sound mixer problem

how , sound mixer example is mixing the buffer......

the code is...


void CMixerThread::FillBufferL()
  {
  // wait for access to shared data
  iShared.iMutex.Wait();

  TInt volume = iShared.iMainVolume;

  // Gather new sample information
  TInt i;
  for( i=0; i<KMaxChannels; i++ )
    {
    if( iShared.iPlayStarted[ i ] )
      {
      iShared.iPlayStarted[ i ] = EFalse;
      TSample& s = iShared.iSample[ i ];

      iAudioData[ i ] = s.iData;
      iAudioPos[ i ] = 0;
      iAudioEnd[ i ] = s.iLength << KAudioShift;
      iRepStart[ i ] = s.iRepStart << KAudioShift;
      iRepEnd[ i ] = s.iRepEnd << KAudioShift;
      }
    }
  // give access to shared data
  iShared.iMutex.Signal();

  // clear buffer
  // has to be done because channels are mixed by adding their values
  // to each other
  Mem::FillZ( iMixBuffer, KBufferSize*4 );

  // mix active channels
  for( i=0; i<KMaxChannels; i++ )
    {
    if( iAudioData[ i ] != NULL )
      {
      TInt* buf = iMixBuffer;
      TInt* bufEnd = buf + KBufferSize;

      TInt16* src = iAudioData[ i ];

      TInt pos = iAudioPos[ i ];
      TInt posEnd = iAudioEnd[ i ];
      TInt repStart = iRepStart[ i ];
      TInt repEnd = iRepEnd[ i ];
      TInt posAdd = ( iShared.iFrequency[ i ] << KAudioShift ) / KSampleRate;
      TInt volume = iShared.iVolume[ i ];

      while( buf < bufEnd )
        {
        TInt sample = ( src[ pos >> KAudioShift ] * volume );
        pos += posAdd;
        if( pos >= posEnd )
          {
          if( repEnd == 0 )
            {
            iAudioData[ i ] = NULL;
            break;
            }
          else
            {
            pos = repStart;
            posEnd = repEnd;
            }
          }
        *buf++ += sample;
        }
      iAudioPos[ i ] = pos;
      iAudioEnd[ i ] = posEnd;
      }
    }

  // convert 32-bit mixing buffer to 16-bit output buffer
  TInt* buf = iMixBuffer;
 
  TInt* bufEnd = buf + KBufferSize;
  TInt16* buf2 = iBuffer;
  while( buf < bufEnd )
    {
    // 32-bit mixer buffer contents are multiplied by main volume
    // shifts are in two phases to prevent overflow and maintain quality
    TInt value = ( ( *buf++ >> 8 ) * volume ) >> 8;

    // Prevent sound from trashing on overboost volume:
    if( value < -0x7fff ) value = -0x7fff;
    if( value > 0x7fff ) value = 0x7fff;

    // and write to buffer
    *buf2++ = (TInt16)value;
    }


pls if anyone know then pls explain

with regards
jai.....