使用FFmpeg实现抽取多媒体文件的音频并按照AAC格式进行保存--附源码

抽取音频文件

注册log与编解码器

av_log_set_level(AV_LOG_INFO);

av_register_all();

打开多媒体文件

打开多媒体文件，并读取头部信息

/**
 * Open an input stream and read the header. The codecs are not opened.
 * The stream must be closed with avformat_close_input().
 *
 * @param ps Pointer to user-supplied AVFormatContext (allocated by avformat_alloc_context).
 *           May be a pointer to NULL, in which case an AVFormatContext is allocated by this
 *           function and written into ps.
 *           Note that a user-supplied AVFormatContext will be freed on failure.
 * @param url URL of the stream to open.
 * @param fmt If non-NULL, this parameter forces a specific input format.
 *            Otherwise the format is autodetected.
 * @param options  A dictionary filled with AVFormatContext and demuxer-private options.
 *                 On return this parameter will be destroyed and replaced with a dict containing
 *                 options that were not found. May be NULL.
 *
 * @return 0 on success, a negative AVERROR on failure.
 *
 * @note If you want to use custom IO, preallocate the format context and set its pb field.
 */
int avformat_open_input(AVFormatContext **ps, const char *url, ff_const59 AVInputFormat *fmt, AVDictionary **options);

/*Open an input stream and read the header*/
int ret = avformat_open_input(&fmt_ctx, "/work/test/test.mp4", NULL, NULL);
if (ret < 0) {
    av_log(NULL, AV_LOG_ERROR, "can't open file.\n");
    return -1;
}

打开一个文件用于写入音频文件

char *pDst = NULL;
pDst = "test.aac";
//  write audio data to AAC file
FILE *dst_fd = fopen(pDst, "wb");
if (dst_fd == NULL) {
    av_log(NULL, AV_LOG_ERROR, "open dst_fd failed.\n");
    avformat_close_input(&fmt_ctx);
    return -1;
}

输出多媒体信息

输出多媒体信息就是讲多媒体的信息以及metadata信息打印出来

/*
 * Print detailed information about the input or output format
 **/
av_dump_format(fmt_ctx, 0, "/work/test/test.mp4", 0);

确保输入的多媒体文件中有帧信息

  // 2. get stream
/*Read packets of a media file to get stream information.*/
ret = avformat_find_stream_info(fmt_ctx, NULL);
if (ret < 0) {
    av_log(NULL, AV_LOG_ERROR, "failed to find stream information.");
    avformat_close_input(&fmt_ctx);
    fclose(dst_fd);
    return -1;
}

提取音频信息

提取音频信息，并按照AAC格式将音频信息写入文件

AVPacket pkt;
/*Initialize optional fields of a packet with default values.*/
av_init_packet(&pkt);
pkt.data = NULL;
pkt.size = 0;
// 循环将音频信息写入AAC文件
int len = -1;
/*保存原始数据，播放时需要添加AAC的音频格式说明的头*/
while (av_read_frame(fmt_ctx, &pkt) >= 0) {
    if (pkt.stream_index == audio_index) {
        /*每帧开头都要写*/
        char adts_header_buf[7];
        // 为每帧音频添加AAC头部信息
        adts_header(adts_header_buf, pkt.size);
        fwrite(adts_header_buf, 1, 7, dst_fd);
        len = fwrite(pkt.data, 1, pkt.size, dst_fd);
        cout << pkt.size << endl;
        if (len != pkt.size) {
            av_log(NULL, AV_LOG_ERROR, "waning, length is not equl size of pkt.\n");
            return -1;
        }
    }
    /*Wipe the packet.*/
    av_packet_unref(&pkt);
}

// 添加ADTS头部信息
void adts_header(char *szAdtsHeader, int dataLen) {

    int audio_object_type = 2;
    int sampling_frequency_index = 7;
    int channel_config = 2;

    int adtsLen = dataLen + 7;

    szAdtsHeader[0] = 0xff;         //syncword:0xfff                          高8bits
    szAdtsHeader[1] = 0xf0;         //syncword:0xfff                          低4bits
    szAdtsHeader[1] |= (0 << 3);    //MPEG Version:0 for MPEG-4,1 for MPEG-2  1bit
    szAdtsHeader[1] |= (0 << 1);    //Layer:0                                 2bits
    szAdtsHeader[1] |= 1;           //protection absent:1                     1bit

    szAdtsHeader[2] =
            (audio_object_type - 1) << 6;            //profile:audio_object_type - 1                      2bits
    szAdtsHeader[2] |=
            (sampling_frequency_index & 0x0f) << 2; //sampling frequency index:sampling_frequency_index  4bits
    szAdtsHeader[2] |= (0 << 1);                             //private bit:0                                      1bit
    szAdtsHeader[2] |=
            (channel_config & 0x04) >> 2;           //channel configuration:channel_config               高1bit

    szAdtsHeader[3] = (channel_config & 0x03) << 6;     //channel configuration:channel_config      低2bits
    szAdtsHeader[3] |= (0 << 5);                      //original：0                               1bit
    szAdtsHeader[3] |= (0 << 4);                      //home：0                                   1bit
    szAdtsHeader[3] |= (0 << 3);                      //copyright id bit：0                       1bit
    szAdtsHeader[3] |= (0 << 2);                      //copyright id start：0                     1bit
    szAdtsHeader[3] |= ((adtsLen & 0x1800) >> 11);           //frame length：value   高2bits

    szAdtsHeader[4] = (uint8_t) ((adtsLen & 0x7f8) >> 3);     //frame length:value    中间8bits
    szAdtsHeader[5] = (uint8_t) ((adtsLen & 0x7) << 5);       //frame length:value    低3bits
    szAdtsHeader[5] |= 0x1f;                                 //buffer fullness:0x7ff 高5bits
    szAdtsHeader[6] = 0xfc;
}

AAC结构说明参考
[AAC说明可以参考]: https://blog.youkuaiyun.com/leixiaohua1020/article/details/50535042

结束释放资源

音频写好之后，释放掉申请的资源

/*Close an opened input AVFormatContext*/
avformat_close_input(&fmt_ctx);
if (dst_fd != NULL)
    fclose(dst_fd);

完整代码

//
// Created by andrew on 2020/11/1.
//
#include <iostream>
using namespace std;
extern "C" {
#include <libavutil/log.h>
#include <libavformat/avformat.h>
}

void adts_header(char *szAdtsHeader, int dataLen) {

    int audio_object_type = 2;
    int sampling_frequency_index = 7;
    int channel_config = 2;

    int adtsLen = dataLen + 7;

    szAdtsHeader[0] = 0xff;         //syncword:0xfff                          高8bits
    szAdtsHeader[1] = 0xf0;         //syncword:0xfff                          低4bits
    szAdtsHeader[1] |= (0 << 3);    //MPEG Version:0 for MPEG-4,1 for MPEG-2  1bit
    szAdtsHeader[1] |= (0 << 1);    //Layer:0                                 2bits
    szAdtsHeader[1] |= 1;           //protection absent:1                     1bit

    szAdtsHeader[2] =
            (audio_object_type - 1) << 6;            //profile:audio_object_type - 1                      2bits
    szAdtsHeader[2] |=
            (sampling_frequency_index & 0x0f) << 2; //sampling frequency index:sampling_frequency_index  4bits
    szAdtsHeader[2] |= (0 << 1);                             //private bit:0                                      1bit
    szAdtsHeader[2] |=
            (channel_config & 0x04) >> 2;           //channel configuration:channel_config               高1bit

    szAdtsHeader[3] = (channel_config & 0x03) << 6;     //channel configuration:channel_config      低2bits
    szAdtsHeader[3] |= (0 << 5);                      //original：0                               1bit
    szAdtsHeader[3] |= (0 << 4);                      //home：0                                   1bit
    szAdtsHeader[3] |= (0 << 3);                      //copyright id bit：0                       1bit
    szAdtsHeader[3] |= (0 << 2);                      //copyright id start：0                     1bit
    szAdtsHeader[3] |= ((adtsLen & 0x1800) >> 11);           //frame length：value   高2bits

    szAdtsHeader[4] = (uint8_t) ((adtsLen & 0x7f8) >> 3);     //frame length:value    中间8bits
    szAdtsHeader[5] = (uint8_t) ((adtsLen & 0x7) << 5);       //frame length:value    低3bits
    szAdtsHeader[5] |= 0x1f;                                 //buffer fullness:0x7ff 高5bits
    szAdtsHeader[6] = 0xfc;
}

/*
 * 从多媒体文件中抽取媒体信息
 * */

int main(int argc, char *argv[]) {

    AVFormatContext *fmt_ctx = NULL;
    av_log_set_level(AV_LOG_INFO);
    /*所有进行操作前，先执行以下，否则需要自己制定类型*/
    av_register_all();

    // 1. 读取多媒体文件
    char *pSrc = NULL;
    char *pDst = NULL;
    pSrc = (char *) "/work/test/test.mp4";
    pDst = "test.aac";

    /*Open an input stream and read the header*/
    int ret = avformat_open_input(&fmt_ctx, "/work/test/test.mp4", NULL, NULL);
    if (ret < 0) {
        av_log(NULL, AV_LOG_ERROR, "can't open file.\n");
        return -1;
    }
    //  write audio data to AAC file
    FILE *dst_fd = fopen(pDst, "wb");
    if (dst_fd == NULL) {
        av_log(NULL, AV_LOG_ERROR, "open dst_fd failed.\n");
        avformat_close_input(&fmt_ctx);
        return -1;
    }


    // 2. get stream
    /*Read packets of a media file to get stream information.*/
    ret = avformat_find_stream_info(fmt_ctx, NULL);
    if (ret < 0) {
        av_log(NULL, AV_LOG_ERROR, "failed to find stream information.");
        avformat_close_input(&fmt_ctx);
        fclose(dst_fd);
        return -1;
    }

    /*
     * Print detailed information about the input or output format
     * */
    av_dump_format(fmt_ctx, 0, "/work/test/test.mp4", 0);

    int audio_index = -1;
    audio_index = av_find_best_stream(fmt_ctx, AVMEDIA_TYPE_AUDIO, -1, -1, NULL, 0);
    if (audio_index < 0) {
        av_log(NULL, AV_LOG_ERROR, "ret = %d\n", ret);
        avformat_close_input(&fmt_ctx);
        fclose(dst_fd);
        return -1;
    }

    AVPacket pkt;
    /*Initialize optional fields of a packet with default values.*/
    av_init_packet(&pkt);
    pkt.data = NULL;
    pkt.size = 0;

    int len = -1;
    /*保存原始数据，播放时需要添加AAC的音频格式说明的头*/
    while (av_read_frame(fmt_ctx, &pkt) >= 0) {
        if (pkt.stream_index == audio_index) {
            /*每帧开头都要写*/
            char adts_header_buf[7];
            adts_header(adts_header_buf, pkt.size);
            fwrite(adts_header_buf, 1, 7, dst_fd);
            len = fwrite(pkt.data, 1, pkt.size, dst_fd);
            cout << pkt.size << endl;
            if (len != pkt.size) {
                av_log(NULL, AV_LOG_ERROR, "waning, length is not equl size of pkt.\n");
                return -1;
            }
        }
        /*Wipe the packet.*/
        av_packet_unref(&pkt);
    }

    /*Close an opened input AVFormatContext*/
    avformat_close_input(&fmt_ctx);
    if (dst_fd != NULL)
        fclose(dst_fd);

    return 0;
}