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(¶ms);
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(¶ms);
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(¶ms);
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;
}
编译的参数：g++ xxx.cpp -o xxx -lasound
本文来自CSDN博客，转载请标明出处：http://blog.csdn.net/ruancsdn/archive/2004/10/14/135911.aspx
