SQL & PL/SQL :: Returns Comments Of All Objects Of Schema Using Metadata Api

A function returns the comments of all objects of the schema using metadata api. I used DATABASE_EXPORT as object_type for open function. which name (eg: base_object_name, base_object_schema...) should i use to get comments of single object. My code is..

1 CREATE OR REPLACE function f_depen_obj
2 return clob
3 as
4 a number;
5 b number;
[code].....

I got comments of all objects when i commented 11th,12th lines. Now i want the comments of EMP table of SCOTT schema. I got the following error ..

SQL> select f_depen_obj from dual;
ERROR:
ORA-31603: object "EMP" of type TABLE not found in schema "EMP"
ORA-06512: at "SYS.DBMS_METADATA", line 1546
ORA-06512: at "SYS.DBMS_METADATA", line 1583
ORA-06512: at "SYS.DBMS_METADATA", line 1901
ORA-06512: at "SYS.DBMS_METADATA", line 3806
ORA-06512: at "SYS.DBMS_METADATA", line 3784
ORA-06512: at "SCOTT.F_DEPEN_OBJ", line 17

no rows selected...i got output (comments of single object) when i use COMMENT instead of DATABASE_EXPORT in open function(line no: 9). But i need to use DATABASE_EXPORT as a object_type. Which name is suitable for set_filter procedure instead of 'NAME'?