Q7. Why do black bars still appear on some HD channnel?

Answer:

Even though a television station may be broadcasting in a high definition format, they may still make use of standard(or analog) video signals as a source. In order to accommodate the wide screen format of high definition digital transmissions,vertical side bars are inserted by the broadcaster. By default, Panasonic plasmas display 720P and 1080i high definition transmissions in their native FULL aspect mode.

Note:
For further information, please see "Q5. What does the Aspect button control?"