????? ALSA 標準是一個先進的linux聲音體系。它包含內(nèi)核驅(qū)動集合，API庫和工具對Linux聲音進行支持。ALSA 包含一系列內(nèi)核驅(qū)動對不同的聲卡進行支持，還提供了libasound的API庫。用這些進行寫程序不需要打開設備等操作，所以編程人員在寫程序的時候不會被底層的東西困擾。與此相反OSS/Free 驅(qū)動在內(nèi)核層次調(diào)用，需要指定設備名和調(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，所以大多數(shù)的程序不需要改動。 ALSA 擁有調(diào)用插件的能力對新設備提供擴展，包括那些用軟件模擬出來的虛擬設備。 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。
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二.ALSA 體系：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的聲卡和對存在的聲卡進行查詢。
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), 一種標準電子音樂指令集。 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)議和時間。
l???????? 記時接口: 為支持聲音的同步事件提供訪問聲卡上的定時器。
l???????? 音序器接口：一個比原始MIDI接口高級的MIDI編程和聲音同步高層接口。它可以處理很多的MIDI協(xié)議和定時器。
l???????? 混音器接口：控制發(fā)送信號和控制聲音大小的聲卡上的設備。
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三.聲卡的緩存和數(shù)據(jù)的傳輸：????? 一塊聲卡有一個聲卡內(nèi)存用來存儲記錄的樣本。當它被寫滿時就產(chǎn)生中斷。內(nèi)核驅(qū)動就使用DMA將數(shù)據(jù)傳輸?shù)絻?nèi)存中。同樣地，當在播放時就將內(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。
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四：典型的聲音程序結(jié)構(gòu)：??????? 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
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五.一些例子： 1.顯示一些PCM的類型和格式:?
#include
#include
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int main()
{
?????? std::cout << "ALSA library version: " << SND_LIB_VERSION_STR << std::endl;
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?????? std::cout << "PCM stream types: " << std::endl;
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?????? 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;
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?????? 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;
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?????? 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;
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?????? 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;
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?????? 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;
??????
}
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2.打開PCM設備和設置參數(shù)?
#include
#include
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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;
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?????? 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);
?????? }
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?????? snd_pcm_hw_params_alloca(¶ms);
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?????? snd_pcm_hw_params_any(handle, params);
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?????? snd_pcm_hw_params_set_access(handle, params, SND_PCM_ACCESS_RW_INTERLEAVED);
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?????? snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_S16_LE);
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?????? snd_pcm_hw_params_set_channels(handle, params, 2);
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?????? val = 44100;
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?????? snd_pcm_hw_params_set_rate_near(handle, params, &val, &dir);
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?????? if ( (rc = snd_pcm_hw_params(handle, params)) < 0)
?????? {
????????????? std::cerr << "unable to set hw parameters: " << snd_strerror(rc) << std::endl;
????????????? exit(1);
?????? }
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?????? std::cout << "PCM handle name = " << snd_pcm_name(handle) << std::endl;
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?????? std::cout << "PCM state = " << snd_pcm_state_name(snd_pcm_state(handle)) << std::endl;
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?????? snd_pcm_hw_params_get_access(params, (snd_pcm_access_t *)&val);
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?????? std::cout << "access type = " << snd_pcm_access_name((snd_pcm_access_t)val) << std::endl;
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?????? 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;
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? ??? 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;
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? ??? snd_pcm_hw_params_get_channels(params, &val);
? ??? std::cout << "channels = " << val << std::endl;
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????? snd_pcm_hw_params_get_rate(params, &val, &dir);
? ??? std::cout << "rate = " << val << " bps" << std::endl;
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?????? snd_pcm_hw_params_get_period_time(params, &val, &dir);
? ??? std::cout << "period time = " << val << " us" << std::endl;
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? ??? snd_pcm_hw_params_get_period_size(params, &frames, &dir);
? ??? std::cout << "period size = " << static_cast
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?????? 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;
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? ??? snd_pcm_hw_params_get_periods(params, &val, &dir);
? ??? std::cout << "periods per buffer = " << val << " frames" << std::endl;
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?????? 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;
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? ??? snd_pcm_hw_params_get_tick_time(params, &val, &dir);
? ??? std::cout << "tick time = " << val << " us" << std::endl;
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? ??? val = snd_pcm_hw_params_is_batch(params);
? ??? std::cout << "is batch = " << val << std::endl;
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? ??? val = snd_pcm_hw_params_is_block_transfer(params);
? ??? std::cout << "is block transfer = " << val << std::endl;
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? ??? val = snd_pcm_hw_params_is_double(params);
? ??? std::cout << "is double = " << val << std::endl;
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?????? val = snd_pcm_hw_params_is_half_duplex(params);
? ??? std::cout << "is half duplex = " << val << std::endl;
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?????? val = snd_pcm_hw_params_is_joint_duplex(params);
? ??? std::cout << "is joint duplex = " << val << std::endl;
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?????? val = snd_pcm_hw_params_can_overrange(params);
? ??? std::cout << "can overrange = " << val << std::endl;
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? ??? val = snd_pcm_hw_params_can_mmap_sample_resolution(params);
? ??? std::cout << "can mmap = " << val << std::endl;
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? ??? val = snd_pcm_hw_params_can_pause(params);
? ??? std::cout << "can pause = " << val << std::endl;
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?????? val = snd_pcm_hw_params_can_resume(params);
? ??? std::cout << "can resume = " << val << std::endl;
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? ??? val = snd_pcm_hw_params_can_sync_start(params);
? ??? std::cout << "can sync start = " << val << std::endl;
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????? snd_pcm_close(handle);
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? ??? return 0;
}
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3.一個簡單的聲音播放程序?
#include
#include
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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;
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?????? 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);
?????? }
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?????? snd_pcm_hw_params_alloca(¶ms);
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?????? snd_pcm_hw_params_any(handle, params);
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?????? snd_pcm_hw_params_set_access(handle, params, SND_PCM_ACCESS_RW_INTERLEAVED);
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?????? snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_S16_LE);
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?????? snd_pcm_hw_params_set_channels(handle, params, 2);
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?????? val = 44100;
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?????? snd_pcm_hw_params_set_rate_near(handle, params, &val, &dir);
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?????? frames = 32;
?????? snd_pcm_hw_params_set_period_size_near(handle, params, &frames, &dir);
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?????? if ( (rc = snd_pcm_hw_params(handle, params)) < 0)
?????? {
????????????? std::cerr << "unable to set hw paramseters: " << snd_strerror(rc) << std::endl;
????????????? exit(1);
?????? }
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?????? snd_pcm_hw_params_get_period_size(params, &frames, &dir);
?????? size = frames * 4;
?????? buffer = new char[size];
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?????? snd_pcm_hw_params_get_period_time(params, &val, &dir);
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?????? loops = 5000000 / val;
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?????? 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;
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????????????? 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;
?????? }
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?????? snd_pcm_drain(handle);
?????? snd_pcm_close(handle);
?????? free(buffer);
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?????? return 0;
}
4.一個簡單的記錄聲音的程序?
#include
#include
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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;
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?????? 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);
?????? }
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?????? snd_pcm_hw_params_alloca(¶ms);
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?????? snd_pcm_hw_params_any(handle, params);
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?????? snd_pcm_hw_params_set_access(handle, params, SND_PCM_ACCESS_RW_INTERLEAVED);
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?????? snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_S16_LE);
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?????? snd_pcm_hw_params_set_channels(handle, params, 2);
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?????? val = 44100;
?????? snd_pcm_hw_params_set_period_size_near(handle, params, &frames, &dir);
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?????? if ( (rc = snd_pcm_hw_params(handle, params)) < 0)
?????? {
????????????? std::cerr << "unable to set hw parameters: " << snd_strerror(rc) << std::endl;
????????????? exit(1);
?????? }
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?????? snd_pcm_hw_params_get_period_size(params, &frames, &dir);
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?????? size = frames * 4;
?????? buffer = new char[size];
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?????? snd_pcm_hw_params_get_period_time(params, &val, &dir);
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?????? loops = 5000000 / val;
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?????? 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;
?????? }
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?????? snd_pcm_drain(handle);
?????? snd_pcm_close(handle);
?????? free(buffer);
??????
?????? return 0;
}
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編譯的參數(shù)：g++ xxx.cpp -o xxx -lasound
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