View source for Упражнение 7. Аудио контролер

== Цел на упражнението ==
Запознаване със средствата за работа със звук.

Alsa Advanced Linux Sound Architecture
Състои се от набор от драйвери, API функции и инструменти.

принципна схема на работа на аудио проложение

отваряне на интерфейса за запис или възпроизвеждане
задаване на параметрите на хардуера
(тип на достъп, формат на данните, канали, честота на семплиране ...)
докато има данни: прихващане на PCM данни или възпроизвеждане на PCM данни
затваряне на интерфейса

libasound2-dev

<code><pre>
/* Use the newer ALSA API */
#define ALSA_PCM_NEW_HW_PARAMS_API

#include <alsa/asoundlib.h>

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

  /* Open PCM device for playback. */
  rc = snd_pcm_open(&handle, "default",
                    SND_PCM_STREAM_PLAYBACK, 0);
  if (rc < 0) {
    fprintf(stderr,
            "unable to open pcm device: %s\n",
            snd_strerror(rc));
    exit(1);
  }

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

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

  /* Set the desired hardware parameters. */

  /* Interleaved mode */
  snd_pcm_hw_params_set_access(handle, params,
                      SND_PCM_ACCESS_RW_INTERLEAVED);

  /* Signed 16-bit little-endian format */
  snd_pcm_hw_params_set_format(handle, params,
                              SND_PCM_FORMAT_S16_LE);

  /* Two channels (stereo) */
  snd_pcm_hw_params_set_channels(handle, params, 2);

  /* 44100 bits/second sampling rate (CD quality) */
  val = 44100;
  snd_pcm_hw_params_set_rate_near(handle, params, &val, &dir);

  /* Set period size to 32 frames. */
  frames = 32;
  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 < 0) {
    fprintf(stderr,
            "unable to set hw parameters: %s\n",
            snd_strerror(rc));
    exit(1);
  }

  /* Use a buffer large enough to hold one period */
  snd_pcm_hw_params_get_period_size(params, &frames,
                                    &dir);
  size = frames * 4; /* 2 bytes/sample, 2 channels */
  buffer = (char *) malloc(size);

  /* We want to loop for 5 seconds */
  snd_pcm_hw_params_get_period_time(params,
                                    &val, &dir);
  /* 5 seconds in microseconds divided by
   * period time */
  loops = 50000000 / val;

  while (loops > 0) {
    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 < 0) {
      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_drain(handle);
  snd_pcm_close(handle);
  free(buffer);

  return 0;
}
</pre></code>


<code><pre>
#define ALSA_PCM_NEW_HW_PARAMS_API

#include <alsa/asoundlib.h>

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

  /* Open PCM device for recording (capture). */
  rc = snd_pcm_open(&handle, "default",
                    SND_PCM_STREAM_CAPTURE, 0);
  if (rc < 0) {
    fprintf(stderr,
            "unable to open pcm device: %s\n",
            snd_strerror(rc));
    exit(1);
  }

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

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

  /* Set the desired hardware parameters. */

  /* Interleaved mode */
  snd_pcm_hw_params_set_access(handle, params,
                      SND_PCM_ACCESS_RW_INTERLEAVED);

  /* Signed 16-bit little-endian format */
  snd_pcm_hw_params_set_format(handle, params,
                              SND_PCM_FORMAT_S16_LE);

  /* Two channels (stereo) */
  snd_pcm_hw_params_set_channels(handle, params, 2);

  /* 44100 bits/second sampling rate (CD quality) */
  val = 44100;
  snd_pcm_hw_params_set_rate_near(handle, params,
                                  &val, &dir);

  /* Set period size to 32 frames. */
  frames = 32;
  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 < 0) {
    fprintf(stderr,
            "unable to set hw parameters: %s\n",
            snd_strerror(rc));
    exit(1);
  }

  /* Use a buffer large enough to hold one period */
  snd_pcm_hw_params_get_period_size(params,
                                      &frames, &dir);
  size = frames * 4; /* 2 bytes/sample, 2 channels */
  buffer = (char *) malloc(size);

  /* We want to loop for 5 seconds */
  snd_pcm_hw_params_get_period_time(params,
                                         &val, &dir);
  loops = 5000000 / val;

  while (loops > 0) {
    loops--;
    rc = snd_pcm_readi(handle, buffer, frames);
    if (rc == -EPIPE) {
      /* EPIPE means overrun */
      fprintf(stderr, "overrun occurred\n");
      snd_pcm_prepare(handle);
    } else if (rc < 0) {
      fprintf(stderr,
              "error from read: %s\n",
              snd_strerror(rc));
    } else if (rc != (int)frames) {
      fprintf(stderr, "short read, read %d frames\n", rc);
    }
    rc = write(1, buffer, size);
    if (rc != size)
      fprintf(stderr,
              "short write: wrote %d bytes\n", rc);
  }

  snd_pcm_drain(handle);
  snd_pcm_close(handle);
  free(buffer);

  return 0;
}

</pre></code>

<code><pre>
#include <stdio.h> 
#include <stdlib.h> 
#include <string.h> 
#include <errno.h> 
#include <alsa/asoundlib.h>
#include <math.h> 
 
static char *device = "hw:0";                     /* playback device */ 
static snd_pcm_format_t format = SND_PCM_FORMAT_S16;    /* sample format */ 
static unsigned int rate = 44100;                       /* stream rate */ 
static unsigned int channels = 2;                       /* count of channels */ 
static unsigned int buffer_time = 500000;               /* ring buffer length in us */ 
static unsigned int period_time = 100000;               /* period time in us */ 
static double freq = 200.100;                               /* sinusoidal wave frequency in Hz */ 
 
static snd_pcm_sframes_t buffer_size; 
static snd_pcm_sframes_t period_size; 
static snd_output_t *output = NULL; 

static void generate_sine(const snd_pcm_channel_area_t *areas,  
                          snd_pcm_uframes_t offset, 
                          int count, double *_phase) 
{ 
        static double max_phase = 2. * M_PI; 
        double phase = *_phase;
        
       /* generate secret number: */
        double step = max_phase*freq/(double)rate; 
        
        double res; 
        unsigned char *samples[channels], *tmp; 
        
        int steps[channels]; 
        unsigned int chn, byte; 
        
        int ires; 
        unsigned int maxval = (1 << (snd_pcm_format_width(format) - 1)) - 1; 
        int bps = snd_pcm_format_width(format) / 8;  /* bytes per sample */ 
         
        /* verify and prepare the contents of areas */ 
        for (chn = 0; chn < channels; chn++) { 
                if ((areas[chn].first % 8) != 0) { 
                        printf("areas[%i].first == %i, aborting...\n", chn, areas[chn].first); 
                        exit(EXIT_FAILURE); 
                } 
                samples[chn] = /*(signed short *)*/(((unsigned char *)areas[chn].addr) + (areas[chn].first / 8)); 
                if ((areas[chn].step % 16) != 0) { 
                        printf("areas[%i].step == %i, aborting...\n", chn, areas[chn].step); 
                        exit(EXIT_FAILURE); 
                } 
                steps[chn] = areas[chn].step / 8; 
                samples[chn] += offset * steps[chn]; 
        } 
        /* fill the channel areas */ 
        while (count-- > 0) { 
                res = sin(phase) *sin(phase) * maxval; 
                ires = res; 
                tmp = (unsigned char *)(&ires); 
                for (chn = 0; chn < channels; chn++) { 
                        for (byte = 0; byte < (unsigned int)bps; byte++) 
                                *(samples[chn] + byte) = tmp[byte]; 
                        samples[chn] += steps[chn]; 
                } 
                phase += step; 
                if (phase >= max_phase) 
                        phase -= max_phase; 
        } 
        *_phase = phase; 
} 
 
static int set_hwparams(snd_pcm_t *handle, 
                        snd_pcm_hw_params_t *params, 
                        snd_pcm_access_t access) 
{ 
        unsigned int rrate; 
        snd_pcm_uframes_t size; 
        int err, dir; 
 
        /* choose all parameters */ 
        snd_pcm_hw_params_any(handle, params); 
        /* set hardware resampling */ 
        snd_pcm_hw_params_set_rate_resample(handle, params, 1); 
        /* set the interleaved read/write format */ 
        err = snd_pcm_hw_params_set_access(handle, params, access); 
        /* set the sample format */ 
        err = snd_pcm_hw_params_set_format(handle, params, format); 
        /* set the count of channels */ 
        err = snd_pcm_hw_params_set_channels(handle, params, channels); 
        /* set the stream rate */ 
        rrate = rate; 
        err = snd_pcm_hw_params_set_rate_near(handle, params, &rrate, 0); 
        if (err < 0) { 
                printf("Rate %iHz not available for playback: %s\n", rate, snd_strerror(err)); 
                return err; 
        } 
        if (rrate != rate) { 
                printf("Rate doesn't match (requested %iHz, get %iHz)\n", rate, err); 
                return -EINVAL; 
        } 
        /* set the buffer time */ 
        snd_pcm_hw_params_set_buffer_time_near(handle, params, &buffer_time, &dir); 
        snd_pcm_hw_params_get_buffer_size(params, &size); 
        
        buffer_size = size; 
        /* set the period time */ 
        err = snd_pcm_hw_params_set_period_time_near(handle, params, &period_time, &dir); 
        if (err < 0) { 
                printf("Unable to set period time %i for playback: %s\n", period_time, snd_strerror(err)); 
                return err; 
        } 
        err = snd_pcm_hw_params_get_period_size(params, &size, &dir); 
        if (err < 0) { 
                printf("Unable to get period size for playback: %s\n", snd_strerror(err)); 
                return err; 
        } 
        period_size = size; 
        /* write the parameters to device */ 
        err = snd_pcm_hw_params(handle, params); 
        if (err < 0) { 
                printf("Unable to set hw params for playback: %s\n", snd_strerror(err)); 
                return err; 
        } 
        return 0; 
} 
 
static int set_swparams(snd_pcm_t *handle, snd_pcm_sw_params_t *swparams) 
{ 
        /* get the current swparams */ 
        snd_pcm_sw_params_current(handle, swparams); 
        /* start the transfer when the buffer is almost full: */ 
        /* (buffer_size / avail_min) * avail_min */ 
        snd_pcm_sw_params_set_start_threshold(handle, swparams, (buffer_size / period_size) * period_size); 
        /* allow the transfer when at least period_size samples can be processed */ 
        snd_pcm_sw_params_set_avail_min(handle, swparams, period_size); 
        /* write the parameters to the playback device */ 
        snd_pcm_sw_params(handle, swparams); 
        return 0; 
} 
 
/* 
 *   Underrun and suspend recovery 
 */ 
  
static int xrun_recovery(snd_pcm_t *handle, int err) 
{ 
        if (err == -EPIPE) {    /* under-run */ 
                err = snd_pcm_prepare(handle); 
                if (err < 0) 
                        printf("Can't recovery from underrun, prepare failed: %s\n", snd_strerror(err)); 
                return 0; 
        } else if (err == -ESTRPIPE) { 
                while ((err = snd_pcm_resume(handle)) == -EAGAIN) 
                        sleep(1);       /* wait until the suspend flag is released */ 
                if (err < 0) { 
                        err = snd_pcm_prepare(handle); 
                        if (err < 0) 
                                printf("Can't recovery from suspend, prepare failed: %s\n", snd_strerror(err)); 
                } 
                return 0; 
        } 
        return err; 
} 
 
/* 
 *   Transfer method - write only 
 */ 
 
static int write_loop(snd_pcm_t *handle, 
                      signed short *samples, 
                      snd_pcm_channel_area_t *areas) 
{ 
        double phase = 0; 
        signed short *ptr; 
        int err, cptr; 
 
        while (1) { 
                generate_sine(areas, 0, period_size, &phase); 
                ptr = samples; 
                cptr = period_size; 
                while (cptr > 0) { 
                        err = snd_pcm_writei(handle, ptr, cptr); 
                        if (err == -EAGAIN) 
                                continue; 
                        if (err < 0) { 
                                if (xrun_recovery(handle, err) < 0) { 
                                        printf("Write error: %s\n", snd_strerror(err)); 
                                        exit(EXIT_FAILURE); 
                                } 
                                break;  /* skip one period */ 
                        } 
                        ptr += err * channels; 
                        cptr -= err; 
                } 
        } 
} 
  
int main(int argc, char *argv[]) 
{ 
	srand(time(NULL));
        snd_pcm_t *handle; 
        snd_pcm_hw_params_t *hwparams; 
        snd_pcm_sw_params_t *swparams; 
        signed short *samples; 
        unsigned int chn; 
        snd_pcm_channel_area_t *areas; 
 
        snd_pcm_hw_params_alloca(&hwparams); 
        snd_pcm_sw_params_alloca(&swparams); 
 
 
        if ( snd_output_stdio_attach(&output, stdout, 0)){ 
                printf("Output failed: %s\n", snd_strerror(0)); 
                return 0; 
        } 
 
        printf("Playback device is %s\n", device); 
        printf("Stream parameters are %iHz, %s, %i channels\n", rate, snd_pcm_format_name(format), channels); 
        printf("Sine wave rate is %.4fHz\n", freq); 
       
        snd_pcm_open(&handle, device, SND_PCM_STREAM_PLAYBACK, 0);
        set_hwparams(handle, hwparams, SND_PCM_ACCESS_RW_INTERLEAVED);
        set_swparams(handle, swparams);
       
        samples = malloc((period_size * channels * snd_pcm_format_width(format)) / 8); 
        areas = calloc(channels, sizeof(snd_pcm_channel_area_t)); 
         
        for (chn = 0; chn < channels; chn++) { 
                areas[chn].addr = samples; 
                areas[chn].first = chn * snd_pcm_format_width(format); 
                areas[chn].step = channels * snd_pcm_format_width(format); 
        } 
 
        write_loop(handle, samples, areas); 
    
        free(areas); 
        free(samples); 
        snd_pcm_close(handle); 
        return 0; 
} 
</pre></code>		
	


[[Category:Компютърна периферия]]


Антидот
 "Музиката, която не изобразява нищо е само шум."
[http://en.wikipedia.org/wiki/Jean_le_Rond_d'Alembert|Ж. Даламбер]