When we copied movies two decades ago, the very oldest video cassette recorders (VCRs) were in high demand. If you’re wondering just how old these VCRs were, think “coal-powered”. The reason was a curious mechanism preventing the copying of movies, and to understand it, we need to look a little bit into how the technology worked in that era. Images on a TV were displayed using a single electron beam racing very fast across a chemically coated screen that produced color when hit by the beam. It raced first from left to right, and in horizontal lines, from top to bottom, painting an image on the fluorescent coating. Then, the beam returned back to top-left and started painting the next image in the movie. The beam painted about 50 such images a second, and the act of returning back to top-left is called a “vertical retrace”.

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