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Colour recovery (or colour restoration) is a process which can restore lost colour to television programmes which were originally transmitted from the colour video tape which the original master was recorded on during final production prior broadcast, but for which only
black and white copies remain archived.
This should not be confused with
colourisation, in which colour is artificially added to source material that was always black-and-white (or where no colour information was preserved in the black-and-white copy), or used to enhance poor-quality original sources. Colour recovery is a newer process
and is fundamentally different from colourisation.
Most of the work has been done on
PAL
Phase Alternating Line (PAL) is a colour encoding system for analogue television. It was one of three major analogue colour television standards, the others being NTSC and SECAM. In most countries it was broadcast at 625 lines, 50 fields (25 ...
signals due to the BBC, but the concept is not fundamentally restricted to this system.
Colour recovery can be based on combining colour information from lower-quality recordings, or in the case of BBC's new method, the particular effects that are created when colour source material is encoded into the common
analogue television
Analog television is the original television technology that uses analog signals to transmit video and audio. In an analog television broadcast, the brightness, colors and sound are represented by amplitude, phase and frequency of an analog s ...
format PAL and then played back. In early colour televisions, until the mid-1980s, this could lead to a problem known as
dot crawl
Dot crawl is a visual defect of color analog video standards when signals are transmitted as composite video, as in terrestrial broadcast television. It consists of moving checkerboard patterns which appear along horizontal color transitions (ver ...
because the encoding of colour information could interfere with the underlying signal. This causes a form of distortion in the output signal displayed on the screen. This pattern is evident even if the resulting image is recorded on film, even in black-and-white film. Colour recovery looks for these tell-tale patterns and uses them to decode the original colours.
, colour recovery has successfully been applied to episodes of the BBC TV programmes ''
Doctor Who'',
''
Dad's Army
''Dad's Army'' is a British television sitcom about the United Kingdom's Home Guard during the Second World War. It was written by Jimmy Perry and David Croft, and originally broadcast on BBC1 from 31 July 1968 to 13 November 1977. It ran fo ...
'',
''
Are You Being Served?
''Are You Being Served?'' is a British sitcom created and written by executive producer David Croft (Croft also directed some episodes) and Jeremy Lloyd, with contributions from Michael Knowles and John Chapman, for the BBC. Set in London ...
'' and ''
The Morecambe & Wise Show''.
Background
Due to the well-documented practice of
wiping
Lost television broadcasts are mostly those early television programs which cannot be accounted for in studio archives (or in personal archives) usually because of deliberate destruction or neglect.
Common reasons for loss
A significant prop ...
, many original videotape recordings of colour programmes were lost. However, in the case of the
BBC #REDIRECT BBC #REDIRECT BBC
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...
, many
telerecorded black and white film copies of affected programmes survived. 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
), black and white film copies were the preferred medium for selling programmes overseas.
This practice ultimately led to many programmes which were originally made and transmitted in colour only existing in black and white form after the practice of wiping finally ceased.
Methods of colour recovery
From off-air recordings
During the 1970s, various off-air
NTSC
The first American standard for analog television broadcast was developed by National Television System Committee (NTSC)National Television System Committee (1951–1953), Report and Reports of Panel No. 11, 11-A, 12–19, with Some supplement ...
video-recordings were made by American and Canadian ''Doctor Who'' fans, which were later returned to the BBC.
Whilst the quality of these early domestic video recordings was not suitable for broadcast, the lower-definition
chrominance
Chrominance (''chroma'' or ''C'' for short) is the signal used in video systems to convey the color information of the picture (see YUV color model), separately from the accompanying luma signal (or Y' for short). Chrominance is usually represente ...
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
The ''Doctor Who'' Restoration Team is a loose collection of '' Doctor Who'' fans, many within the television industry, who restore ''Doctor Who'' episodes for release on DVD.
The Restoration Team was formed in 1992 when a small group of ''Doctor ...
.
Several colour ''Doctor Who'' serials were subsequently released on
VHS.
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).
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.
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).
Thus, the chrominance (colour) signal is typically 'shoe-horned' into the same channel as the luminance (brightness) signal,
modulated
In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the ''carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informatio ...
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 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 ...
on black & white TV sets.
This is normally considered a nuisance in analogue broadcasting.
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,
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
In signal processing, a band-stop filter or band-rejection filter is a filter that passes most frequencies unaltered, but attenuates those in a specific range to very low levels. It is the opposite of a band-pass filter. A notch filter is a ...
and is lost.) The idea to recover this information was originally suggested by BBC researcher James Insell.
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. Due to the alternating property of PAL, it is possible to narrow the colours down to 4 possibilities, requiring much less guesswork compared to e.g. NTSC. 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 at the informal ''
Colour Recovery Working Group''
was put to use, finally resulting in the broadcast and release of colour-recovered episodes of ''Dad's Army'' and ''Doctor Who'',
and subsequently two episodes of ''
The Morecambe & Wise Show'' as well as the "Party Political Broadcast (Choreographed)" sketch from ''
Monty Python's Flying Circus''
''Example of the chroma dot reconstruction:''
File:Mouse pad Remicade Infliximab.jpg, Original picture
File:mousepad_in.jpg, Black and white picture with PAL chroma dots
File:Mousepad quad.gif, Quadrant assignment for colour reconstruction (Russel's software)
File:mousepad_out.jpg, Reconstructed Colour picture (Russel's software)
See also
*
*
Reverse Standards Conversion
References
External links
A visual comparison showing the resulting differences between traditional ''colourisation'' and ''Colour recovery''
{{DEFAULTSORT:Colour Recovery
Film and video technology