A fascinating and technically impressive demonstration of video restoration techniques on the BBC the other night - using just a black & white film copy of a 1969 episode of classic sitcom Dad’s Army, they restored the original colour. Not with artificial colourisation, the actual colours as they were recorded at the time (well, approximately).
The background is that in those days the BBC was in the habit of overwriting videotapes after a programme had been shown once or twice. A lot of material has consequently been lost forever. This show was shot on colour videotape, but the only surviving copy was a black & white film made by pointing a film camera at a monochrome monitor (for possible distribution to countries who, at the time, were still on black & white TV).
So how did they do it? Well, colour information in analogue TV (NTSC, PAL etc.) is in effect encoded on top of the black & white picture and, without filtering, appears as fine grained interference patterns when viewed on a monochrome monitor. It was found to be possible to extract sufficient colour information from the B&W film to reproduce the original colours fairly accurately. A bit of cleaning up and frame interpolation to restore the original interlaced video look, and they ended up with something that looked remarkably close to how it must have done when first broadcast.
Here’s the clip they used to introduce the restored colour episode, explaining the technique (YouTube quality, so you don’t quite see the video-like effect that they ultimately achieved):
