CSO

CSO is the common name for a special effects procedure championed by producer Barry Letts during the Jon Pertwee era. It was a somewhat primitive forerunner to CGI, wherein a special effect could be mixed into a live shot to suggest that an actor in the foreground was interacting with a fantastical element in the background. As the name implies it involves separating the color from one shot then overlaying that shot onto another one. Outside the BBC the process is generally known as chroma key.