I remember reading about the efforts to restore colour to material that had only been preserved in black and white. Before magnetic tape for video really becam

I remember reading about the efforts to restore colour to material that had only been preserved in black and white. Before magnetic tape for video really became feasible (1970s), to make a record of television broadcasts, the BBC (and many other broadcasters) would film a quality TV monitor with a film camera. Black-and-white film recordings exist of a number of Doctor Who episodes originally broadcast in colour in the late 60s and early 70s. Black-and-white analog televisions, like used in the BBC telecine recorder, receive the same signal as a colour TV. They receive a signal with colour information, encoded at very high frequency within the otherwise black-and-white compatible TV signal. On a black and white TV, that high frequency colour information, shows up as fine variations in texture with repeating patterns. Shift the phase of the colour carrier, and the colour changes for an appropriately equipped colour decoder, and the pattern shifts on a B&W TV. It's almost imperceptible really, which was the hack that allowed compatible colour TV. But it's there on the B&W display. And it's preserved on the film recording, too. That colour data can be recovered with appropriate software: https://en.wikipedia.org/wiki/Colour_recovery#From_chroma_crawl https://en.wikipedia.org/wiki/Colour_recovery#From_chroma_cr... https://www.theguardian.com/technology/2008/dec/11/digital-video-restoration-dad-s-army https://www.theguardian.com/technology/2008/dec/11/digital-v...