• av_init_packet()
  • av_find_best_stream()
  • av_read_frame()/av_pack_unref() ```cpp

    include

    include

    include

void adts_header(char *aac_header, int profile, int sample_rate, int channels, int frame_len) { int sampling_frequency_index = 4; //44100

  1. //sync word
  2. aac_header[0] = 0xff;         //syncword:0xfff                          高8bits
  3. aac_header[1] = 0xf0;         //syncword:0xfff                          低4bits
  4.                               //ID
  5. aac_header[1] |= (0 << 3);    //ID : 0 for MPEG-4 ;  1 for MPEG-2  
  6.                               //layer
  7. aac_header[1] |= (0 << 1);    //Layer:0  always:00
  8.                               //protection absent
  9. aac_header[1] |= 1;           //protection absent:1    前两个字节的最低位为 1 
  10.                               // also : aac_header[0] = 0xff; aac_header[1] = 0xf1;
  11.                               //profile
  12. aac_header[2] = (profile) << 6;  //profile:profile  2bits
  13.                                   //sampling_frequency_index
  14. aac_header[2] |= (sampling_frequency_index & 0x0f) << 2; //sampling_frequency_index 4bits 只有4bit要 &0x0f，清空高4位 
  15.                                                          //private_bit
  16. aac_header[2] |= (0 << 1);        //private_bit: 0   1bits      
  17.                                   //channels
  18. aac_header[2] |= (channels & 0x04) >> 2;  //channel 高1bit: &0000 0100取channels的最高位
  19. aac_header[3] |= (channels & 0x03) << 6;  //&0000 0011取channels的低2位
  21. aac_header[3] |= (0 << 5);               //original：0                1bit
  22. aac_header[3] |= (0 << 4);               //home：0                    1bit
  23. aac_header[3] |= (0 << 3);               //copyright id bit：0        1bit
  24. aac_header[3] |= (0 << 2);               //copyright id start：0      1bit
  26.                                          //frame len 一个ADTS帧的长度包括ADTS头和AAC原始流
  27. int adtsLen = frame_len + 7;
  28. aac_header[3] |= ((adtsLen & 0x1800) >> 11);           //frame length：value   高2bits 
  29. aac_header[4] = (uint8_t)((adtsLen & 0x7f8) >> 3);     //frame length:value    中间8bits
  30. aac_header[5] = (uint8_t)((adtsLen & 0x7) << 5);       //frame length:value    低3bits
  31.                                                        //buffer fullness 0x1FF 说明是码率可变的码流
  32. aac_header[5] |= 0x1f;                                 //buffer fullness:0x1ff 高5bits
  33. aac_header[6] = 0xfc;      //111111 00                  //buffer fullness:0x1ff 低6bits
  34. //buffer fullness = 1FF, number_of_raw_data_blocks_in_frame = 0, 表示说ADTS帧中有一个AAC数据块
  35. return;

}

int main(int argc, char argv[]) { char src = NULL; char dst = NULL; int ret = -1; int audio_index = -1; int len = 0; AVPacket pkt; AVFormatContext fmt_ctx = NULL;

  1. av_log_set_level(AV_LOG_INFO);
  3. av_register_all();
  5. //1、read two params from console
  6. if (argc < 3) {
  7.     av_log(NULL, AV_LOG_ERROR, "the count of params should be more than three!\n");
  8.     return -1;
  9. }
  11. src = argv[1];
  12. dst = argv[2];
  13. if (src == NULL || dst == NULL) {
  14.     av_log(NULL, AV_LOG_ERROR, "src or dst is null!\n");
  15.     return -1;
  16. }
  18. ret = avformat_open_input(&fmt_ctx, src, NULL, NULL);
  19. if (ret < 0) {
  20.     av_log(NULL, AV_LOG_ERROR, "Can't open file: %s\n", av_err2str(ret));
  21.     return -1;
  22. }
  23. av_dump_format(fmt_ctx, 0, "./test.mp4", 0);
  25. FILE *dst_fd = fopen(dst, "wb");
  26. if (!dst_fd) {
  27.     av_log(NULL, AV_LOG_ERROR, "Can't open out file !\n");
  28.     avformat_close_input(&fmt_ctx);
  29.     return -1;
  30. }
  32. //2、get frame
  33. ret = av_find_best_stream(fmt_ctx, AVMEDIA_TYPE_AUDIO, -1, -1, NULL, 0); //第2个参数是获取流的类型;第3个参数是流的索引号，不知道直接写-1;第4个参数是相关的流的索引号，比如音频对应的视频流的索引号，不知道直接写-1;第5个参数是对应的编解码器,不知道直接写NULL
  34.   if (ret < 0) {
  35.     av_log(NULL, AV_LOG_ERROR, "Can't find the best stream!\n");
  36.     avformat_close_input(&fmt_ctx);
  37.     fclose(dst_fd);
  38.     return -1;
  39. }
  40. audio_index = ret;
  42. av_init_packet(&pkt);
  43. while (av_read_frame(fmt_ctx, &pkt) >= 0) {
  44.     if (pkt.stream_index == audio_index) {
  45.         //3、write audio data to aac file 
  46.         char adts_header_buf[7] = {0};
  47.         adts_header(adts_header_buf, 2, 44100, 2, pkt.size);
  48.         fwrite(adts_header_buf, 1, 7, dst_fd);
  49.         len = fwrite(pkt.data, 1, pkt.size, dst_fd);
  50.         if (len != pkt.size) {
  51.             av_log(NULL, AV_LOG_WARNING, "warning, length of data is not equal size of pkt!\n");
  52.         }
  53.     }
  54.     av_packet_unref(&pkt);
  55. }
  56. av_packet_unref(&pkt);
  58. if (fmt_ctx) {
  59.     avformat_close_input(&fmt_ctx);
  60. }
  61. if (dst_fd) {
  62. fclose(dst_fd);
  63. }
  64. return 0;

}

编译： gcc -lavutil -lavformat -lavcodec ``` 注：mp4里的aac数据没有adts，从MP4里抽取的aac数据需要自己添加adts头。