Don't forget how they found a way to squish closed caption information into analog broadcast. The electron beam traces a path on the CRT display by drawing the odd lines from top to bottom (which draws half the image) and then the even lines (the rest of the owl) from top to bottom. While the electron beam is repositioning itself from the bottom of the screen to the top of the screen, there is a brief period of time where other data can be transmitted. That's where closed caption data was shoehorned in.

Teletext used the vertical blanking interval too. It consisted of numbered pages of text, each number input on the numpad on the remote control as a form of hypertext. Page after page was transmitted after one-another, repeatedly. It was sometimes used for subtitles, with some pages with a transparent background. Better receivers cached pages so you wouldn't have to wait for its next transmission when going to another page...

Digital television formats adopted the framing from analogue formats and sends the same data in digital form within the vertical blanking interval. Many channels have stopped offering teletext. One network here in Sweden still uses it to deliver news, and I often prefer that format because the articles are concise and distraction-free.

BTW. I was once asked to hack together a system for using data in the vblank period to control relays at a remote site.

Technology Connections does a really good video on this on YouTube