I view Sky Digital with a 4:3 televison, and as a result get to watch a lot of programmes that have black bars at the top and bottom. However I have noticed that these bars are not always even, ie; they have a step at some point in the line. This seems to be completely random, no channel is worse than the others and one programme will have the uneven bars, the next programme (on the same channel) will be perfect. Sometimes it can change within the same programme, eg: football, when the camera changes to a different shot. Any idea what causes this?