mp3.c

MP3解析文件头的源码

eeprom word BitRateValue[16]= {0, 32, 40, 48, 56, 64, 80, 96, 112,  
                               128, 160, 192, 224, 256, 320, 0 
                              }; 
eeprom byte MpegLayer[4]= {0, 3, 2, 1}; 
eeprom byte SampleRateValue[3]={44, 48, 32}; 
eeprom byte TimeInfo[7]= {'[',' ',' ',':',' ',' ',']'}; //time info. 

dword mp3StreamHeader;  //32 bits for saving mp3 stream header. 
byte  mp3BitRate;       //bitrate of a mp3 file. 
byte  mp3SampleRate;    //sample rate of a song. 
byte  mpegLayer;        //layer of a song; eg:mp2 or mp3. 
  
byte  mp3Pos;           //Global 
//This function is used to decode BitRate and SampleRate of a song 
//In this function,we read 512 bytes of a file to find stream header. 
byte DecodeMP3StreamHeader( ) { 
  byte i=0; 
  mp3StreamHeader=0; 
  mp3BitRate=0; 
  mp3SampleRate=0; 
  mpegLayer=0; 
   
  readPos=0;   //need to read a sector, set readPos=0 
  sectorPos=0;             
  //Read the first 512 bytes of MP3 file. This is where we will look for mp3 stream header. 
  ATA_Read_Sectors_LBA(clust2LBA(currentCluster),256); 
  fileoffset=512; // we have read 256 bytes of the file. 

  mp3Pos=0; 
  //we actually read 509 bytes so that we don't cross array 
  //boundary when we are doing following instructions in the loop. 
  while (mp3Pos<509) { 
   if (sectorBuffer.data[mp3Pos]==0xff) {//Check if it is "ff". 
     mp3Pos++; //check next byte. 
     if ((sectorBuffer.data[mp3Pos] & 0xf0)==0xf0){ //if it is true,we found header. 
      mp3Pos--; 
      //Load the bytes into mp3StreamHeader for calculating. 
      for(i=0; i<4; i++) mp3StreamHeader=(mp3StreamHeader<<8)+sectorBuffer.data[mp3Pos+i]; 
      mpegLayer=(mp3StreamHeader & 0x00060000)>>17; 
      mp3BitRate=(mp3StreamHeader & 0x0000f000)>>12; 
      mp3SampleRate=(mp3StreamHeader & 0x00000c00)>>10; 
      i=mp3BitRate; mp3BitRate=BitRateValue[i]; 
      i=mp3SampleRate; mp3SampleRate=SampleRateValue[i]; 
      i=mpegLayer; mpegLayer=MpegLayer[i];  
      return 1;   //decode successfully. 
     } 
   } 
    mp3Pos++; 
  }   . 
  return 0; //we don't find stream header at beginning 512 bytes.Failed! return 0 
}