def ReadPTS(buffer):
        """
        read PTS (PES timestamp) at the buffer beginning (5 Bytes)
        """
        high = ((byte2int(buffer) & 0xF) >> 1)
        med = (indexbytes(buffer, 1) << 7) + (indexbytes(buffer, 2) >> 1)
        low = (indexbytes(buffer, 3) << 7) + (indexbytes(buffer, 4) >> 1)
        return ((int(high) << 30) + (med << 15) + low) / 90000