The Unofficial Colour Recovery Wiki

Although in principle the technique described here can recover the colour from a black-and-white telerecording, there are unfortunately a number of reasons why the results may not be as good as one would ideally like:

  1. The chroma patterning on which the process relies has the required form only when the chroma content in the two video fields is the same (or similar). This will often be the case, but in the case of fast movement, or an out-of-phase telecine insert, there may be significant differences between the fields. This can cause the process to work badly or not at all in those regions.
  2. Mechanical imperfections in the film recorder, such as the film not having come fully to rest after being advanced during field blanking, or slight movement between the two fields, also cause the subcarrier patterns to be distorted. This can affect the entire frame, or just a band across the picture near the top or bottom of the frame.
  3. Poor CRT focus, excessive spot-wobble and/or optical limitations in the film recorder can attenuate the high spatial frequencies (particularly the 216 c/aph vertical frequency) to an extent that performance is seriously impaired.
  4. Noise in the source video material and/or film grain, especially when combined with low colour saturation, can make the chroma content difficult to isolate. Similarly, diagonal luminance edges crosstalking into the chroma channel can mask or distort the true chroma content.
  5. Disjoint coloured areas (i.e. two or more coloured areas separated by regions with little or no colour) cause the process to fail because the disambiguation operation relies on 'propagating' phase information from one part of a frame to the rest. The process only works well when most of the frame contains chroma.
  6. Geometry errors in excess of about 3% may cause the process to fail, since the spatial frequencies of the chroma patterning may fall outside the passband of the various two-dimensional filters. In particular, source material must be accurately scanned; if excessively zoomed in or out performance will be seriously degraded.

Because of these and other effects one can only determine how successful the process will be in a particular case by trying it. The film recorders used seem to have been quite variable, resulting in significant differences in Colour Recovery performance, even between episodes of the same series or story.