Colour recovery

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 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]

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 at the informal Colour Recovery Working Group[11] was put to use, finally resulting in the broadcast and release of colour-recovered episodes of Dad's Army and Doctor Who,[3] 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:

• 
  Original picture
• 
  Black and white picture with chroma dots
• 
  Quadrant calculation for Colour reconstruction
• 
  Reconstructed Colour picture (reconstructed using software written by Richard Russell)