Getting a millennial to explain CRTs is the free space in our bingo card.

Okay, so: tube televisions are a glass bulb containing vacuum. There’s an electron beam at the back, and electromagnets steer that beam in straight lines left to right, from the top of the screen to the bottom, every single frame. But in-between each straight line there’s a brief “blanking interval” where the beam has to move to the next line. The video signal itself goes dark during these periods. There’s a longer “vertical blank” when the beam moves from the bottom-right corner back to the upper-left.

VHS encodes one sequence of lines, one frame, from the top edge of the tape to the bottom edge of the tape. The tape is pulled around a drum, and inside the drum, there’s a read-write head spinning perpendicular. If the tape stopped then it’d overwrite the same vertical stripe across the tape, top to bottom, over and over. But because the tape is moving around the drum, it writes each stripe diagonally, slightly separated from the previous stripe.

It’s okay to lose some signal between the bottom of one stripe and the top of the next, because there’s not supposed to be any signal. That’s the vertical blank.

Magnetic media has limited “response.” You can only cram so much signal onto it before everything smears together, and you lose information. When VHS moves the tape more slowly, each stripe is closer to vertical, and also closer to its adjacent stripes.

Limited response is the reason for all of this. Industrial video recording just records linearly - like audio cassettes. But to do that, the tape has to move much faster, to avoid losing information. Studio-quality tapes come on enormous reels. VHS is compact, and moves at a more manageable speed, because the head moves quickly, even though the tape does not.