I’ve often idly wondered how teletext worked. How was digital information somehow stuffed into or alongside an analogue television transmission? Or VHS copy protection. How was it possible to add copy protection to an analogue medium? The answer is the vertical blanking interval (VBI).
The VBI is the time between drawing the last line of one frame and the first line of the next, and it was originally required because the magnetic field used to deflect the electron beam in CRTs took time to reset from one corner of the screen to the other. Over time, this interval was used to carry information not required for the primary signal, including subtitles, teletext and – in the case of video cassettes – copy protection.
On Macrovision’s VHS copy protection, from Wikipedia:
Macrovision’s legacy analog copy protection (ACP) works by implanting a series of excessive voltage pulses within the off-screen VBI lines of video. These pulses were included physically within pre-existing recordings on VHS and Betamax, and were generated upon playback by a chip in DVD players and digital cable or satellite boxes. A DVD recorder receiving an analog signal featuring these pulses would detect them and display a message saying that the source is “copy-protected” followed by aborting the recording. VCRs, in turn, react to these excessive voltage pulses by compensating with their automatic gain control circuitry, causing the recorded picture to wildly change brightness, rendering it annoying to watch. The system was only effective on VCRs made at around the mid-1980s and later.