Several boxes such as the CM7000 and DTVpal and Zinwell ZAT show "garbage" at the top of the image. These lines are mostly digital data such as closed captions being sent "above" the actual image on old NTSC analog signals. They are part of the signal's vertical blanking interval (VBI) and normally are hidden by the plastic edge of your set.
While watching NBC's Today show, they displayed a 4:3 videotape surrounded by a graphic on the left & right side of the image. The pillars on the left and right looked flawless, but the center part of the image had a visible VBI.
This indicates to me that it's not the box that is the problem (otherwise the VBI would be imposed across the entire width of the image). It is the *broadcaster* incorrectly framing the image, making visible part of the 4:3 analog videotape that was supposed to be invisible.