I did some tests on my SD cable channels, and the results are pretty mixed.
Some channels have simple letterboxing of 16x9 content:
Some letterbox, but put their logo in the letterbox area:
Sometimes a postage stamp is built-in, because they took original 4x3 content and pillarboxed it for the HD channel, then take that and letterbox it again for the SD version:
Some channels put their logo in the letterbox, but overlap it partially with the content:
Worse, they’ll even show other content overlapped along with the logo:
It’s almost impossible to deal with all these cases automatically. Even as a human I’m not sure what the proper crop for each of these situations should be.
The problem is that the cable company is broadcasting video with black bars added. They expect that the video will be displayed with the bars, and that’s why they’ve started putting other stuff in that area.
Interestingly enough, the SCTE (society of cable tech engineers) even came up with a broadcast standard that transmits BAR DATA, specifying exactly which areas of the picture are black bars. Unfortunately, no one actually uses it.