What you’re about to read is a simplified version of how digital projection works. The thing is, that it’s all true, just simplified a bit.

Digital Cinema?

So digital cinema refers to when a cinema or movie theatre puts on a performance of a film (more commonly known as a showing) which is projected from a digital source.

Up until half a decade ago, it was very common for most cinemas to be using traditional projection units to show films. We’ve all seen this types of projectors: they’re the ones with reels of film provided on celluloid.

Reel to reel camera example

Example of a reel to reel projector. Image from http://www.film-center.com/bh1552.html

The idea here is that the film is supplied as a series of images which are burnt onto a semi-transparent reel of celluloid. The celluloid is passed between a light source (usually a bulb) and a lens. The lens magnifies the images and they would be projected onto a sheet/wall.

Most films were provided across multiple reels (Reel A was the first half of the film, and Reel B the second half), this goes all the way back to when films had intermissions, just like stage plays do. The intermission allowed the viewers to take a break for refreshment whilst the projectionist put Reel B in place. Later, the reels would be combined ether before the performance began or swapped over during the performance.

The captions are a little off (and you might have to click through to full YouTube to get them), but this scene from Fight Club explains a little about how it used to be done:

However, since the early 90’s there has been a push to supply films in a digital format; mainly to reduce the amount of work required to show them, and to reduce the chances of the medium breaking down (celluloid is very, very easy to break, stretch or destroy – just ask the BBC). This meant that a common format for distributing digital films between cinemas was developed.

Enter the DCP

DCP stands for Digital Cinema Package.

Essentially it’s a file format for digital films, which can be used by all cinemas. This file format means that digital films are no longer distributed via expensive celluloid, they’re now distributed via hard drives.

The average film can be stored on off the shelf hard drives (as long as they support up to 250GB/s transfer rate), and tend to max out at a file size of 300 GB (which is the size of 6 Blu-Ray disks).

At the time of writing, a compatible hard drive costs £39.99. Ref: Amazon

Films need to be transferred to the server (a computer that is connected to the projector) or can be played through the server’s USB port, and this is usually done via USB. Once the film is transferred to the server, it can remain there until it is no longer needed.

This all means that a cinema, regardless of the number of screens and projectors it has, needs to only ever receive one copy of a film. Which reduces the distribution costs of a film (especially since you don’t need any kind of special hardware to transport the films around).

For those interested in finding more about DCP, I’d recommend starting with this Wiki article: http://en.wikipedia.org/wiki/Digital_Cinema_Package

What can DCP do?

So, DCP is a file format. Much like MP3, DOC, TXT or MOV. This means that it is a container (or bucket) for the data to sit in. Containers work by separating the data in a file into known chunks. An example would be the MP3 file format:

  • The first section of the file contains meta data (song title, artist name, album art, etc)
  • The next section of the file contains the compressed audio (what you actually listen to)
  • The final section contains some error checking values (used by your computer to ensure that the file is complete and ready for playing)

Bear in mind that this is an extremely simplified version of the MP3 file format, though.

DCP refers to everything in terms of “reels”, so a DCP file may have many video reels, audio reels and subtitle reels. This maps to how consumer digital video technology works :Blu-Ray video has support for 1 video track [what you see], 99 audio tracks [what you can hear, usually separated by format or language] and 99 subtitle tracks [each with a label for the language].

Comparing this to 33mm film (non-digital film projection): one video track (the images burnt onto the celluloid), one audio track (provided next to the images, again burnt onto the celluloid) and no optional extras. If a studio wanted to provide subtitles, they would have to make a separate print of the film, print the subtitles onto the images and supply that. Very few films had captions/subtitles in the language of the film for the reason that this was VERY expensive to do.

Film studios or film distributors will usually send a DCP with one video track, at least one audio track, and one subtitle track. More often that not, multiple audio tracks are supplied (usually an audio descriptive track, and maybe a dubbed audio track if the film has dialogue in a foreign language).

This means that before a film performance begins, there is ample accessibility for all. It’s simply a matter of selecting a few parameters, or pushing a few buttons – not difficult at all.

A lot of cinemas even offer autism friendly showings, where the lights aren’t dimmed as much as usual, the film isn’t as loud, and customers are permitted to bring their own snacks and (for children with autism) toys too.

So before a film performance is scheduled, the cinema management know that they can cater for folks who might require subtitles or an audio description.

Many cinemas offer a headphone plugin point by each seat for the audio descriptive track so that a specific screening is not required.

Technical Issues

With all this in mind, it is easy to think that putting on a performance of a film with suitable support for the accessibility requirements would be simple.

Well, it is.

Seriously, to show a film with subtitles all that the projectionist need do is enable the correct subtitle track before the performance begins. To be fair, the subtitles can be selected whilst the film is playing but that’s not the best option.

The same can be said or a film with an audio descriptive track.

It’s just like flipping through the audio and subtitle options on a DVD or Blu-Ray film. In fact its easier, because there are functions on the projectors interface to do that for you. All you need to do is hit the subtitles button and the projector will select the appropriate subtitle track.

This is because subtitle tracks in DCPs have extra information (called Meta Data) to tell the projector what language it is in (much like DVDs and Blu-Rays do with their subtitle tracks), and digital projectors (if set up correctly) will know what country they are in, and what language is spoken in that country.

 So Why No Subs?

This is a little tricky to answer at first (mainly because I haven’t done enough data gathering), but most cinemas are reluctant to offer many film performances with subtitles/audio description (at least during “peak times”) because they fear that it would scare off customers who are not hard of hearing/hard of sight/have accessibility issues.

Due to this, many cinemas will offer a subtitled/audio descriptive performance of a film during the daytime, or at the weekend (if you can find them at all).

For those looking for subtitled showings of films, in the UK, I’d recommend the following site: http://www.yourlocalcinema.com/

Even then, I hear (no pun intended) that many of these showings, which are catering to an almost completely deaf/hard of hearing audience, will suffer “technical faults” whereby the subtitles wont be shown. But, as I’ve just mentioned (in the previous section, in fact) this is not a technical fault and even if it were, the person who could fix the issue (the projectionist) is in the perfect place (the projection room) to fix the issue.

I feel like I’m getting on a it of a rant now, but I’ve covered the basics of what digital film, so I think I’ll leave it there for now.

Before I go though, give this a share with any folks you know who might be interested in film and (if you are hearing – that is, if you have no hearing issues) leave a comment as to whether you would watch a film, at the cinema, that had subtitles enabled.

I do. in fact, I did when my girlfriend and I went to see The Imitation Game.

Right, I’d better be off. Have fun.

Related Post

Jamie is a .NET developer specialising in ASP.NET MVC websites and services, with a background in WinForms and Games Development.

When not programming using .NET, he is either learning about .NET Core (and usually building something cross platform with it), speaking Japanese to anyone who’ll listen, learning about languages, writing for this blog, or writing for a blog about Retro Gaming (which he runs with his brother)


Jamie is a .NET developer specialising in ASP.NET MVC websites and services, with a background in WinForms and Games Development.

When not programming using .NET, he is either learning about .NET Core (and usually building something cross platform with it), speaking Japanese to anyone who’ll listen, learning about languages, writing for this blog, or writing for a blog about Retro Gaming (which he runs with his brother)

  • Paper View

    I think that you are incorrect about cinema reels – I’m sure they had to be changed every 20 minutes. The intermissions were between films, as back then there were usually 2 films shown and maybe some shorts also. The DCP is only used for transporting via physical media, it is then converted back to a DCDM at the theatre. Are all the subtitles/audio in the DCP, or just the ones that are packaged up from the original DCDM (I would think it would be pointless to package German, French, Spanish audio into the DCP if only going to a UK cinema)? Also, strictly speaking it should be called captioning (same language) rather than subtitling (text in a different language to the audio).

    • Hey Karim,

      I can honestly say that I only have anecdotal information (and the clip from Fight Club) to go on for reel to reel projection, so I’ll take your word for it. If they had to change the reels that often, then that just proves how expensive and inefficient it will have become in the end.

      Indeed you are right about DCP only being used to transport a film (as it would be silly to playback a compressed format on such a scale). I’m going to guess that the subtitles and captions for the region would be included – as you say, it would be a bit silly providing subtitles that weren’t relevant to a particular region… that being said, DVD and Blu-Ray films usually contain all manner of subtitle languages, and most films have international releases, so it might be more trouble than it’s worth to filter the subtitle files on a rushed international release. Then again, I don’t know.

      You’re also right about my use of “subtitles”, as it should be “captions”.

      Other than that, the basic premise of the post remains the same: captions are provided for all digital cinema releases, but they just aren’t used for a number of reasons relating to a perceived notion that hearing folks would not be willing to sit through a film with captions on. I’ve not collected any hard, empirical evidence, but a large majority of those I asked said that it would not bother them if a cinema was showing a captioned film.