MIT MDR-200 Downscaling - Bright Line Artifact

I've moved my STB to my old RPTV for a bit while I do some repair work in my basement. I've been watching downsampled to 480i letterboxed on a 4:3 set for a couple of days. Here's my issue: When watching 1080i content in this format I'm seeing a bright white line about 5 pixels high on the bottom of the letterbox. If this TV didn't already have massive Closed Captioning burn-in and other failings I'd be much more concerned, but I'm wondering what might be causing this. I've had similar artifacts playing back .ts files with some players and it was related to them producing an 1088 pixel high output frame from the 1080 signal and not blanking the bottom 8 pixels because of some 16-byte boundary issues in the software. This line doesn't look that thick, but it looks to be the same sort of thing (i.e. it doesn't change with source or content). It might be there all the time and just overscanned so I don't see it when watching 16:9 but I can't test that at the moment because my 16:9 display is inactive. I'm going to drag my 20" monitor from the computer room into a spot where I can connect it to the box as soon as I can, but in the meantime i figured I'd throw it out there and see if anyone else had experienced this behaviour.


-MM