Linux下ALSA声卡编程.docx

Linux下ALSA声卡编程 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 #include 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 #include 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(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 #include 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) 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 #include 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, SN