A digital cinema server is a computer in a cinema's projection room that plays the film from a digital file instead of a reel of film. It sends the picture to the projector and the sound to the cinema's sound system. The term also covers the network that delivers new films to thousands of cinemas, by satellite, over the internet or on hard drives. In India, digital cinema replaced film prints in most cinemas during the 2000s and early 2010s.
Why did cinemas change? A film print is a large reel of plastic film. Each copy is expensive to make and to transport, and it wears out after some weeks of shows. So small towns often got a film weeks after the big cities. A digital copy costs very little to make. So a film can open on thousands of screens across the country on the same Friday.
The film reaches the cinema as a Digital Cinema Package, or DCP. It is a set of files: the picture, the sound, the subtitles and a list that says how to play them. A feature film's DCP is usually between 100 and 300 GB, because the picture is only lightly compressed, with a video codec built for high quality. The files are encrypted, which means they are scrambled so that nobody can play them without a key.
The key arrives separately, in a small file called a key delivery message, or KDM. Each KDM works only on one particular server, and only between two dates and times. Here is a typical case. A new Telugu film is releasing on 1,500 screens on a Friday. Its encrypted DCP of about 200 GB is sent to the cinemas by satellite on the Monday and Tuesday before, and each server stores it. On Thursday, each cinema receives a KDM for its own server, valid from the first show on Friday at 6 am for four weeks. Before that moment, the file cannot be played even by the cinema. If the film runs for longer, the distributor sends new KDMs.
While it plays the film, the server also hides a forensic watermark in the picture and the sound. It is an invisible mark that records which screen played the film, and when. If someone records the film on a phone in the cinema and puts the copy online, the mark shows which cinema and which show it came from.
The server also plays the advertisements and trailers before the film, in the order set by the cinema's schedule. The shows on that schedule are the ones sold through the online ticketing platform. A film mastered in immersive audio needs a server and sound processor that support that format.