FFmpeg之音频重采样

为什么要重采样

从设备采集的音频数据与编码器要求的数据不一致
扬声器要求的音频数据与要播放的音频数据不一致
更方便运算（回音消除须使用单声道，需要先转换）

比如说语音识别，需要很低的采样率就可以了，高了增加了数据量，毫无用处，这时候就需要进行音频重采样，重采样可以改变音频采样值或采样格式。

swr_init()

/*设置用户参数后初始化上下文。 @note必须使用AVOption API配置上下文。** @see av_opt_set_int()* @see av_opt_set_dict()** @param[in,out]   s Swr context to initialize* @return AVERROR error code in case of failure.*/
int swr_init(struct SwrContext *s);

swr_free()

/*** 释放给定的SwrContext并将指针设置为NULL。** @param[in] s a pointer to a pointer to Swr context*/
void swr_free(struct SwrContext **s);

swr_alloc_set_opts（）

/*** 如果需要，分配SwrContext并设置/重置公共参数。 * * 此函数不要求使用swr_alloc（）分配% s。关于 * 另一方面，swr_alloc（）可以使用swr_alloc_set_opts（）来设置参数 * 在分配的上下文上** @param s               existing Swr context if available, or NULL if not* @param out_ch_layout   output channel layout (AV_CH_LAYOUT_*)* @param out_sample_fmt  output sample format (AV_SAMPLE_FMT_*).* @param out_sample_rate output sample rate (frequency in Hz)* @param in_ch_layout    input channel layout (AV_CH_LAYOUT_*)* @param in_sample_fmt   input sample format (AV_SAMPLE_FMT_*).* @param in_sample_rate  input sample rate (frequency in Hz)* @param log_offset      logging level offset* @param log_ctx         parent logging context, can be NULL** @see swr_init(), swr_free()* @return NULL on error, allocated context otherwise*/
struct SwrContext *swr_alloc_set_opts(struct SwrContext *s,int64_t out_ch_layout, enum AVSampleFormat out_sample_fmt, int out_sample_rate,int64_t  in_ch_layout, enum AVSampleFormat  in_sample_fmt, int  in_sample_rate,int log_offset, void *log_ctx);

av_samples_alloc_array_and_samples()

/**为nb_samples分配一个数据指针数组，samples缓冲区 采样，并相应地填充数据指针和行大小。** This is the same as av_samples_alloc(), but also allocates the data* pointers array.** @see av_samples_alloc()*/
int av_samples_alloc_array_and_samples(uint8_t ***audio_data, int *linesize, int nb_channels,int nb_samples, enum AVSampleFormat sample_fmt, int align);

swr_convert()

/** Convert audio.** in和in_count可以设置为0，* 如果提供的输入空间多于输出空间，则输入将被缓冲。* 您可以使用swr_get_out_samples（）检索* 对于给定数量的输出样本的所需数量的上限* 输入样本。只要可能，转换将直接运行而不进行复制。** @param s         allocated Swr context, with parameters set* @param out       output buffers, only the first one need be set in case of packed audio* @param out_count amount of space available for output in samples per channel* @param in        input buffers, only the first one need to be set in case of packed audio* @param in_count  number of input samples available in one channel** @return number of samples output per channel, negative value on error*/
int swr_convert(struct SwrContext *s, uint8_t **out, int out_count,const uint8_t **in , int in_count);

例子：

#include <string.h>
#include <stdio.h>
extern "C"{#include "libavutil/avutil.h"#include "libavdevice/avdevice.h"#include "libavformat/avformat.h"#include "libavcodec/avcodec.h"#include "libswresample/swresample.h"
}
#include <string>
#include <iostream>
using namespace std;
static int rec_status = 0;void set_status(int status){rec_status = status;
}SwrContext* init_swr(){SwrContext *swr_ctx = NULL;//channel, number/swr_ctx = swr_alloc_set_opts(NULL,                //ctxAV_CH_LAYOUT_STEREO, //输出channel布局AV_SAMPLE_FMT_S16,   //输出的采样格式44100,               //采样率AV_CH_LAYOUT_STEREO, //输入channel布局AV_SAMPLE_FMT_FLT,   //输入的采样格式44100,               //输入的采样率0, NULL);if(!swr_ctx){}if(swr_init(swr_ctx) < 0){}return swr_ctx;
}void rec_audio() {int ret = 0;char errors[1024] = {0, };// 原地址缓冲区uint8_t **src_data = NULL;int src_linesize = 0;// 目标地址缓冲区uint8_t **dst_data = NULL;int dst_linesize = 0;//ctxAVFormatContext *fmt_ctx = NULL;AVDictionary *options = NULL;//pakcetAVPacket pkt;//[[video device]:[audio device]]char *devicename = "hw:1";//set log levelav_log_set_level(AV_LOG_DEBUG);//start recordrec_status = 1;//register audio deviceavdevice_register_all();//get formatAVInputFormat *iformat = av_find_input_format("alsa");//open deviceif((ret = avformat_open_input(&fmt_ctx, devicename, iformat, &options)) < 0 ){av_strerror(ret, errors, 1024);fprintf(stderr, "Failed to open audio device, [%d]%s\n", ret, errors);return;}//create filechar *out = "../source/audio.pcm";FILE *outfile = fopen(out, "wb+");SwrContext* swr_ctx = init_swr();//4096/4=1024/2=512//创建输入缓冲区av_samples_alloc_array_and_samples(&src_data,         //输出缓冲区地址&src_linesize,     //缓冲区的大小2,                 //通道个数512,               //单通道采样个数AV_SAMPLE_FMT_FLT, //采样格式0);//创建输出缓冲区av_samples_alloc_array_and_samples(&dst_data,         //输出缓冲区地址&dst_linesize,     //缓冲区的大小1,                 //通道个数512,               //单通道采样个数AV_SAMPLE_FMT_S16, //采样格式0);cout << "src_linesize" << src_linesize << endl;cout << "dst_linesize" << dst_linesize << endl;//read data from devicewhile((ret = av_read_frame(fmt_ctx, &pkt)) == 0 &&rec_status) {av_log(NULL, AV_LOG_INFO,"packet size is %d(%p)\n",pkt.size, pkt.data);//进行内存拷贝，按字节拷贝的memcpy((void*)src_data[0], (void*)pkt.data, pkt.size);-//重采样swr_convert(swr_ctx,                    //重采样的上下文dst_data,                   //输出结果缓冲区512,                        //每个通道的采样数(const uint8_t **)src_data, //输入缓冲区512);                       //输入单个通道的采样数//write file//fwrite(pkt.data, 1, pkt.size, outfile);fwrite(dst_data[0], 1, dst_linesize, outfile);fflush(outfile);av_packet_unref(&pkt); //release pkt}//close filefclose(outfile);//释放输入输出缓冲区if(src_data){av_freep(&src_data[0]);}av_freep(src_data);if(dst_data){av_freep(&dst_data[0]);}av_freep(dst_data);//释放重采样的上下文swr_free(&swr_ctx);//close device and release ctxavformat_close_input(&fmt_ctx);av_log(NULL, AV_LOG_DEBUG, "finish!\n");return;
}int main(int argc, char *argv[])
{rec_audio();return 0;
}

对应的编译文件为：

cmake_minimum_required(VERSION 3.5)project(ffmpeg_learn)#设置编译选项
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")set(ffmpeg_libs_DIR /usr/lib/x86_64-linux-gnu)
set(ffmpeg_headers_DIR /usr/include/x86_64-linux-gnu)set(EXECUTABLE_OUTPUT_PATH ${PROJECT_SOURCE_DIR}/bin)#对于find_package找不到的外部依赖库，可以用add_library添加
# SHARED表示添加的是动态库
# IMPORTED表示是引入已经存在的动态库
add_library( avcodec SHARED IMPORTED )#指定所添加依赖库的导入路径
set_target_properties( avcodec PROPERTIES IMPORTED_LOCATION ${ffmpeg_libs_DIR}/libavcodec.so )add_library( avfilter SHARED IMPORTED )
set_target_properties( avfilter PROPERTIES IMPORTED_LOCATION ${ffmpeg_libs_DIR}/libavfilter.so )add_library( avdevice SHARED IMPORTED )
set_target_properties( avdevice PROPERTIES IMPORTED_LOCATION ${ffmpeg_libs_DIR}/libavdevice.so )add_library( swresample SHARED IMPORTED )
set_target_properties( swresample PROPERTIES IMPORTED_LOCATION ${ffmpeg_libs_DIR}/libswresample.so )add_library( swscale SHARED IMPORTED )
set_target_properties( swscale PROPERTIES IMPORTED_LOCATION ${ffmpeg_libs_DIR}/libswscale.so )add_library( avformat SHARED IMPORTED )
set_target_properties( avformat PROPERTIES IMPORTED_LOCATION ${ffmpeg_libs_DIR}/libavformat.so )add_library( avutil SHARED IMPORTED )
set_target_properties( avutil PROPERTIES IMPORTED_LOCATION ${ffmpeg_libs_DIR}/libavutil.so )# 添加头文件路径到编译器的头文件搜索路径下，多个路径以空格分隔
include_directories( ${ffmpeg_headers_DIR} )# 添加一个可执行目标，名称可自己指定，本例是直接用工程名称命名的
# 该可执行目标是由SRC_LIST中所列出的源文件生成
add_executable(${PROJECT_NAME} src/samlpel_audio1.cpp)
# directory of opencv library
# link_directories(${ffmpeg_libs_DIR} )# 链接目标文件与依赖库
target_link_libraries( ${PROJECT_NAME}  avcodec avformat avutil swresample swscale  avfilter avdevice)add_executable(resample_audio src/resample_audio.cpp)
# directory of opencv library
# link_directories(${ffmpeg_libs_DIR} )# 链接目标文件与依赖库
target_link_libraries( resample_audio  avcodec avformat avutil swresample swscale  avfilter avdevice)