一. 概述

ALSA是Advanced Linux Sound Architecture 的縮寫，目前已經(jīng)成為了linux的主流音頻體系結(jié)構(gòu)。

在內(nèi)核設(shè)備驅(qū)動(dòng)層，ALSA提供了alsa-driver，同時(shí)在應(yīng)用層，ALSA為咱們提供了alsa-lib，應(yīng)用程序只要調(diào)用alsa-lib提供的API，便可以完成對(duì)底層音頻硬件的控制。

二.常用命令

aplay -l 顯示實(shí)際聲卡序號(hào)

查看聲卡：

cat /proc/asound/cards

錄音：

arecord -D hw:0,0 -c 2 -r 44100 -f S16_LE test.wav

播放:

aplay -Dplughw:0,0 test.wav

打開耳機(jī)功能

amixer cset numid=1,iface=MIXER,name='Playback Path' 3

打開MIC（麥克風(fēng)）功能：

amixer cset numid=2,iface=MIXER,name='Capture MIC Path' 1

三.音頻參數(shù)理解

聲道

單聲道：mono

雙聲道：stereo。最常見的類型，包含左聲道以及右聲道

采樣率

音頻采樣，是把聲音從模擬信號(hào)轉(zhuǎn)換為數(shù)字信號(hào)。采樣率，就是每秒對(duì)聲音進(jìn)行采集的次數(shù)，同樣也是所得的數(shù)字信號(hào)的每秒樣本數(shù)

采樣越高，聲音的還原就越真實(shí)越自然，人對(duì)頻率的識(shí)別范圍是 20HZ - 22000HZ, 如果每秒鐘能對(duì)聲音做 22000 個(gè)采樣, 回放時(shí)就足可以滿足人耳的需求. 所以 22050 的采樣頻率是常用的, 根據(jù)奈奎斯特采樣定理44100Hz是不失真的情況下的采樣率， 超過48000的采樣對(duì)人耳已經(jīng)沒有意義。文章來源地址http://www.zghlxwxcb.cn/news/detail-597055.html

四.用戶空間打開PCM設(shè)備錄制聲音

#define ALSA_PCM_NEW_HW_PARAMS_API

#include <alsa/asoundlib.h>

int main() {
    long loops;
    int rc;
    int size;
    unsigned int val;
    int dir;
    char *buffer;
    snd_pcm_t *handle;
    snd_pcm_hw_params_t *params;
    snd_pcm_uframes_t frames;

    /*以錄制模式打開*/
    /* Open PCM device for recording (capture). */
    rc = snd_pcm_open( &handle, "default", SND_PCM_STREAM_CAPTURE, 0);
    if (rc < 0) {
        fprintf(stderr, "unable to open pcm device");
        exit(EXIT_FAILURE);
    }

    /*分配一個(gè)參數(shù)對(duì)象*/
    /* Allocate a hardware parameters object. */
    snd_pcm_hw_params_alloca(&params);
    /*初始化參數(shù)對(duì)象*/
    /* Fill it in with default values. */
    rc = snd_pcm_hw_params_any(handle, params);
    if (rc < 0) {
        printf("Err\n");
    }
    /* Set the desired hardware parameters. */

    /*交錯(cuò)模式*/
    /* Interleaved mode */
    rc = snd_pcm_hw_params_set_access(handle, params,
                          SND_PCM_ACCESS_RW_INTERLEAVED);
    if (rc < 0) {
        printf("Err\n");
    }
    /*PCM格式*/
    /* Signed 16-bit little-endian format */
    rc = snd_pcm_hw_params_set_format(handle, params,
                                  SND_PCM_FORMAT_S16_LE);
    if (rc < 0) {
        printf("Err\n");
    }
    /*設(shè)置通道數(shù)*/
    /* Two channels (stereo) */
    rc = snd_pcm_hw_params_set_channels(handle, params, 2);
    if (rc < 0) {
        printf("Err\n");
    }
    /*設(shè)置采樣率*/
    /* 44100 bits/second sampling rate (CD quality) */
    val = 44100;
    rc = snd_pcm_hw_params_set_rate_near(handle, params,
                                &val, &dir);
    if (rc < 0) {
        printf("Err\n");
    }
    /*沒周期的幀數(shù)*/
    /* Set period size to 32 frames. */
    frames = 32;
    rc = snd_pcm_hw_params_set_period_size_near(handle,
                            params, &frames, &dir);
    if (rc < 0) {
        printf("Err\n");
    }
    /* Write the parameters to the driver */
    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 */
    rc = snd_pcm_hw_params_get_period_size(params,
                                          &frames, &dir);
    if (rc < 0) {
        printf("Err\n");
    }
    size = frames * 4; /* 2 bytes/sample, 2 channels */
    buffer = (char *) malloc(size);

    /* We want to loop for 5 seconds */
    rc = 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) {
          /* EPIPE means overrun */
          fprintf(stderr, "overrun occurred/n");
          //把PCM流置于PREPARED狀態(tài)，這樣下次我們向該P(yáng)CM流中數(shù)據(jù)時(shí)，它就能重新開始處理數(shù)據(jù)。
          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);
        }
        rc = write(1, buffer, size);
        if (rc != size)
          fprintf(stderr,
                  "short write: wrote %d bytes/n", rc);
    }

    //調(diào)用snd_pcm_drain把所有掛起沒有傳輸完的聲音樣本傳輸完全
    rc = snd_pcm_drain(handle);
    //關(guān)閉該音頻流，釋放之前動(dòng)態(tài)分配的緩沖區(qū)，退出
    rc = snd_pcm_close(handle);
    free(buffer);

    return 0;
}

五.用戶空間打開PCM設(shè)備播放聲音

#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. */
    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);
    }

    /*分配一個(gè)參數(shù)對(duì)象*/
    /* Allocate a hardware parameters object. */
    snd_pcm_hw_params_alloca(&params);

    /*初始化參數(shù)對(duì)象*/
    /* Fill it in with default values. */
    snd_pcm_hw_params_any(handle, params);

    /* Set the desired hardware parameters. */

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

    /*設(shè)置PCM格式*/
    /* Signed 16-bit little-endian format */
    snd_pcm_hw_params_set_format(handle, params,
                              SND_PCM_FORMAT_S16_LE);

    /*設(shè)置通道數(shù)*/
    /* Two channels (stereo) */
    snd_pcm_hw_params_set_channels(handle, params, 2);

    /*設(shè)置采樣率*/
    /* 44100 bits/second sampling rate (CD quality) */
    val = 44100;
    snd_pcm_hw_params_set_rate_near(handle, params,
                                  &val, &dir);

    /* Set period size to 32 frames. */
    frames = 32;
    snd_pcm_hw_params_set_period_size_near(handle,
                              params, &frames, &dir);

    /* Write the parameters to the driver */
    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 */
    snd_pcm_hw_params_get_period_size(params, &frames, &dir);
    size = frames * 4; /* 2 bytes/sample, 2 channels */
    buffer = (char *) malloc(size);

    /* We want to loop for 5 seconds */
    snd_pcm_hw_params_get_period_time(params,
                                    &val, &dir);
    /* 5 seconds in microseconds divided by
    * period time */
    loops = 5000000 / val;

    while (loops > 0) {
        loops--;
        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);
        }
        rc = snd_pcm_writei(handle, buffer, frames);
        if (rc == -EPIPE) {
            /* EPIPE means underrun */
            fprintf(stderr, "underrun occurred/n");
            //把PCM流置于PREPARED狀態(tài)，這樣下次我們向該P(yáng)CM流中數(shù)據(jù)時(shí)，它就能重新開始處理數(shù)據(jù)。
            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);
        }
    }

    //調(diào)用snd_pcm_drain把所有掛起沒有傳輸完的聲音樣本傳輸完全
    snd_pcm_drain(handle);
    //關(guān)閉該音頻流，釋放之前動(dòng)態(tài)分配的緩沖區(qū)，退出
    snd_pcm_close(handle);
    free(buffer);

  return 0;
}