Alsa-lib实现播放wav音频

1.alsa-lib移植

例程中使用的alsa-lib库是1.2.6版本的，目前最新的release版本已经到1.2.7版本了，可以根据自身需求下载。

具体下载链接可以访问下面alsa官网的下载链接

https://www.alsa-project.org/files/pub/lib/

下载完alsa-lib库后，我们解压后进入文件夹然后执行下列命令,进行交叉编译

mkdir builds
./configure --prefix=$PWD/builds --host=arm cc=arm-linux-gnueabihf-gcc
make
make install

所有的lib库和头文件都会在builds文件夹下,执行file + lib库，查看交叉编译是否完成，下列是32bit 小端模式，表示交叉编译完成

/project/alsa-lib-1.2.6$ file build/lib/libasound.so.2.0.0
build/lib/libasound.so.2.0.0: ELF 32-bit LSB shared object, ARM, EABI5 version 1 (SYSV), dynamically linked, BuildID[sha1]=568ab75e8ab65df19ba5f34b536694ee4449ce6f, not stripped

2.播放wav音频

1.实现流程

alsa-lib具体的播放音频步骤如下，可以参照官网的例程

1.相关变量定义

2.申请hwparams变量内存 snd_pcm_hw_params_alloca(&pcm->hwparams)

3.打开pcm设备snd_pcm_open

4.初始化hwparams的结构体空间

5.设置数据的属性

1.设置数据的读写模式（一般音频双通道分交错访问模式）

2.设置数据的采样格式（每bit的样本长度）

3.设置数据的通道数

4.设置数据的采样速率

5.设置数据的采样周期

每帧的大小 = 采样格式 * 通道数 / 8bit

特定的周期内的帧数 = 周期大小 * 周期数 / 每帧的大小

比如某个音频的采样格式是SND_PCM_FORMAT_S16_LE ，通道数是 2 ，所以一帧的音频大小是 16 x 2 /8 = 4个字节，

周期数为20，周期大小为8192，这个周期内帧数一共有 20 x 8192 >> 2 = 40960 帧

6.申请存放buff的空间（按帧数计算）snd_pcm_hw_params_set_buffer_size

7.读取音频数据

8.调用snd_pcm_writei 进行播放

2.具体实现

具体的代码在，这个例程只实现了播放wav文件，后续继续更新其他功能

git@github.com:lilixinxin/Alsa_example.git

alsa-lib的封装的结构体

typedef struct pcm_params {//pcm name like plughw:0,0char pcm_card_name[64];//pcm device handlesnd_pcm_t *pcm_handle;//pcm stream directionsnd_pcm_stream_t stream_direction;//specify configution for pcmsnd_pcm_hw_params_t *hwparams;//channel numint channel_num;//周期数int periods;//sample format bit per samplesnd_pcm_format_t sample_format;//PCM access type 交错模式或者交错snd_pcm_access_t access_type;//采样率int sample_rate;//每个周期的大小int period_szie;//frames 帧数int frames;
}Pcm_params;

初始化alsa 相关参数

//初始化alsa device
int Alsa_Dev_init(Pcm_params *pcm)
{int ret;//allocate snd_pcm_hw_params_tsnd_pcm_hw_params_alloca(&pcm->hwparams);//open pcm deviceif(snd_pcm_open(&pcm->pcm_handle, pcm->pcm_card_name, pcm->stream_direction, 0) < 0){fprintf(stderr, "Error opening PCM device %s\n", pcm->pcm_card_name);return -1;}/* Init hwparams with full configuration space */if (snd_pcm_hw_params_any(pcm->pcm_handle, pcm->hwparams) < 0) {fprintf(stderr, "Can not configure this PCM device.\n");return -1;}//设置数据访问模式if (snd_pcm_hw_params_set_access(pcm->pcm_handle, pcm->hwparams, pcm->access_type) < 0) {fprintf(stderr, "Can not set hw params access\n");return -1;}//set sample format bit per sampleif(snd_pcm_hw_params_set_format(pcm->pcm_handle, pcm->hwparams, pcm->sample_format) < 0){fprintf(stderr, "snd_pcm_hw_params_set_format fail\n");return -1;}//set channel numif(snd_pcm_hw_params_set_channels(pcm->pcm_handle, pcm->hwparams, pcm->channel_num) < 0){fprintf(stderr, "snd_pcm_hw_params_set_channels fail\n");return -1;}//set periodif(snd_pcm_hw_params_set_periods(pcm->pcm_handle, pcm->hwparams, pcm->periods, 0) < 0){fprintf(stderr, "snd_pcm_hw_params_set_periods fail\n");return -1;}//set sample rateif(snd_pcm_hw_params_set_rate_near(pcm->pcm_handle, pcm->hwparams, &pcm->sample_rate, 0) < 0){fprintf(stderr, "snd_pcm_hw_params_set_rate_near fail\n");return -1;}//每帧大小  每bit样本长度 * channel_num / 8bit//buffer size 按帧计算器 周期数 * 每个周期时间/* Set buffer size (in frames). The resulting latency is given by *//* latency = periodsize * periods / (rate * bytes_per_frame)     *///pcm->frames = (pcm->period_szie * pcm->periods) >> 2;if (snd_pcm_hw_params_set_buffer_size(pcm->pcm_handle, pcm->hwparams, pcm->frames) < 0) {fprintf(stderr, "Error setting buffersize.\n");return(-1);}/* Apply HW parameter settings to *//* PCM device and prepare device  */if (snd_pcm_hw_params(pcm->pcm_handle, pcm->hwparams) < 0) {fprintf(stderr, "Error setting HW params.\n");return(-1);}return 0;}
//播放
int Alsa_Playback(Pcm_params *pcm, char **buf)
{int pcmreturn;while ((pcmreturn = snd_pcm_writei(pcm->pcm_handle, buf, pcm->frames)) < 0) {snd_pcm_prepare(pcm->pcm_handle);fprintf(stderr, "<<<<<<<<<<<<<<< Buffer Underrun >>>>>>>>>>>>>>>\n");}return 0;
}

实现播放

  //申请buff空间int buffsize = pcm.period_szie * pcm.periods;buff = (char *)malloc(buffsize);if(buff == NULL){fprintf(stderr, "malloc fail\n");return -1;}while(1){ret = fread(buff, 1, buffsize, fp);printf("read buff len, :%d\n", ret);if(ret == 0){printf("end of music file input! \n");exit(1);}if(ret < 0){printf("read pcm from file! \n");exit(1);}while ((ret = snd_pcm_writei(pcm.pcm_handle, buff, pcm.frames)) < 0) {snd_pcm_prepare(pcm.pcm_handle);fprintf(stderr, "<<<<<<<<<<<<<<< Buffer Underrun >>>>>>>>>>>>>>>\n");}//Alsa_Playback(&pcm, &buff);}