一. 介紹
ALSA 標準是一個先進的linux聲音體系。它包含內核驅動集合,API庫和工具對Linux聲音進行支持。ALSA 包含一系列內核驅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 驅動在內核層次調用,需要指定設備名和調用ioctl。爲提供向後兼容, ALSA 提供內核模塊模仿 OSS/Free 驅動,所以大多數的程序不需要改動。 ALSA 擁有調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。
二.ALSA 體系:
ALSA API 被主要分爲以下幾種接口:
l 控制接口:提供靈活的方式管理註冊的聲卡和對存在的聲卡進行查詢。
l PCM接口:提供管理數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。這些原始藉口直接工作在 The MIDI 事件上,程序員只需要管理協議和時間。
l 記時接口: 爲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協議和定時器。
l 混音器接口:控制發送信號和控制聲音大小的聲卡上的設備。
三.聲卡的緩存和數據的傳輸:
一塊聲卡有一個聲卡內存用來存儲記錄的樣本。當它被寫滿時就產生中斷。內核驅動就使用DMA將數據傳輸到內存中。同樣地,當在播放時就將內存中的聲音樣本使用DMA傳到聲卡的內存中!
聲卡的緩存是環狀的,這裏只討論應用程序中的內存結構:ALSA將數據分成連續的片段然後傳到按單元片段傳輸。
四:典型的聲音程序結構:
- 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.顯示一些PCM的類型和格式:
- #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.打開PCM設備和設置參數
- #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;
- }
3.一個簡單的聲音播放程序
- #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;
- }
4.一個簡單的記錄聲音的程序
- #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;
- }
編譯的參數:g++ xxx.cpp -o xxx -lasound