All six 625-line transmission tapes of 'The Time Monster' were wiped within five years of transmission.
However, TV Ontario in Canada subsequently returned their tapes of the story, converted to the North American 525-line NTSC video standard by BBC Enterprises in the mid-'70s.
As can be seen from this sequence, the pictures from these tapes are quite soft and washed out.
Movement is both smeary and juddery, due to the way the early converter blended and repeated pictures in the conversion process.
Engineers at BBC Research and Development produced a system called Reverse Standards Conversion to help restore these tapes.
A complex mathematical process analyses the 60 pictures per second on the NTSC tapes, unpicking the blended and repeated pictures, and outputting an approximation of the original 50 pictures per second that would have been seen on the PAL transmission tapes.
The movement is significantly better now, but there are high levels of picture noise.
Despite this, the RSC output provides a good starting point for the picture restoration.
After colour correction, noise reduction and manual re-touching, the clip is significantly improved over the NTSC original.
In 1983, a 625-line copy of episode six was discovered on a tape used to train BBC studio crews.
Although taped from a colour episode, it had been recorded on a low-band broadcast format designed to record monochrome pictures.
Although some colour can be seen, it is not usable in this form.
However, the detail levels are much higher than the NTSC conversion and the movement rendition is just as good as the original.
By taking just the black and white part of this tape and combining it with just the colour part of the restored RSC output, a much improved full-colour version of the episode can be produced.