The Makeup of Digital Cinema Server Systems

The entire Digital Cinema System is built upon a data file-based design where all of the content is made up of data stored in files that are organized around the image frames; thus the “file” is the most basic component of the system.
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Digital Cinema server systems, in production for several years now, are reaching hundreds of placements in cinemas around the world. Digital Cinema harnesses disk based post-production, compilation, transport, distribution, and play-out technologies into an end-to-end solution.

The operational and workflow requirements for these disk based systems require specific parameters be met which are outlined in the Digital Cinema System Specification (Version 1.1), as presented by the Digital Cinema Initiatives LLC to the Member Representatives Committee in April 2007. This month we’ll explore those server technologies and workflows applicable to cinema systems, while introducing the reader to some of the terminologies relevant to Digital Cinema. The exhaustive fine details of the entire specification (from post production to projection) can be found in the 146 page DCI specification.

DIGITAL CINEMA SYSTEM

The entire Digital Cinema System is built upon a data file-based design where all of the content is made up of data stored in files that are organized around the image frames; thus the “file” is the most basic component of the system.

Early in the preparation of a release for Digital Cinema are processes such as the digitizing a motion picture film, manipulating color or other image characteristics, and conforming those final creative adjustments prior to distribution; all part of what is called the Digital Intermediate (DI) process. These various DI elements are carried to the first stage of preparing that material for presentation to the cinema projection system. This workflow results in the production of a Digital Source Master (DSM) which is then used for conversion into a Digital Cinema Distribution Master (DCDM).

How the content for the DSM is developed is up to the content provider which will utilize high bandwidth, high performance disk recorders and storage to create all the elements of a master quality level production. The DSM effectively becomes a film duplication master, a home video master, or a master for archival purposes whose elements are comprised of video essence, audio essence, and any additional data necessary to eventually produce the DCDM.

Data sets may be stored on removable drive digital disk recorders and a digital tape master for library and archive purposes. Specifications indicate that bit depth and image resolution be kept at a high level allowing the master DSM to be used for future versioning or repurposing of the content.

The stage that produces the DCDM is the output of the Digital Cinema post-production process—which should not to be confused with the feature post-production process—which creates the DSM. The purpose of the DCDM is for the exchange of images, audio and subtitles to encoding systems and ultimately to the Digital Cinema playback system.

The DCDM is a collection of raw data file formats that meet a standardized format. The files become the representation of images, audio and other information, which provides a complete and standardized way to communicate movies (compositions) between studio, post production and exhibition.

This master set of files is given a quality control check to verify that each item, such as synchronization, are correct, and that the composition is complete—since it is this format which the cinema projection system will display once it is decoded at the output of the play-out server. The DCDM files will be played back directly to the final devices, e.g., the projector and sound system, in their native decrypted, uncompressed, and unpackaged form.

The DCDM image file format is required to conform to the SMPTE Material eXchange Format (MXF), with the audio file format based on Broadcast Wave (BWF). Given the platform is film centric, the DCDM image structure must support a frame rate of precisely 24.000 Hz; however it can also support a frame rate of 48.000 Hz for 2K image content only. Each individual DCDM master’s frame rate is required to remain constant with metadata carried in the image data file format to indicate that frame rate.

Files within the DCDM set are required to carry information that provides for frame-based synchronization between each file. At a minimum, they must include information stating a “start of file” plus a continuous frame count which keeps audio, image, subtitle and other information in sync from the start of the package through the end. This synchronization requirement allows files to be packaged and delivered—for example as encapsulated-IP over satellite—in nonreal time, and then properly reassembled in a cache server at the receiving point.

With a complete and readied DCDM, the next stage is the preparation of the Digital Cinema Package (DCP); a compressed, encrypted and packaged set of files ready for distribution. When the DCP arrives at the theater, it may be placed onto central storage where it is eventually unpackaged, decrypted and decompressed to create the DCDM, whose image is visually indistinguishable from the original DCDM image.

The DCP is produced such that it can be multicast or delivered in bulk to the various theaters’ central storage well ahead of the scheduled showing. The transport mechanism might be a several hundred gigabyte disk drive which is mass duplicated and shipped to the theater house; or it might be encapsulated-IP and transmitted over satellite (for example on DVB-S2) that is received on sub-2 meter antennas at each theatre where it is then cached into a central storage server matching the requirements of the distribution company who created the DCP.

A STORE AND FORWARD SYSTEM


(click thumbnail)Fig. 1: Digital Cinema process workflowThe Digital Cinema system is basically a store-and-forward distribution system. Files can therefore be managed, processed and transported in nonreal time, which could be slower than real time or faster than real time. In the central storage caching distribution model described, as the DCP’s package of files are validated on the cache, they then await another transfer to the theatre play-out system’s local storage subsystem, where it will later be converted to the play-out version of the DCDM. Fig. 1 shows the basic flow of these various sets of data packages.

The play-out servers and projector systems are generally owned, managed and operated by the theater itself. The transformation of the DCP to the DCDM files is coordinated according to the contract booking for the actual features themselves.

Since playback and projection of the Digital Cinema content is in real time, purpose built play-out file servers, capable of meeting all the requirements of the Digital Cinema System specification, are usually assigned to each theater screen in the cinema multiplex. Components in the theater play-out system may include central storage (for packaged content in a multiplex that may be stored in one location), plus one or more local storage components and one or more media blocks.

The local storage file server holds packed content for playback, while the media block is the hardware that converts that packaged content into the DCDM as streaming data which is finally sent to the projection and sound systems accordingly. Local storage and media block components may be physically together or separated from each other.

Some distributors may deliver physical transportable hard drives to the theater where a secure transfer is made between the transportable drive and the storage subsystem by means specific to the digital cinema server manufacturer. This method is often a high speed USB connection that migrates files from one drive to another.

A local storage system must provide enough throughput to support a continuous stream of 307 Mbps for the compressed JPEG2000 image, plus 16 channels of 24-bit at 96 kHz uncompressed AES3 audio, and subtitle data to allow for non-interrupted Digital Cinema playback. At a minimum each local storage system is required to provide for retaining three features, including 20 minutes of pre-show content, with 3,000 sub pictures in PNG file format, and 3,000 Timed Text lines (as captions), per screen.


(click thumbnail)Fig. 2: Digital Cinema media blockThe heart of the play-out system is the media block, a subsystem that handles the security functions which process plain text essence or security data (such as decryption keys). See Fig. 2. Media blocks may be placed only within physically secure perimeters called Secure Processing Blocks (SPB). Media blocks may be integrated with the projection system, or they may be isolated from the project system, connected over a dual-link HD-SDI system compliant with SMPTE 372M, the 1.5 Gbps digital interface for 1920x1080 and 2048x1080 picture formats, and that it is required to be encrypted using an open international standard (i.e., AES) with a 128-bit key.

The assembly of the actual show schedules is handled by the local theater through a GUI that allows the theater operator to set the times for play-out, the order of any interstitial advertising, promos, etc. Feedback between the media block and the network operations center is handled via a secure connection, either over a private-network or a VSAT return. Information such as the health of the system, the acknowledgement of the proper key exchanges and an audit of all the actual file exchange or play-out activities is carried back to the managing entities.

Servers continue to be an extremely important part in the Digital Cinema workflow. The servers, storage components and security-management systems involved in this highly evolving portion of the world’s entertainment community are providing new opportunities for the communication and display of moving images—for Digital Cinema as well as for other venues certain to emerge over time.