system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Old Cathode TVs would sometimes display an image that would roll up the screen and come back up on the bottom. What was happening that would cause that? And furthermore why did slapping the TV on the side sometimes fix it?
gpt
In old broadcasts, pictures were generated one line at a time, roughly 500 lines per frame, with a blank interval between each picture. Both the transmitter and the receiver had to be synchronized with the start of each frame.

When you saw the screen roll it was because the TV became unsynchronized with the broadcast and it was trying to find the frame. Most, if not all early TVs had a knob that allowed this to be tuned, while later TVs had better circuits to handle this. 

Modern broadcasts don't require it.