Colour recovery

During the 1970s, various off-air NTSC video recordings were made by American and Canadian Doctor Who fans that were later returned to the BBC.

[4][6][7] Whilst the quality of these early domestic video recordings were not suitable for broadcast, the lower-definition chrominance signal could be retrieved from them.

[4] By the early 1990s, cheaply available, sufficiently powerful computer hardware and software made this task practical for the first time.

Black-and-white televisions do not decode this extra colour information in the subcarrier, using only the luminance to provide a monochrome picture.

This timing must be recovered as the phase of the chroma dots, which is represented by their horizontal position on the screen, determines the hue of the reconstructed colours.

Distortions in the geometry of the telerecordings resulting from recording from a curved CRT screen onto film means that a transformation must be applied 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 employed, 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.