HDV is the little brother to broadcast High Def such as HDCAM or D5. It shoots/records in the 16×9 aspect ratio which, for the math inclined is the same as 1.78 : 1 But standard def television uses an aspect ratio of 1.33 :1
So in down-converting the HDV to standard, you basically need to force the shape of a longer rectangle into a the shape of a regular TV which has a more square shape. (see examples below)
The two ways of doing this are to hack off the sides of the rectangle (also called a “center extaction”), which still provides excellent resolution . But in that case, the sides of the picture will be gone. For example, if there is a dinner table scene and one of the actors is far to either side of the frame, they will be cut off.
However, if you want to see ALL the image, then the long rectangle needs to be shrunk down to fit inside the square. This means there is no image at the tops and bottoms of the frame in the standard def tv. It’s letterboxed. The black bars at the top and bottom are just “empty”areas where there is no picture because it has been shrunken down.
Letterbox is widely accepted on broadcast tv these days. Many network shows run this way due to the complications of trying to have different versions for high def and standard def. People are used to it and the filmmaker’s compositions are preserved. So shoot/record/ dub your HDV into letterbox when you put it on a regular tv. It looks cooler, and the dude at the dinner table isn’t cut off.