Chroma dots

Chroma dots are visual artefacts caused by displaying an unfiltered analogue color video signal on a black-and-white television or monitor. They are commonly found on black-and-white recordings of television programs originally made in color. Chroma dots were once regarded as undesirable picture noise, but recent advances in computer technology have allowed them to be used to reconstruct the original color signal from black-and-white recordings, providing a means to re-color material where the original color copy is lost.

Analogue color video signals comprise two components: chrominance and luminance. The luminance component describes the brightness of each part of the picture, while the chrominance component describes the color tone. When displayed on a black-and-white monitor, the luminance signal produces a normal black-and-white image, while the chrominance signal manifests as a fine pattern of dots of varying size and intensity overlaid over the black-and-white picture. A related phenomenon is dot crawl, which can produce visual artefacts in color pictures.

In the early days of color television, it was common practice for broadcasters to produce black-and-white film copies of color programs for sale and transmission in territories lacking color broadcast facilities or employing different color television systems. During the telerecording process, it was normal practice to insert a filter circuit between the color video output and the black-and-white monitor input in order to remove the color signal and prevent the formation of chroma dots. In many cases, however, the filter was not used and the chroma dot patterning is permanently burned into the resulting film recording.

In 1994, James Insell, a BBC engineer, noticed that when playing back a copy of a black-and-white telerecording through color video equipment, spurious color was generated by the presence of chroma dots in the picture. He theorised that it might be possible to use the chroma dots to reconstruct the original color signal, and in 2007 set up a working group to carry out further research.

In 2008, it was announced that members of the working group had successfully managed to restore the Dad's Army episode "Room at the Bottom" using information from the chroma dot patterning. The process has since been used to restore other programs including the pilot episode of Are You Being Served? and various episodes of Doctor Who. (Planet of the Daleks, episode 3; Invasion of the Dinosaurs, episode 1; and episodes 2-6 of The Mind of Evil) The working group hope that the technique may enable the restoration of many other programs for which no color copy is known to exist, however the results are dependent on whether chroma dot patterning is present and the quality of the black-and-white recording.

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