c - 用 ALSA 录音

Question

我正在尝试“录制”一个音频文件，我读过这个 doc ，实际上我想在文件中将“采样”值记录为双倍值，这是我使用的代码(不起作用，我不知道为什么没有):

/* Use the newer ALSA API */
#define ALSA_PCM_NEW_HW_PARAMS_API
#include <alsa/asoundlib.h>
#include <stdio.h>

int main() {
  long loops;
   int rc;
  int size,z = 0 ;
  unsigned int val;
  double* buffer;
  int dir;
  snd_pcm_t *handle;
  snd_pcm_hw_params_t *params;
  snd_pcm_uframes_t frames;

  FILE* output = NULL;
  output =fopen("recod_values.txt","w");

  /* 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);

  /* MONO ! channel */
  snd_pcm_hw_params_set_channels(handle, params,1);

  /* 96000 bits/second sampling rate  */
  val = 96000;
  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 * 1; /* 2 bytes/sample, 2 channels */
  buffer = (double*) 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);

    for ( z = 0 ; z < size;z++){ 
      fprintf(output,"%lf \n",buffer[z]);

     } 

  }

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

  return 0;
}

得到的结果是假的，这是我得到的一部分:

    22164315735966424535139159791393084768500567327664428456919459225160782460290374318673978007739264159481013271070331047993933279328468540357215794465042587451392.000000 
6279123284769190191779385445935961906015983426866033011058871857355906568111563202057623448515972440403603734829703874073506103294822799230919004382628287132004967906916958208.000000 
0.000000 
0.000000 
0.000000 
1144284986495925317233642104161717490326555645567187694497268067386154959544116448945812917900397813140052794333237528352248562790473524666519326385472977850060639877322124157668710305854399264313107197566174852391468506111174989083416952045568.000000 
486016490646530877454846463864566567777058339173509861936688900655372986604461971881776121791820561656186713737600239967251495507590158168644511518013315486066797390056338302864276615681456563025411108154944185463301467774199971222422456597209993418331127808.000000 
97502070478605056015384266450931746345849383201940068533539715554348065640767209105810488184602958812662140616258325471782047626172052091225732327584237457213932719329242675004694018348986553910427648.000000 
0.000000 
0.000000 
0.000000 
0.000000 
0.000000 

知道如何使它看起来更好吗？提前致谢 !

更新使用Audacity导入输出文件后，结果或声音与录制的声音完全无关!是我做错了吗？我的意思是这部分代码:

loops = 5000000/ val;

  while (loops > 0) {
    loops--;
    rc = snd_pcm_readi(handle, buffer, frames);

    for ( z = 0 ; z < size;z++){ 
      fprintf(output,"%lf \n",buffer[z]);

     } 

  }

Answer 1

snd_pcm_readi 以您使用 snd_pcm_hw_params_set_format 设置的格式写入示例，因此您必须确保 buffer 的类型匹配这。double 的等效项是 SND_PCM_FORMAT_FLOAT64。

此外，malloc 需要一个以 bytes 为单位的大小，因此您必须使用

size = frames * sizeof(double);

打印样本时，您的字节数多于样本数，因此 size 是错误的计数器。返回的样本数在rc中:

for (z = 0; z < rc; z++)
  fprintf(output, "%lf\n", buffer[z]);