rc));
         exit();
     }

     /* Use a buffer large enough to hold one period */
     snd_pcm_hw_params_get_period_size(params, &frames, &dir);

     size = frames *  * ; /* 2 bytes/sample, 2 channels */

     buffer = (char *) malloc(size);
     if(buffer == NULL) {
         fprintf(stderr, "Not enough Memory!\n");
         exit();
     }

     /* We want to loop for 15 seconds */
     snd_pcm_hw_params_get_period_time(params, &val, &dir);

     /* 15 seconds in microseconds divided by period time */
     loops =  / val;

     for (size_t i = ; i < size; i += ) {
         // Generate a 500Hz tone
         *((int16_t *)(buffer + i)) = (uint16_t)(16384.0*sin(i*2.0*M_PI/size));
         *((int16_t *)(buffer + i + )) = (uint16_t)(16384.0*sin(i*2.0*M_PI/size));
         // Generate a 2kHz tone
         *((int16_t *)(buffer + i)) += (uint16_t)(16384.0*sin(i*8.0*M_PI/size));
         *((int16_t *)(buffer + i + )) += (uint16_t)(16384.0*sin(i*8.0*M_PI/size));
     }

     while (loops > ) {
         loops--;
         //rc = read(0, buffer, size);
         //if (rc == 0) {
         //    fprintf(stderr, "end of file on input\n");
         //    break;
         //} else if (rc != size) {
         //    fprintf(stderr, "short read: read %d bytes\n", rc);
         //}

         rc = snd_pcm_writei(handle, buffer, frames);
         if (rc == -EPIPE) {
             /* EPIPE means underrun */
             fprintf(stderr, "underrun occurred\n");
             snd_pcm_prepare(handle);
         } else if (rc < ) {
             fprintf(stderr,"error from writei: %s\n", snd_strerror(rc));
         } else if (rc != (int)frames) {
             fprintf(stderr,"short write, write %d frames\n", rc);
         }
     }
     snd_pcm_drop(handle);
     snd_pcm_drain(handle);
     snd_pcm_close(handle);
     free(buffer);
     return ;
 }