ALSA 標(biāo)準(zhǔn)是一個(gè)先進(jìn)的linux聲音體系。它包含內(nèi)核驅(qū)動(dòng)集合，API庫和工具對(duì)Linux聲音進(jìn)行支持。ALSA 包含一系列內(nèi)核驅(qū)動(dòng)對(duì)不同的聲卡進(jìn)行支持，還提供了libasound的API庫。用這些進(jìn)行寫程序不需要打開設(shè)備等操作，所以編程人員在寫程序的時(shí)候不會(huì)被底層的東西困擾。與此相反OSS/Free 驅(qū)動(dòng)在內(nèi)核層次調(diào)用，需要指定設(shè)備名和調(diào)用ioctl。為提供向后兼容， ALSA 提供內(nèi)核模塊模仿 OSS/Free 驅(qū)動(dòng)，所以大多數(shù)的程序不需要改動(dòng)。 ALSA 擁有調(diào)用插件的能力對(duì)新設(shè)備提供擴(kuò)展，包括那些用軟件模擬出來的虛擬設(shè)備。 ALSA 還提供一組命令行工具包括 mixer, sound file player 和工具控制一些特別的聲卡的特別的作用。
ALSA API 被主要分為以下幾種接口：
l 控制接口：提供靈活的方式管理注冊(cè)的聲卡和對(duì)存在的聲卡進(jìn)行查詢。
l PCM接口：提供管理數(shù)字音頻的捕捉和回放。
l 原始 MIDI 接口: 支持 MIDI (Musical Instrument Digital Interface), 一種標(biāo)準(zhǔn)電子音樂指令集。 這些 API 提供訪問聲卡上的 MIDI 總線。這些原始借口直接工作在 The MIDI 事件上，程序員只需要管理協(xié)議和時(shí)間。
l 記時(shí)接口: 為支持聲音的同步事件提供訪問聲卡上的定時(shí)器。
l 音序器接口：一個(gè)比原始MIDI接口高級(jí)的MIDI編程和聲音同步高層接口。它可以處理很多的MIDI協(xié)議和定時(shí)器。
l 混音器接口：控制發(fā)送信號(hào)和控制聲音大小的聲卡上的設(shè)備。
一塊聲卡有一個(gè)聲卡內(nèi)存用來存儲(chǔ)記錄的樣本。當(dāng)它被寫滿時(shí)就產(chǎn)生中斷。內(nèi)核驅(qū)動(dòng)就使用DMA將數(shù)據(jù)傳輸?shù)絻?nèi)存中。同樣地，當(dāng)在播放時(shí)就將內(nèi)存中的聲音樣本使用DMA傳到聲卡的內(nèi)存中！
聲卡的緩存是環(huán)狀的，這里只討論應(yīng)用程序中的內(nèi)存結(jié)構(gòu)：ALSA將數(shù)據(jù)分成連續(xù)的片段然后傳到按單元片段傳輸。
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
#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;
}
#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;
}
#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;
}
#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;
}
編譯的參數(shù)：g++ xxx.cpp -o xxx -lasound
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