Linux ALSA (Advanced Linux Sound Architecture)

相关资料网站：

http://www.alsa-project.org/main/index.php/Main_Page

http://www.alsa-project.org/alsa-doc/alsa-lib/index.html

http://www.alsa-project.org/~tiwai/writing-an-alsa-driver/

http://www.alsa-project.org/alsa-doc/alsa-lib/group___p_c_m___h_w___params.html

编译时需要使用:
-lasound

录音功能:

/*********************************************************************
ALSA 简单的录音功能 capture 捕获
*********************************************************************/

/* Use the newer ALSA API */
#define ALSA_PCM_NEW_HW_PARAMS_API

#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;

    /* Open PCM device for recording (capture). */
    /* 打开 PCM capture 捕获设备 */
    rc = snd_pcm_open(&handle, "default",
                        SND_PCM_STREAM_CAPTURE, 0);
    if (rc < 0) {
        fprintf(stderr,
                "unable to open pcm device: %s\n",
                snd_strerror(rc));
        exit(1);
    }

    /* Allocate a hardware parameters object. */
    /* 分配一个硬件参数结构体 */
    snd_pcm_hw_params_alloca(&params);

    /* Fill it in with default values. */
    /* 使用默认参数 */
    snd_pcm_hw_params_any(handle, params);

    /* Set the desired hardware parameters. */

    /* Interleaved mode */
    snd_pcm_hw_params_set_access(handle, params,
                          SND_PCM_ACCESS_RW_INTERLEAVED);

    /* Signed 16-bit little-endian format */
    /* 16位 小端 */
    snd_pcm_hw_params_set_format(handle, params,
                                  SND_PCM_FORMAT_S16_LE);

    /* Two channels (stereo) */
    /* 双通道 */
    snd_pcm_hw_params_set_channels(handle, params, 2);

    /* 44100 bits/second sampling rate (CD quality) */
    /* 采样率 */
    val = 44100;
    snd_pcm_hw_params_set_rate_near(handle, params,
                                      &val, &dir);

    /* Set period size to 32 frames. */
    /* 一个周期有 32 帧 */
    frames = 32;
    snd_pcm_hw_params_set_period_size_near(handle,
                                  params, &frames, &dir);

    /* Write the parameters to the driver */
    /* 参数生效 */
    rc = snd_pcm_hw_params(handle, params);
    if (rc < 0) {
        fprintf(stderr,
                "unable to set hw parameters: %s\n",
                snd_strerror(rc));
        exit(1);
    }

    /* Use a buffer large enough to hold one period */
    /* 得到一个周期的数据大小 */
    snd_pcm_hw_params_get_period_size(params,
                                          &frames, &dir);
    /* 16位 双通道，所以要 *4 */
    size = frames * 4; /* 2 bytes/sample, 2 channels */
    buffer = (char *) malloc(size);

    /* We want to loop for 5 seconds */
    /* 等到一个周期的时间长度 */
    snd_pcm_hw_params_get_period_time(params,
                                             &val, &dir);
    loops = 5000000 / val;

    while (loops > 0) {
        loops--;
        /* 捕获数据 */
        rc = snd_pcm_readi(handle, buffer, frames);
        if (rc == -EPIPE) {
            /* EPIPE means overrun */
            fprintf(stderr, "overrun occurred\n");
            snd_pcm_prepare(handle);
        } else if (rc < 0) {
            fprintf(stderr,
                  "error from read: %s\n",
                  snd_strerror(rc));
        } else if (rc != (int)frames) {
            fprintf(stderr, "short read, read %d frames\n", rc);
        }

        /* 写入到标准输出中去 */
        rc = write(1, buffer, size);
        if (rc != size)
          fprintf(stderr,
                  "short write: wrote %d bytes\n", rc);
    }

    snd_pcm_drain(handle);
    snd_pcm_close(handle);
    free(buffer);

    return 0;
}

放音功能：

/******************************************************************
ALSA 简单的 playback 例子
******************************************************************/

#define ALSA_PCM_NEW_HW_PARAMS_API

#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;

    /* Open PCM device for playback. */
    /* 打开 PCM playback 设备 */
    rc = snd_pcm_open(&handle, "default",
    SND_PCM_STREAM_PLAYBACK, 0);
    if (rc < 0) {
        fprintf(stderr,
                "unable to open pcm device: %s\n",
                snd_strerror(rc));
        exit(1);
    }

    /* Allocate a hardware parameters object. */
    /* 分配一个硬件参数结构体 */
    snd_pcm_hw_params_alloca(&params);

    /* Fill it in with default values. */
    /* 使用默认参数 */
    snd_pcm_hw_params_any(handle, params);

    /* Set the desired hardware parameters. */
    /* 设置硬件参数 */

    /* Interleaved mode */
    snd_pcm_hw_params_set_access(handle, params,
                                SND_PCM_ACCESS_RW_INTERLEAVED);

    /* Signed 16-bit little-endian format */
    /* 数据格式为 16位 小端 */
    snd_pcm_hw_params_set_format(handle, params,
                                SND_PCM_FORMAT_S16_LE);

    /* Two channels (stereo) */
    /* 两个声道 */
    snd_pcm_hw_params_set_channels(handle, params, 2);

    /* 44100 bits/second sampling rate (CD quality) */
    /* 采样率为 44100 */
    val = 44100;
    snd_pcm_hw_params_set_rate_near(handle, params,
              &val, &dir);

    /* Set period size to 32 frames. */
    /* 设置一个周期为 32 帧 */
    frames = 32;
    snd_pcm_hw_params_set_period_size_near(handle,
          params, &frames, &dir);

    /* Write the parameters to the driver */
    /* 把前面设置好的参数写入到playback设备 */
    rc = snd_pcm_hw_params(handle, params);
    if (rc < 0) {
        fprintf(stderr,
            "unable to set hw parameters: %s\n",
            snd_strerror(rc));
    exit(1);
    }

    /* Use a buffer large enough to hold one period */
    /* 得到一个周期的数据长度 */
    snd_pcm_hw_params_get_period_size(params, &frames,
                &dir);

    /* 因为我们是16位 两个通道，所以要 *2*2 也就是 *4 */
    size = frames * 4; /* 2 bytes/sample, 2 channels */
    buffer = (char *) malloc(size);

    /* We want to loop for 8 seconds */
    /* 得到一个周期的时间长度 */
    snd_pcm_hw_params_get_period_time(params,
                &val, &dir);
    /* 5 seconds in microseconds divided by
    * period time */
    loops = 8000000 / val;

    while (loops > 0) {
        loops--;
        /* 从标准输入中获取数据 */
        rc = read(0, buffer, size);
        if (rc == 0) {
            fprintf(stderr, "end of file on input\n");
            break;
        } else if (rc != size) {
            fprintf(stderr,
                "short read: read %d bytes\n", rc);
        }

        /* 播放这些数据 */
        rc = snd_pcm_writei(handle, buffer, frames);
        if (rc == -EPIPE) {
            /* EPIPE means underrun */
            fprintf(stderr, "underrun occurred\n");
            snd_pcm_prepare(handle);
        } else if (rc < 0) {
            fprintf(stderr,
                    "error from writei: %s\n",
                    snd_strerror(rc));
        }  else if (rc != (int)frames) {
            fprintf(stderr,
                    "short write, write %d frames\n", rc);
        }
    }

    snd_pcm_drain(handle);
    snd_pcm_close(handle);
    free(buffer);

    return 0;
}

编译运行

$gcc playback.c -o playback -lasound
$gcc capture.c -o capture -lasound
$ ./capture > a.raw
$ ./playback < a.raw

ARM LINUX ALSA

lsa-lib库：进入网站http://www.alsa-project.org/选择下载
alsa-lib-1.1.6.tar.bz2

tar xvf alsa-lib-1.1.6.tar.bz2

cd alsa-lib-1.1.6

./configure --host=arm-linux CC=arm-linux-gnueabihf-gcc --enable-shared=yes --enable-static=yes

$make

/alsa-lib-1.1.6/src/.libs/libasound.la


    arm-linux-gnueabihf-gcc playback.c -o arm_playback -I. -L. -lasound  -lpthread -lm -lrt -ldl

    arm-linux-gnueabihf-gcc capture.c -o arm_capture -I./ -L. -lasound  -lpthread  -lm -lrt -ldl

ALSA的体系结构:

官方主页 http://www.alsa-project.org/

主要跟编程相关是
alsa-lib. ALSA 应用库(最常用)

alsa-driver 一些常见芯片的ALSA驱动代码,一般内核会集成.

alsa-firmware 一些DSP或ASIC的专用的微码(运在芯片之上,启动时由LINUX装入到硬件中).

alsa-utils 一般ALSA小的测试工具.如aplay/arecord 播放和录音小程序.

alsa-oss 用alsa接口模拟旧的oss接口.

其中alsa-driver,alsa-firwware是内核开发者所接触的东西,对于已经正常运行硬件,通常意味着这一部分已经整合到内核当中,无需修改.
而alsa-utils主要是测试一些小工具.
因此对于一个应用程序开发者,或者嵌入式应用开发者,接触到主要是alsa-lib编译出来的库libasound.

如果安装ALSA,在/proc会看到相应设备:
用cat /proc/asound/devices

正常情况下,在你的/dev/snd会看到如下设备结点(有例外,就是内核驱动调整了结点位置)
用ls -l /dev/snd

以下是一些用alsa-utils测试样例
播放wave文件
aplay /mnt/nfs/test.wav

变频播放,(以是以 44 KHz来播放音频)
aplay -D rate_44k /mnt/nfs/test.wav

录音,以20秒的间隔(-d 20),立体声(-c 2),频率是 8000Hz来录制Wave格式音频
arecord -d 20 -c 2 -t wav -r 8000 -f “Signed 16 bit Little Endian” /mnt/nfs/test.wav

测试混音播放(先是播放test1.wav,然后再同时播放test2.wav)
aplay -D plug:dmix_44k /mnt/nfs/test1.wav &
aplay -D plug:dmix_44k /mnt/nfs/test2.wav

设置放音增益（0 to 3）
amixer set Master 1

设置录音音量(0-31)
amixer set Line 10