Mp3 文件结构全解析 (二)

typedef FrameHeader
{
unsigned int sync:11;                        //同步信息
unsigned int version:2;                      //版本
unsigned int layer: 2;                           //层
unsigned int error protection:1;           // CRC校验
unsigned int bitrate_index:4;              //位率
unsigned int sampling_frequency:2;         //采样频率
unsigned int padding:1;                    //帧长调节
unsigned int private:1;                       //保留字
unsigned int mode:2;                         //声道模式
unsigned int mode extension:2;        //扩充模式
unsigned int copyright:1;                           // 版权
unsigned int original:1;                      //原版标志
unsigned int emphasis:2;                  //强调模式
}HEADER, *LPHEADER;

position = lseek(fd, tagsize-10, 0);  printf("seek to get audio frame , position = %d\n", position);
  int nFrames, nFileSampleRate;  unsigned char ucHeaderByte1, ucHeaderByte2, ucHeaderByte3, ucHeaderByte4;  float fBitRateSum=0;syncWordSearch:  while( position < file_size)  {    if (read(fd, &ucHeaderByte1, sizeof(ucHeaderByte1)) < 0) {      perror("Read File: ");      exit(1);    }    position ++;    //printf("111:%d\n", ucHeaderByte1);    if( ucHeaderByte1 == 0xFF )    {      if (read(fd, &ucHeaderByte2, sizeof(ucHeaderByte2)) < 0) {        perror("Read File: ");        exit(1);      }      position ++;      unsigned char ucByte2LowerNibble = ucHeaderByte2 & 0xF0;      if( ucByte2LowerNibble == 0xF0 || ucByte2LowerNibble == 0xE0 )      {          ++nFrames;          printf("Found frame %d at offset = %ld B\n",nFrames, position);          //printf("Header Bits:\n");          //get the rest of the header:          if (read(fd, &ucHeaderByte3, sizeof(ucHeaderByte3)) < 0) {            perror("Read File: ");            exit(1);          }          position ++;          if (read(fd, &ucHeaderByte4, sizeof(ucHeaderByte4)) < 0) {            perror("Read File: ");            exit(1);          }          position ++;          //print the header:          //printBits(sizeof(ucHeaderByte1),&ucHeaderByte1);          //printBits(sizeof(ucHeaderByte2),&ucHeaderByte2);          //printBits(sizeof(ucHeaderByte3),&ucHeaderByte3);          //printBits(sizeof(ucHeaderByte4),&ucHeaderByte4);          //get header info:          int nFrameSamplingFrequency = findFrameSamplingFrequency(ucHeaderByte3);          int nFrameBitRate = findFrameBitRate(ucHeaderByte3);          int nMpegVersionAndLayer = findMpegVersionAndLayer(ucHeaderByte2);
          if( nFrameBitRate==0 || nFrameSamplingFrequency == 0 || nMpegVersionAndLayer==0 )          {//if this happens then we must have found the sync word but it was not actually part of the header              --nFrames;              printf("Error: not a header\n\n");              goto syncWordSearch;          }          fBitRateSum += nFrameBitRate;          if(nFrames==1){ nFileSampleRate = nFrameSamplingFrequency; }          int nFramePadded = findFramePadding(ucHeaderByte3);          //calculate frame size:          int nFrameLength = (144 * (float)nFrameBitRate /                                            (float)nFrameSamplingFrequency ) + nFramePadded;          printf("\tFrame Length: %d Bytes \n\n", nFrameLength);
          //lnPreviousFramePosition=ftell(ifMp3)-4; //the position of the first byte of this frame
          //move file position by forward by frame length to bring it to next frame:          position = lseek(fd, position + nFrameLength-4, 0);      }    }  }  float fFileAveBitRate= fBitRateSum/nFrames;  printmp3details(nFrames,nFileSampleRate,fFileAveBitRate);