Linux ALSA声卡介绍及使用

一. 介绍

      ALSA 标准是一个先进的linux声音体系。它包含内核驱动集合，API库和工具对Linux声音进行支持。ALSA 包含一系列内核驱动对不同的声卡进行支持，还提供了libasound的API库。用这些进行写程序不需要打开设备等操作，所以编程人员在写程序的时候不 会被底层的东西困扰。与此相反OSS/Free 驱动在内核层次调用，需要指定设备名和调用ioctl。为提供向后兼容， ALSA 提供内核模块模仿 OSS/Free 驱动，所以大多数的程序不需要改动。 ALSA 拥有调用插件的能力对新设备提供扩展，包括那些用软件模拟出来的虚拟设备。 ALSA 还提供一组命令行工具包括  mixer, sound file player 和工具控制一些特别的声卡的特别的作用。
 
二.ALSA 体系：

ALSA API 被主要分为以下几种接口：
l         控制接口：提供灵活的方式管理注册的声卡和对存在的声卡进行查询。
l         PCM接口：提供管理数字音频的捕捉和回放。
l         原始 MIDI 接口: 支持 MIDI (Musical Instrument Digital Interface), 一种标准电子音乐指令集。 这些 API 提供访问声卡上的 MIDI 总线。这些原始借口直接工作在 The  MIDI 事件上，程序员只需要管理协议和时间。
l         记时接口: 为支持声音的同步事件提供访问声卡上的定时器。
l         音序器接口：一个比原始MIDI接口高级的MIDI编程和声音同步高层接口。它可以处理很多的MIDI协议和定时器。
l         混音器接口：控制发送信号和控制声音大小的声卡上的设备。
 
三.声卡的缓存和数据的传输：

      一块声卡有一个声卡内存用来存储记录的样本。当它被写满时就产生中断。内核驱动就使用DMA将数据传输到内存中。同样地，当在播放时就将内存中的声音样本 使用DMA传到声卡的内存中！
      声卡的缓存是环状的，这里只讨论应用程序中的内存结构：ALSA将数据分成连续的片段然后传到按单元片段传输。
 
四：典型的声音程序结 构：

        open interface for capture or playback
        set hardware parameters
        (access mode, data format, channels, rate, etc.)
        while there is data to be processed:
        read PCM data (capture)
        or write PCM data (playback)
        close interface
 
五.一些例子：

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;
}

 

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本文标题：Linux ALSA声卡介绍及使用
本文作者：poseidonqiu 本文来源:blog.chinaunix.net
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