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Digital Cinema Specification Released

Several of the key developers of the DCI Digital Cinema Specification were on hand at the recent Digital Cinema Summit to explain the nuances of this new spec. The work specifies two resolutions for theatrical presentation: 4096 ´ 2160 (4K) and 2048 ´ 1080 (2K). Key to this level of quality were the choice of JPEG2000 for video compression, linear (uncompressed) audio, and a new expanded color space that can allow reproduction beyond that of human vision. Informally called “cap xyz,” the X’Y’Z’ color space uses 12 bits/color and can apply to laser projectors currently under development. (The prime (΄) designation refers to the fact that the color primaries are defined after gamma is applied.) At a maximum allowed bit rate of 250 Mb/s, a typical 2K ´ 24fps film would require about 150GB of storage.

Using wavelet functions and a layered compression scheme, the standard affords a reproduction “essentially identical to the original material.” Up to 16 channels of linear audio are supported, using 48 or 96 kHz sampling at 24 bits. Audio also supports a second language, commentary, and other features. Subtitles are supported, together with the use of projector-resident fonts, as well as PNG graphics.

According to Michael Marcellin, professor, Electrical and Computer Engineering, University of Arizona (Tucson, AZ) (www.arizona.edu), while there is ongoing work developing JPEG2000, the core coding specified in DCI – a constrained subset of JPEG2000 – is complete and is “essentially royalty-free.” The progressive-only coding scheme is scalable, can be lossless, and allows different resolutions to be separately and simultaneously coded. The standard also supports “progression,” where the different versions of the image can be decoded according to resolution, quality, spatial region, color component, or even a combination of these. Technically, the coding method utilizes a hierarchical wavelet transform with horizontal/vertical and low-pass/high-pass sub-sampling and bitplane coding.

The linear XYZ color space was picked to match the original axes of the CIE 1931 colorimetry definition. The standard is constrained to a flat field (i.e., no anamorphic stretch) for sampling and coding. Glenn Kennel of TI (Plano, TX) (www.dlp.com) described the characteristics of the color space definition; because the color primaries are outside of the gamut of the human visual system, the color space should be “future proof.”

After a gamma of 2.6 is applied, RGB matrixing, as defined in SMPTE RP177, is performed on the linear primaries. While peak white as delivered by film to the screen is specified by SMPTE at 12f‑L, experiments done by DCI show that this value is actually closer to 14f-L with modern film emulsions, and is actually measured at 16f-L in the “open-gate” condition.

While the gamut mapping needed to reproduce current material on future wider-gamut displays is straightforward (involving a simple re-matrixing), the reverse situation (i.e., reproducing future material captured in XYZ space on today’s displays) is much more complex – and is currently undefined. Because “hard clipping” at the display’s color boundaries would cause an undesirable change to the image’s color histogram, additional work must be done to define a color transfer function, with perhaps hue-preserving soft clipping. (Current projector manufacturers are advised to provide the capability of at least a lookup-table upgrade.) In a hallway conversation, Peter Symes of SMPTE and Thomson Grass Valley Group (Paris, France) (www.thomsongrassvalley.com) agreed that mapping down to a smaller color space will be problematic, saying that the actual conversion “will require subjective input to be developed – perhaps experimentally – and must involve technologists, producers and cinematographers.”

The DCI Specification also describes the electronic “packaging” of the program material, which is similar to that used for DVD. The goal of the specification is to have a final picture presentation that is better than an answer print (the first complete print combination of audio and video used for the producer's examination). The completed files are encapsulated in smaller contiguous “reels,” using the Material Exchange Format (MXF). With such an arrangement, producers can actually send replacement reels to theaters, even after the original release. In order to properly control these program assets, a “screen management system” is proposed that complements the theater management systems already in use.

Security is a significant part of the DCI Specification, where the generation and management of keys are important tools to combat in-theater camcorder-based theft, thought to be the biggest security hole in cinema today – much bigger than any potential digital theft. In order to protect the program material, the projector is part of a “secure media block,” with all incoming material encrypted. Any unauthorized tampering with files, the file system, server, or an exposed interface results in a “black screen.”

In conclusion, numerous praises were made concerning the DCI Specification, and its potential impact on the 175,000 motion picture screens worldwide. Supported by all seven major studios, it is said to be a “stable system,” with a “long future.” In addition to the color gamut work, some additional issues must be resolved, including the need for consistency across projector makes. So far, most of the DCI work has involved TI, with Sony now becoming more involved. An exploration of the issues surrounding the different gamuts of Xenon lamps and lasers was also suggested. Will theater images match those achieved in postproduction? They can, it was suggested, if color metadata is used, and if theaters pay attention to alignment and calibration.

Entertainment Technology Center at USC, Charles Swartz, 213-743-1530, cswartz@etcenter.org

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