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"Digital movies sometimes differ in size, even if the films are of the same length. How is that possible?"

According to DCI recommendations a digital cinema file (Digital Cinema Package [DCP]) includes an image file, a sound file, optional additional 'dub' sound files and/or subtitle files, plus a number of meta data files that ensure accurate projection. The image file is the largest file in the DCP.

Before the film is made into a DCP the image file is compressed using JPEG2000 compression, reducing the file size to enable easy transport and processing by play out servers. Before compression the image files of a film are about 1.5 TB in size. Depending on the amount of compression that can be applied, the final file size will be approximately 35 to 250 GB. The source material is decisive in this. Naturally the compression may not become visible. The method of capturing (35mm film, digital), post production (special effects) and colour all affect the materials that are delivered for DCP mastering and therefore the compression that can be applied.

The resolution (the number of pixels) of images can vary. For digital projection in cinemas this is 2K (2211840 pixels) or 4K (8847360 pixels). Naturally 4K content requires a lot more data, even after compression.

A 3D film is generally 'large' because two images are needed. One for the left eye and one for right eye. The sound file can contain multiple tracks. For stereo sound there are two tracks, for a 5.1 mix six. Subtitle files have almost no influence on the size of a DCP. Their file size is only a few kilobytes.

René Stapel
Nedcipro
24-2-2011

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