ubuntu alsa

今天要在linux下搞音頻編程，在網上查閱了一下資料，網上很多資料都是在linux下直接對/dev/dsp進行編程的，因為在以往的linux系統中，我們是可以通過cat  xxx.wav /dev/dsp 來很容易的播放一個音頻文件，在應用程序中，也可以直接操作/dev/dsp，實現聲音的播放：打開->寫入(實際上就能播放)->關閉。

然而在換成了ubuntu-12.04 LST后，我發現/dev中根本找不到dsp，之前直接操作/dev/dsp的程序都無法正常運行，而是 can't find /dev/dsp.

經過我再三查找，發現現在基本上都不用dsp了，大多數人都在用ALSA音頻編程（其實我也是個菜鳥）。首先介紹一下一些關於ALSA編程的知識：

 

1、GNU/Linux 系統下三大主流聲卡驅動程序集：

Linux 有三個主流的聲卡驅動程序集：OSS/Lite（也稱為OSS/Free）、OSS/Full

（商業軟件）、ALSA（自由軟件）。

 

OSS/Lite 是現在linux內核中自帶的聲卡驅動程序集，最初由 Hannu Savolainen

開發。后來 Hannu 跑去開發 Open Sound System（也就是上面所說的OSS/Full）。

由於 Hannu 的“逃跑”，RH 資助 Alan Cox 增強 Hannu 開發的驅動程序，並使它們

完全模塊化。現在 Alan Cox 是內核聲卡驅程集的維護人。OSS/Lite 從kernel-2.0開

始並入內核，現在大家使用的聲卡驅程默認都是OSS/Lite中的。

 

OSS/Full 是由 4Front Technologies 開發並銷售的商業軟件。它可以驅動很多

聲卡並且可以用在很多 UNIX 系統中。OSS/Full 完全兼容以前基於 OSS／Lite 開發

的應用程序。作為一個商業軟件，你雖然可以使用它，但是你得不到它的源代碼，並且

你必須為此而付錢。

 

ALSA 是linux內核的下一代標准聲卡驅動集。開始時 Jaroslav Kysela 等人為

Gravis UltraSound Card 開發驅動程序，后來該計划改名為 ALSA「先進的linux音頻

體系」，因為他們認為 ALSA 比原來內核中的 OSS/Lite 驅動程序集更優秀，完全可以

代替 OSS/Lite。他們是對的，ALSA 支持的聲卡比 OSS/Lite 多，完全兼容以前基於

OSS 開發的程序，SMP（多處理器） 與 線程安全設計，並且從 2.5 分支的內核開始，

ALSA 的驅動程序集開始並入內核，大家可以在今年的 2.6 版本的內核中看到使用它們。

 

2、為什么要使用 ALSA 開發音頻程序

首先，ALSA 是 linux 以后聲卡驅動程序的標准，OSS/Lite 遲早會從內核中去除。

開發基於 ALSA 的音頻程序可以保證以后的兼容。

其次，我們簡單比較一下開發基於 OSS 與 ALSA 的方法。

OSS 向應用程序提供了一系列的系統接口。開發基於 OSS 的應用程序需要使用

open,close,ioctl,read,write 等低級系統調用來完成音頻設備的控制、音頻流的輸入

輸出。

而 ALSA 則為應用程序開發人員提供了一個優秀的音頻庫。利用該音頻庫，開發

人員可以方便快捷地開發出自己的應用程序，細節則留給音頻庫內部處理。當然 ALSA

也提供了類似於 OSS 的系統接口，不過 ALSA 的開發者建議應用程序開發者使用音頻

庫而不是驅程API。

 

3、那么我該從何開始呢

第一步當然是安裝 ALSA 驅動程序與音頻庫。

當前 ALSA 有兩個分支，一個是以前的0.5版本，一個是現在的0.9。ALSA的開發者

已經不支持0.5版本了，所以我們要使用0.9。大家可以在 ALSA 的主頁

www.alsa-project.org 上下載安裝。這個頁面上的信息對大家安裝很有用：

www.alsa-project.org/alsa-doc ，建議先瀏覽一下。

 

第二步是參考文檔與例子。

在 ALSA 的文檔頁面上有兩篇為應用程序開發者提供的文章：

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 ]

當然，還有音頻庫API的在線參考：

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

 

  要是你已經完成以上幾步的話，那么你就應該開始開發了。

4. ALSA設備文件結構

我們從alsa在Linux中的設備文件結構開始我們的alsa之旅. 看看我的電腦中的alsa驅動的設備文件結構:

$ 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
$

    我們可以看到以下設備文件:

    controlC0 -->                 用於聲卡的控制，例如通道選擇，混音，麥克風的控制等
    midiC0D0  -->                用於播放midi音頻
    pcmC0D0c --〉               用於錄音的pcm設備
    pcmC0D0p --〉               用於播放的pcm設備
    seq  --〉                        音序器
    timer --〉                       定時器

其中，C0D0代表的是聲卡0中的設備0，pcmC0D0c最后一個c代表capture，pcmC0D0p最后一個p代表playback，這些都是alsa-driver中的命名規則。從上面的列表可以看出，我的聲卡下掛了6個設備，根據聲卡的實際能力，驅動實際上可以掛上更多種類的設備，在include/sound/core.h中，定義了以下設備類型：

#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，我們更關心的是pcm和control這兩種設備。

5.一些例子，這些例子在官方文檔也有，請自行查閱

1.）顯示一些PCM的類型和格式:

 #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.）打開PCM設備和設置參數

 #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.）一個簡單的聲音播放程序

 #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.）一個簡單的記錄聲音的程序

 #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：編譯的時候記得加上參數，g++ xxx.cpp -o xxx -lasound；如果編譯時出現如下錯誤：alsa/asoundlib.h: No such file or directory

缺少一個庫：

apt-get install libasound2-dev

OK!