Chroma dots are visual artefacts caused by displaying an unfiltered analogue colour video signal on a black and white television or monitor. They are commonly found on black and white recordings of television programmes originally made in colour. 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 colour signal from black and white recordings, providing a means to colourise material where the original colour copy is lost.

Background

Analogue colour video signals comprise two components: chrominance

Chrominance

Chrominance is the signal used in video systems to convey the color information of the picture, separately from the accompanying luma signal . Chrominance is usually represented as two color-difference components: U = B' − Y' and V = R' − Y'...

 and luminance

Luma

Luma may refer to :* Luma , a genus of plants in the myrtle family* Amomyrtus luma, a species of tree in the myrtle family* Luma , a signal used in video technology* A star species from Super Mario Galaxy or Super Mario Galaxy 2....

. The luminance component describes the brightness of each part of the picture, whilst the chrominance component describes the colour tone. When displayed on a black and white monitor the luminance signal produces a normal black and white image, whilst 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

Dot crawl

Dot crawl is the popular name for a visual defect of color analog video standards when signals are transmitted as composite video, as in terrestrial broadcast television. It consists of animated checkerboard patterns which appear along vertical color transitions...

, which can produce visual artefacts in colour pictures.

History

In the early days of colour television, it was common practice for broadcasters to produce black and white film copies of colour programmes for sale and transmission in territories lacking colour broadcast facilities or (employing different colour television systems). During the telerecording process, it was normal practice to insert a filter circuit between the colour video output and the black and white monitor input in order to remove the colour 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.

Use in colourisation

In 1994 James Insell, a BBC

BBC

The British Broadcasting Corporation is a British public service broadcaster. Its headquarters is at Broadcasting House in the City of Westminster, London. It is the largest broadcaster in the world, with about 23,000 staff...

 engineer, noticed that when playing back a copy of a black and white telerecording through colour video equipment, spurious colour 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 colour 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 colourise the Dad's Army

Dad's Army

Dad's Army is a British sitcom about the Home Guard during the Second World War. It was written by Jimmy Perry and David Croft and broadcast on BBC television between 1968 and 1977. The series ran for 9 series and 80 episodes in total, plus a radio series, a feature film and a stage show...

 episode "Room at the Bottom" using information from the chroma dot patterning. The process has since been used to colourise other programmes including the pilot episode of Are You Being Served?

Are You Being Served?

Are You Being Served? is a British sitcom broadcast from 1972 to 1985. It was set in the ladies' and gentlemen's clothing departments of Grace Brothers, a large, fictional London department store. It was written mainly by Jeremy Lloyd and David Croft, with contributions by Michael Knowles and John...

 and episode three of the Doctor Who

Doctor Who

Doctor Who is a British science fiction television programme produced by the BBC. The programme depicts the adventures of a time-travelling humanoid alien known as the Doctor who explores the universe in a sentient time machine called the TARDIS that flies through time and space, whose exterior...

 serial Planet of the Daleks

Planet of the Daleks

Planet of the Daleks is a serial in the British science fiction television series Doctor Who, which was first broadcast in six weekly parts from April 7 to May 12, 1973.-Synopsis:...

. The working group hope that the technique may enable the restoration many of other programmes for which no colour 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.