Colour recovery

Colour recovery (or colour restoration) is a process which can restore lost colour, specifically to television programmes which were originally transmitted in colour, but for which only black & white copies remain archived.^[1] Not to be confused with colourisation, colour recovery is a newer process ^[2] and is fundamentally different from colourisation for several reasons.^[2] Firstly, colour recovery can only be performed if the originally transmitted colour signal can be reconstructed or recovered from some source, whereas this is not usually the case for traditional colourisation. Secondly, colourisation can be used to colourise films and programmes that were made in black and white, using still colour photos and/or some educated guesswork to manually choose a colour palette. Conversely, the goal of colour recovery is to reinstate (as closely as possible) the colour signals of programmes originally made in colour as they were first seen. Colour recovery reconstructs the colour information from actual recovered signals and theoretically without depending on guesswork. As of 2010, colour recovery has successfully been applied to episodes of the BBC TV programmes Doctor Who,^[2] Dad's Army,^[3] and Are You Being Served?.

Background

Due to the well-documented practice of wiping, many original videotape recordings of colour programmes were lost. However, in the case of the BBC, many telerecorded black & white film copies of affected programmes survived. These black & white copies were made for overseas commercial exploitation of BBC programmes. For a variety of technical and practical reasons (for example various incompatible international TV standards, and the then-high cost of videotape over that of film^[4]), black & white film copies were the preferred medium for selling programmes overseas.^[5] This practice ultimately led to many programmes which were originally made and transmitted in colour only existing in black and white form after the practise of wiping finally ceased.

Methods of colour recovery

From off-air recordings

During the 1970s, various off-air NTSC video-recordings were made by American and Canadian Doctor Who fans, which were later returned to the BBC.^[4][6][7] Whilst the quality of these early domestic video recordings was not suitable for broadcast, the lower-definition chrominance signal could be retrieved from them. This signal could be successfully combined with the luminance signal from digitally-scanned existing broadcast-quality monochrome telerecordings to make new colour master copies, suitable for broadcast and sales. In the 1990s this method was carried out by the Doctor Who Restoration Team.^[4] Several colour-restored Doctor Who serials were subsequently released on VHS.^[4] Combining the video-recorded colour signals with the monochrome telerecordings is a non-trivial task, requiring digital processing (for example matching up the different screen sizes of the two recordings).^[4] Thus, it wasn't until the early 1990s that cheaply available, sufficiently powerful computer hardware and software made this task particularly practical at that time.^[4]

From chroma crawl

Black & white TV systems predate colour, and so subsequent analogue colour broadcast systems have been designed with backwards-compatibility in mind (known as a compatible colour system).^[8] Thus, the chrominance (colour) signal is typically 'shoe-horned' into the same channel as the luminance (brightness) signal, modulated on a fixed frequency, known as the colour subcarrier. Black and white televisions do not decode this extra colour information in the subcarrier, using only the luminance to provide a monochrome picture. However, due to limited bandwidth in the video channel, the chrominance and luminance signals bleed into each other considerably, resulting in the colour information showing up visibly as Chroma Crawl, or Chroma dots on black & white TV sets.^[1] This is normally considered a nuisance in analogue broadcasting.^[9] However, since telerecordings were made from black & white TV screens and technicians at the time often decided not to apply a filter to remove this interference,^[2] these patterns are retained even in the existing monochrome film prints and theoretically contain the original colour information. (Occasionally the colour information was filtered out using a notch filter and is lost.) The idea to recover this information was originally suggested by BBC researcher James Insell.^[2]

In practice however, the recovery of this colour information from telerecordings is highly complex for several reasons. Firstly, the colour reference timing signal, known as the colour burst, is absent from telerecordings, as it is nominally off the edge of the visible screen area being recorded. This timing has to effectively be recovered since the phase of the chroma dots, which is represented by their horizontal position on the screen, determines the hue of the reconstructed colours.^[10] Distortions in the geometry of the telerecordings due to the nature of physically recording from a non-flat CRT screen onto film means that a transformation has to be applied in order to infer the original positions of the chroma dots within the broadcast.

However, these technical obstacles were finally overcome in 2008, and software written by developer Richard Russell^[11] at the informal Colour Recovery Working Group^[12] was put to use, finally resulting in the broadcast and release of colour-recovered episodes of Dad's Army and Doctor Who.^[3]

Example of the chroma dot reconstruction:

• 

  Original picture

• 

  Black and white picture with chroma dots

• 

  Quadrant calculation for colour reconstruction

• 

  Reconstructed colour picture (reconstructed with the program of Richard Russell)

See also

• Room at the Bottom#Colour restoration of the original television recording
• Reverse Standards Conversion
• Multi-Colourisation, DVD special feature on Doctor Who: Dalek War DVD Box set

References

External links

• A visual comparison showing the resulting differences between traditional colourisation and colour recovery
• Pictures of a Marconi quick-pull-down 16mm Film Recorder of the kind used to record Dad's Army and Are You Being Served