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Linux下ALSA声卡编程

2014-08-02 16:22 549 查看


Linux下ALSA声卡编程

时间:2010-07-19 23:00来源:CSDN博客
作者:shenbin1430 点击: 1017次
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。



TAG: 声卡 ALSA 声卡驱动 libasound

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：

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。
PCM 接口：提供管理数字音频的捕捉和回放。
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。
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。
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。
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);
}
snd_pcm_hw_params_get_access(params, (snd_pcm_access_t *)&val);
snd_pcm_hw_params_get_format(params, (snd_pcm_format_t*)(&val));
snd_pcm_hw_params_get_subformat(params, (snd_pcm_subformat_t *)&val);
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);
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

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