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 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 format PAL and then played back. In early colour televisions, until the mid-1980s, this could lead to a problem known as dot crawl 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, Are You Being Served? and The Morecambe & Wise Show. 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 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 This practice ultimately led to many programmes which were originally made and transmitted in colour only existing in black and white form after the practice of wiping finally ceased.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 video-recordings were made by American and Canadian Doctor Who fans, which were later returned to the BBC. Whilst