chroma dots

Chroma dots are visual artifacts caused by displaying an unfiltered PAL 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 They are commonly found on black-and-white recordings of television programmes originally made in colour.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 re-colour material where the original colour copy is lost.

''Example of the chroma dot reconstruction:''

File:Mouse pad Remicade Infliximab.jpg, Original picture
File:mousepad_in.jpg, Black and white picture with chroma dots
File:Mousepad quad.gif, Quadrant calculation for Colour reconstruction
File:mousepad_out.jpg, Reconstructed Colour picture (reconstructed using software written by Richard Russell)

Background

Analogue colour 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 colour 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 artifacts 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 restoration

In 1994, James Insell, a BBC
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