linux音频编程指南.htm

linux下的音频编程指导

  int fd;	/* 声音设备的文件描述符 */
  int arg;	/* 用于ioctl调用的参数 */
  int status;   /* 系统调用的返回值 */
  /* 打开声音设备 */
  fd = open("/dev/dsp", O_RDWR);
  if (fd < 0) {
    perror("open of /dev/dsp failed");
    exit(1);
  }
  /* 设置采样时的量化位数 */
  arg = SIZE;
  status = ioctl(fd, SOUND_PCM_WRITE_BITS, &arg);
  if (status == -1)
    perror("SOUND_PCM_WRITE_BITS ioctl failed");
  if (arg != SIZE)
    perror("unable to set sample size");
  /* 设置采样时的声道数目 */
  arg = CHANNELS; 
  status = ioctl(fd, SOUND_PCM_WRITE_CHANNELS, &arg);
  if (status == -1)
    perror("SOUND_PCM_WRITE_CHANNELS ioctl failed");
  if (arg != CHANNELS)
    perror("unable to set number of channels");
  /* 设置采样时的采样频率 */
  arg = RATE;
  status = ioctl(fd, SOUND_PCM_WRITE_RATE, &arg);
  if (status == -1)
    perror("SOUND_PCM_WRITE_WRITE ioctl failed");
  /* 循环，直到按下Control-C */
  while (1) {
    printf("Say something:\n");
    status = read(fd, buf, sizeof(buf)); /* 录音 */
    if (status != sizeof(buf))
      perror("read wrong number of bytes");
    printf("You said:\n");
    status = write(fd, buf, sizeof(buf)); /* 回放 */
    if (status != sizeof(buf))
      perror("wrote wrong number of bytes");
    /* 在继续录音前等待回放结束 */
    status = ioctl(fd, SOUND_PCM_SYNC, 0); 
    if (status == -1)
      perror("SOUND_PCM_SYNC ioctl failed");
  }
}
</PRE></TD></TR></TBODY></TABLE><BR>
            <P><A name=N1025A><SPAN class=smalltitle>4.4 混音器框架</SPAN></A></P>
            <P>下面再给出一个对混音器进行编程的基本框架，利用它可以对各种混音通道的增益进行调节，其所有的功能都是通过读写/dev/mixer设备文件来完成的：</P>
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                <TD class=code-outline><PRE class=displaycode>/*
 * mixer.c
 */
#include &lt;unistd.h&gt;
#include &lt;stdlib.h&gt;
#include &lt;stdio.h&gt;
#include &lt;sys/ioctl.h&gt;
#include &lt;fcntl.h&gt;
#include &lt;linux/soundcard.h&gt;
/* 用来存储所有可用混音设备的名称 */
const char *sound_device_names[] = SOUND_DEVICE_NAMES;
int fd;                  /* 混音设备所对应的文件描述符 */
int devmask, stereodevs; /* 混音器信息对应的位图掩码 */
char *name;
/* 显示命令的使用方法及所有可用的混音设备 */
void usage()
{
  int i;
  fprintf(stderr, "usage: %s &lt;device&gt; &lt;left-gain%%&gt; &lt;right-gain%%&gt;\n"
	  "       %s &lt;device&gt; &lt;gain%%&gt;\n\n"
	  "Where &lt;device&gt; is one of:\n", name, name);
  for (i = 0 ; i &lt; SOUND_MIXER_NRDEVICES ; i++)
    if ((1 &lt;&lt; i) &amp; devmask) /* 只显示有效的混音设备 */
      fprintf(stderr, "%s ", sound_device_names[i]);
  fprintf(stderr, "\n");
  exit(1);
}
int main(int argc, char *argv[])
{
  int left, right, level;  /* 增益设置 */
  int status;              /* 系统调用的返回值 */
  int device;              /* 选用的混音设备 */
  char *dev;               /* 混音设备的名称 */
  int i;
  name = argv[0];
  /* 以只读方式打开混音设备 */
  fd = open("/dev/mixer", O_RDONLY);
  if (fd == -1) {
    perror("unable to open /dev/mixer");
    exit(1);
  }
  
  /* 获得所需要的信息 */
  status = ioctl(fd, SOUND_MIXER_READ_DEVMASK, &amp;devmask);
  if (status == -1)
    perror("SOUND_MIXER_READ_DEVMASK ioctl failed");
  status = ioctl(fd, SOUND_MIXER_READ_STEREODEVS, &amp;stereodevs);
  if (status == -1)
    perror("SOUND_MIXER_READ_STEREODEVS ioctl failed");
  /* 检查用户输入 */
  if (argc != 3 &amp;&amp; argc != 4)
    usage();
  /* 保存用户输入的混音器名称 */
  dev = argv[1];
  /* 确定即将用到的混音设备 */
  for (i = 0 ; i &lt; SOUND_MIXER_NRDEVICES ; i++)
    if (((1 &lt;&lt; i) &amp; devmask) &amp;&amp; !strcmp(dev, sound_device_names[i]))
      break;
  if (i == SOUND_MIXER_NRDEVICES) { /* 没有找到匹配项 */
    fprintf(stderr, "%s is not a valid mixer device\n", dev);
    usage();
  }
  /* 查找到有效的混音设备 */
  device = i;
  /* 获取增益值 */
  if (argc == 4) {
    /* 左、右声道均给定 */
    left  = atoi(argv[2]);
    right = atoi(argv[3]);
  } else {
    /* 左、右声道设为相等 */
    left  = atoi(argv[2]);
    right = atoi(argv[2]);
  }
  
  /* 对非立体声设备给出警告信息 */
  if ((left != right) &amp;&amp; !((1 &lt;&lt; i) &amp; stereodevs)) {
    fprintf(stderr, "warning: %s is not a stereo device\n", dev);
  }
  
  /* 将两个声道的值合到同一变量中 */
  level = (right &lt;&lt; 8) + left;
  
  /* 设置增益 */
  status = ioctl(fd, MIXER_WRITE(device), &amp;level);
  if (status == -1) {
    perror("MIXER_WRITE ioctl failed");
    exit(1);
  }
  /* 获得从驱动返回的左右声道的增益 */
  left  = level &amp; 0xff;
  right = (level &amp; 0xff00) &gt;&gt; 8;
  /* 显示实际设置的增益 */
  fprintf(stderr, "%s gain set to %d%% / %d%%\n", dev, left, right);
  /* 关闭混音设备 */
  close(fd);
  return 0;
}
</PRE></TD></TR></TBODY></TABLE><BR>
            <P>编译好上面的程序之后，先不带任何参数执行一遍，此时会列出声卡上所有可用的混音通道：</P>
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                <TD class=code-outline><PRE class=displaycode>[xiaowp@linuxgam sound]$ ./mixer
usage: ./mixer &lt;device&gt; &lt;left-gain%&gt; &lt;right-gain%&gt;
       ./mixer &lt;device&gt; &lt;gain%&gt;
 
Where &lt;device&gt; is one of:
vol pcm speaker line mic cd igain line1 phin video
</PRE></TD></TR></TBODY></TABLE><BR>
            <P>之后就可以很方便地设置各个混音通道的增益大小了，例如下面的命令就能够将CD输入的左、右声道的增益分别设置为80%和90%：</P>
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                <TD class=code-outline><PRE class=displaycode>[xiaowp@linuxgam sound]$ ./mixer cd 80 90
cd gain set to 80% / 90%
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                <TD><IMG height=1 alt="" src="Linux音频编程指南.files/blue_rule.gif" 
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