- You may be looking for the Short Trips anthology.
A time signature was a unique designation for every being that marked what time period they originated from. When many individuals from across time gathered together, a time centrifuge could be used to forcibly separate them by time signature, at which point they could be more easily returned to their correct era. The Thirteenth Doctor used this process when breaking down a pocket universe that had appeared and attracted individuals from across time and space. (COMIC: Hydra's Gate)