ubuntu alsa

今天要在linux下搞音频编程，在网上查阅了一下资料，网上很多资料都是在linux下直接对/dev/dsp进行编程的，因为在以往的linux系统中，我们是可以通过cat  xxx.wav /dev/dsp 来很容易的播放一个音频文件，在应用程序中，也可以直接操作/dev/dsp，实现声音的播放：打开->写入(实际上就能播放)->关闭。

然而在换成了ubuntu-12.04 LST后，我发现/dev中根本找不到dsp，之前直接操作/dev/dsp的程序都无法正常运行，而是 can't find /dev/dsp.

经过我再三查找，发现现在基本上都不用dsp了，大多数人都在用ALSA音频编程（其实我也是个菜鸟）。首先介绍一下一些关于ALSA编程的知识：

 

1、GNU/Linux 系统下三大主流声卡驱动程序集：

Linux 有三个主流的声卡驱动程序集：OSS/Lite（也称为OSS/Free）、OSS/Full

（商业软件）、ALSA（自由软件）。

 

OSS/Lite 是现在linux内核中自带的声卡驱动程序集，最初由 Hannu Savolainen

开发。后来 Hannu 跑去开发 Open Sound System（也就是上面所说的OSS/Full）。

由于 Hannu 的“逃跑”，RH 资助 Alan Cox 增强 Hannu 开发的驱动程序，并使它们

完全模块化。现在 Alan Cox 是内核声卡驱程集的维护人。OSS/Lite 从kernel-2.0开

始并入内核，现在大家使用的声卡驱程默认都是OSS/Lite中的。

 

OSS/Full 是由 4Front Technologies 开发并销售的商业软件。它可以驱动很多

声卡并且可以用在很多 UNIX 系统中。OSS/Full 完全兼容以前基于 OSS／Lite 开发

的应用程序。作为一个商业软件，你虽然可以使用它，但是你得不到它的源代码，并且

你必须为此而付钱。

 

ALSA 是linux内核的下一代标准声卡驱动集。开始时 Jaroslav Kysela 等人为

Gravis UltraSound Card 开发驱动程序，后来该计划改名为 ALSA「先进的linux音频

体系」，因为他们认为 ALSA 比原来内核中的 OSS/Lite 驱动程序集更优秀，完全可以

代替 OSS/Lite。他们是对的，ALSA 支持的声卡比 OSS/Lite 多，完全兼容以前基于

OSS 开发的程序，SMP（多处理器） 与 线程安全设计，并且从 2.5 分支的内核开始，

ALSA 的驱动程序集开始并入内核，大家可以在今年的 2.6 版本的内核中看到使用它们。

 

2、为什么要使用 ALSA 开发音频程序

首先，ALSA 是 linux 以后声卡驱动程序的标准，OSS/Lite 迟早会从内核中去除。

开发基于 ALSA 的音频程序可以保证以后的兼容。

其次，我们简单比较一下开发基于 OSS 与 ALSA 的方法。

OSS 向应用程序提供了一系列的系统接口。开发基于 OSS 的应用程序需要使用

open,close,ioctl,read,write 等低级系统调用来完成音频设备的控制、音频流的输入

输出。

而 ALSA 则为应用程序开发人员提供了一个优秀的音频库。利用该音频库，开发

人员可以方便快捷地开发出自己的应用程序，细节则留给音频库内部处理。当然 ALSA

也提供了类似于 OSS 的系统接口，不过 ALSA 的开发者建议应用程序开发者使用音频

库而不是驱程API。

 

3、那么我该从何开始呢

第一步当然是安装 ALSA 驱动程序与音频库。

当前 ALSA 有两个分支，一个是以前的0.5版本，一个是现在的0.9。ALSA的开发者

已经不支持0.5版本了，所以我们要使用0.9。大家可以在 ALSA 的主页

www.alsa-project.org 上下载安装。这个页面上的信息对大家安装很有用：

www.alsa-project.org/alsa-doc ，建议先浏览一下。

 

第二步是参考文档与例子。

在 ALSA 的文档页面上有两篇为应用程序开发者提供的文章：

ALSA 0.9.0 HOWTO [ http://www.suse.de/~mana/alsa090_howto.html ]

Howto use the ALSA API [ http://www.op.net/~pbd/alsa-audio.html ]

当然，还有音频库API的在线参考：

http://www.alsa-project.org/alsa-doc/alsa-lib

 

 要是你已经完成以上几步的话，那么你就应该开始开发了。

4. ALSA设备文件结构

我们从alsa在Linux中的设备文件结构开始我们的alsa之旅. 看看我的电脑中的alsa驱动的设备文件结构:

$ cd /dev/snd
$ ls -l

crw-rw----+ 1 root audio 116, 8 2011-02-23 21:38 controlC0
crw-rw----+ 1 root audio 116, 4 2011-02-23 21:38 midiC0D0
crw-rw----+ 1 root audio 116, 7 2011-02-23 21:39 pcmC0D0c
crw-rw----+ 1 root audio 116, 6 2011-02-23 21:56 pcmC0D0p
crw-rw----+ 1 root audio 116, 5 2011-02-23 21:38 pcmC0D1p
crw-rw----+ 1 root audio 116, 3 2011-02-23 21:38 seq
crw-rw----+ 1 root audio 116, 2 2011-02-23 21:38 timer
$

    我们可以看到以下设备文件:

    controlC0 -->                 用于声卡的控制，例如通道选择，混音，麦克风的控制等
    midiC0D0  -->                用于播放midi音频
    pcmC0D0c --〉               用于录音的pcm设备
    pcmC0D0p --〉               用于播放的pcm设备
    seq  --〉                        音序器
    timer --〉                       定时器

其中，C0D0代表的是声卡0中的设备0，pcmC0D0c最后一个c代表capture，pcmC0D0p最后一个p代表playback，这些都是alsa-driver中的命名规则。从上面的列表可以看出，我的声卡下挂了6个设备，根据声卡的实际能力，驱动实际上可以挂上更多种类的设备，在include/sound/core.h中，定义了以下设备类型：

#define SNDRV_DEV_TOPLEVEL  ((__force snd_device_type_t) 0) 
#define SNDRV_DEV_CONTROL   ((__force snd_device_type_t) 1) 
#define SNDRV_DEV_LOWLEVEL_PRE  ((__force snd_device_type_t) 2) 
#define SNDRV_DEV_LOWLEVEL_NORMAL ((__force snd_device_type_t) 0x1000) 
#define SNDRV_DEV_PCM       ((__force snd_device_type_t) 0x1001) 
#define SNDRV_DEV_RAWMIDI   ((__force snd_device_type_t) 0x1002) 
#define SNDRV_DEV_TIMER     ((__force snd_device_type_t) 0x1003) 
#define SNDRV_DEV_SEQUENCER ((__force snd_device_type_t) 0x1004) 
#define SNDRV_DEV_HWDEP     ((__force snd_device_type_t) 0x1005) 
#define SNDRV_DEV_INFO      ((__force snd_device_type_t) 0x1006) 
#define SNDRV_DEV_BUS       ((__force snd_device_type_t) 0x1007) 
#define SNDRV_DEV_CODEC     ((__force snd_device_type_t) 0x1008) 
#define SNDRV_DEV_JACK      ((__force snd_device_type_t) 0x1009) 
#define SNDRV_DEV_LOWLEVEL  ((__force snd_device_type_t) 0x2000)   

 通常，我们更关心的是pcm和control这两种设备。

5.一些例子，这些例子在官方文档也有，请自行查阅

1.）显示一些PCM的类型和格式:

 #include <iostream>
#include <alsa/asoundlib.h>
 
int main()
{
       std::cout << "ALSA library version: " << SND_LIB_VERSION_STR << std::endl;
 
       std::cout << "PCM stream types: " << std::endl;
 
       for (int val=0; val <= SND_PCM_STREAM_LAST; ++val)
              std::cout << snd_pcm_stream_name((snd_pcm_stream_t)val) << std::endl;
       std::cout << std::endl;
 
       std::cout << "PCM access types: " << std::endl;
       for (int val=0; val <= SND_PCM_ACCESS_LAST; ++val)
              std::cout << snd_pcm_access_name((snd_pcm_access_t)val) << std::endl;
       std::cout << std::endl;
 
       std::cout << "PCM subformats: " << std::endl;
       for (int val=0; val <= SND_PCM_SUBFORMAT_LAST; ++val)
              std::cout << snd_pcm_subformat_name((snd_pcm_subformat_t)val) << " (" << snd_pcm_subformat_description((snd_pcm_subformat_t)val) << ")" << std::endl;
       std::cout << std::endl;
 
       std::cout << "PCM states: " << std::endl;
       for (int val=0; val <= SND_PCM_STATE_LAST; ++val)
              std::cout << snd_pcm_state_name((snd_pcm_state_t)val) << std::endl;
       std::cout << std::endl;
 
 
       std::cout << "PCM formats: " << std::endl;
       for (int val=0; val <= SND_PCM_FORMAT_LAST; ++val)
              std::cout << snd_pcm_format_name((snd_pcm_format_t)val) << " (" << snd_pcm_format_description((snd_pcm_format_t)val) << ")" << std::endl;
       std::cout << std::endl;
 
}

2.）打开PCM设备和设置参数

 #include <iostream>
#include <alsa/asoundlib.h>
 
int main()
{
       int                               rc;
       snd_pcm_t*                         handle;
       snd_pcm_hw_params_t*      params;
       unsigned int                  val, val2;
       int                               dir;
       snd_pcm_uframes_t             frames;
 
       if ( (rc = snd_pcm_open(&handle, "default", SND_PCM_STREAM_PLAYBACK, 0)) < 0)
       {
              std::cerr << "unable to open pcm devices: " << snd_strerror(rc) << std::endl;
              exit(1);
       }
 
       snd_pcm_hw_params_alloca(&params);
 
       snd_pcm_hw_params_any(handle, params);
 
       snd_pcm_hw_params_set_access(handle, params, SND_PCM_ACCESS_RW_INTERLEAVED);
 
       snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_S16_LE);
 
       snd_pcm_hw_params_set_channels(handle, params, 2);
 
       val = 44100;
 
       snd_pcm_hw_params_set_rate_near(handle, params, &val, &dir);
 
       if ( (rc = snd_pcm_hw_params(handle, params)) < 0)
       {
              std::cerr << "unable to set hw parameters: " << snd_strerror(rc) << std::endl;
              exit(1);
       }
 
       std::cout << "PCM handle name = " << snd_pcm_name(handle) << std::endl;
 
       std::cout << "PCM state = " << snd_pcm_state_name(snd_pcm_state(handle)) << std::endl;
 
       snd_pcm_hw_params_get_access(params, (snd_pcm_access_t *)&val);
 
       std::cout << "access type = " << snd_pcm_access_name((snd_pcm_access_t)val) << std::endl;
 
       snd_pcm_hw_params_get_format(params, (snd_pcm_format_t*)(&val));
 
       std::cout << "format = '" << snd_pcm_format_name((snd_pcm_format_t)val) << "' (" << snd_pcm_format_description((snd_pcm_format_t)val) << ")" << std::endl;
 
      snd_pcm_hw_params_get_subformat(params, (snd_pcm_subformat_t *)&val);
      std::cout << "subformat = '" <<
    snd_pcm_subformat_name((snd_pcm_subformat_t)val) << "' (" << snd_pcm_subformat_description((snd_pcm_subformat_t)val) << ")" << std::endl;
 
      snd_pcm_hw_params_get_channels(params, &val);
      std::cout << "channels = " << val << std::endl;
 
      snd_pcm_hw_params_get_rate(params, &val, &dir);
      std::cout << "rate = " << val << " bps" << std::endl;
 
       snd_pcm_hw_params_get_period_time(params, &val, &dir);
      std::cout << "period time = " << val << " us" << std::endl;
 
      snd_pcm_hw_params_get_period_size(params, &frames, &dir);
      std::cout << "period size = " << static_cast<int>(frames) << " frames" << std::endl;
 
       snd_pcm_hw_params_get_buffer_time(params, &val, &dir);
      std::cout << "buffer time = " << val << " us" << std::endl;
 
       snd_pcm_hw_params_get_buffer_size(params, (snd_pcm_uframes_t *) &val);
      std::cout << "buffer size = " << val << " frames" << std::endl;
 
      snd_pcm_hw_params_get_periods(params, &val, &dir);
      std::cout << "periods per buffer = " << val << " frames" << std::endl;
 
       snd_pcm_hw_params_get_rate_numden(params, &val, &val2);
      std::cout << "exact rate = " << val/val2 << " bps" << std::endl;
 
      val = snd_pcm_hw_params_get_sbits(params);
      std::cout << "significant bits = " << val << std::endl;
 
      snd_pcm_hw_params_get_tick_time(params, &val, &dir);
      std::cout << "tick time = " << val << " us" << std::endl;
 
      val = snd_pcm_hw_params_is_batch(params);
      std::cout << "is batch = " << val << std::endl;
 
      val = snd_pcm_hw_params_is_block_transfer(params);
      std::cout << "is block transfer = " << val << std::endl;
 
      val = snd_pcm_hw_params_is_double(params);
      std::cout << "is double = " << val << std::endl;
 
       val = snd_pcm_hw_params_is_half_duplex(params);
      std::cout << "is half duplex = " << val << std::endl;
 
       val = snd_pcm_hw_params_is_joint_duplex(params);
      std::cout << "is joint duplex = " << val << std::endl;
 
       val = snd_pcm_hw_params_can_overrange(params);
      std::cout << "can overrange = " << val << std::endl;
 
      val = snd_pcm_hw_params_can_mmap_sample_resolution(params);
      std::cout << "can mmap = " << val << std::endl;
 
      val = snd_pcm_hw_params_can_pause(params);
      std::cout << "can pause = " << val << std::endl;
 
       val = snd_pcm_hw_params_can_resume(params);
      std::cout << "can resume = " << val << std::endl;
 
      val = snd_pcm_hw_params_can_sync_start(params);
      std::cout << "can sync start = " << val << std::endl;
 
      snd_pcm_close(handle);
 
      return 0;
}

3.）一个简单的声音播放程序

 #include <iostream>
#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;
 
       if ( (rc = snd_pcm_open(&handle, "default", SND_PCM_STREAM_PLAYBACK, 0)) < 0)
       {
              std::cerr << "unable to open pcm device: " << snd_strerror(rc) << std::endl;
              exit(1);
       }
 
       snd_pcm_hw_params_alloca(&params);
 
       snd_pcm_hw_params_any(handle, params);
 
       snd_pcm_hw_params_set_access(handle, params, SND_PCM_ACCESS_RW_INTERLEAVED);
 
       snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_S16_LE);
 
       snd_pcm_hw_params_set_channels(handle, params, 2);
 
       val = 44100;
 
       snd_pcm_hw_params_set_rate_near(handle, params, &val, &dir);
 
       frames = 32;
       snd_pcm_hw_params_set_period_size_near(handle, params, &frames, &dir);
 
       if ( (rc = snd_pcm_hw_params(handle, params)) < 0)
       {
              std::cerr << "unable to set hw paramseters: " << snd_strerror(rc) << std::endl;
              exit(1);
       }
 
       snd_pcm_hw_params_get_period_size(params, &frames, &dir);
       size = frames * 4;
       buffer = new char[size];
 
       snd_pcm_hw_params_get_period_time(params, &val, &dir);
 
       loops = 5000000 / val;
 
       while (loops > 0) {
              loops--;
              if ( (rc = read(0, buffer, size)) == 0)
              {
                     std::cerr << "end of file on input" << std::endl;
                     break;
              }
              else if (rc != size)
                     std::cerr << "short read: read " << rc << " bytes" << std::endl;
 
              if ( (rc = snd_pcm_writei(handle, buffer, frames)) == -EPIPE)
              {
                     std::cerr << "underrun occurred" << std::endl;
                     snd_pcm_prepare(handle);
              }
              else if (rc < 0)
                     std::cerr << "error from writei: " << snd_strerror(rc) << std::endl;
              else if (rc != (int)frames)
                     std::cerr << "short write, write " << rc << " frames" << std::endl;
       }
 
       snd_pcm_drain(handle);
       snd_pcm_close(handle);
       free(buffer);
 
       return 0;
}

4.）一个简单的记录声音的程序

 #include <iostream>
#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;
 
       if ( (rc = snd_pcm_open(&handle, "default", SND_PCM_STREAM_CAPTURE, 0)) < 0)
       {
              std::cerr << "unable to open pcm device: " << snd_strerror(rc) << std::endl;
              exit(1);
       }
 
       snd_pcm_hw_params_alloca(&params);
 
       snd_pcm_hw_params_any(handle, params);
 
       snd_pcm_hw_params_set_access(handle, params, SND_PCM_ACCESS_RW_INTERLEAVED);
 
       snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_S16_LE);
 
       snd_pcm_hw_params_set_channels(handle, params, 2);
 
       val = 44100;
       snd_pcm_hw_params_set_period_size_near(handle, params, &frames, &dir);
 
       if ( (rc = snd_pcm_hw_params(handle, params)) < 0)
       {
              std::cerr << "unable to set hw parameters: " << snd_strerror(rc) << std::endl;
              exit(1);
       }
 
       snd_pcm_hw_params_get_period_size(params, &frames, &dir);
 
       size = frames * 4;
       buffer = new char[size];
 
       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)
              {
                     std::cerr << "overrun occurred" << std::endl;
                     snd_pcm_prepare(handle);
              }
              else if (rc < 0)
                     std::cerr << "error from read: " << snd_strerror(rc) << std::endl;
              else if ( rc != (int)frames)
                     std::cerr << "short read, read " << rc << " frames" << std::endl;
              rc = write(1, buffer, size);
              if (rc != size)
                     std::cerr << "short write: wrote " << rc << " bytes" << std::endl;
       }
 
       snd_pcm_drain(handle);
       snd_pcm_close(handle);
       free(buffer);
 
       return 0;
}

 注意：编译的时候记得加上参数，g++ xxx.cpp -o xxx -lasound；如果编译时出现如下错误：alsa/asoundlib.h: No such file or directory

缺少一个库：

apt-get install libasound2-dev

OK!