[Nini]

Those lines are data being transmitted in the vertical blanking interval, usually no more than a line in the overscan area where it wouldn't be visible to the viewer. In the past it's been used to send things like AR flags, teletext and other data streams like subtitling though rarely timecode. If it's on SD broadcasts it's there for compatibility reasons with old tube televisions. I might be wrong on some of this so you can correct me.