@hi_cial so analog video in north america has 525 lines of vertical resolution, of which only 480 are visible (the other 45 are blank lines used to give the deflection coils time to move the electron beam back to the top left of the picture tube); by convention, the invisible lines are partially before the visible ones and partially after them

starting in the late 70s, people experimented with ways of adding captions to television. One of the systems developed was line 21 captions, which placed digital text data on line 21, one of the invisible scanlines before the picture. A decoder box could pull out this data, convert it into text, and overlay it on the image (the format also included information for where in the image the captions should be placed, as well as various colors the text can be, so it's already superior to youtube captions).

in 1990, the US congress passed a law that required every television sold after then to have built-in circuitry to decode line 21 captions and display them, so it became ubiquitous since no one needed any extra hardware. either the same law or a different one passed around the same time (i'm not too sure) also required that line 21 captions be broadcast on every channel (with a few exceptions, i think), including for live media--news stations had to start hiring stenographers to do their captions live

Captions were also required on all prerecorded videotapes made after a certain date, and VCRs (as they just record the raw video signal) do reproduce the captions that were transmitted along with the video they record, so it's quite likely there are captions on your tapes
-F