How Digital Cinema Works

Presentation Systems: The Data-Centric Model

An alternative approach to serving images in the theatre that is better geared to the multiplex cinema is the Data-Centric Model. This model can take significant advantage of new networked storage arrays in the in the data industry. Again focusing on only image and audio paths, the Data-Centric model is presented in the block diagram below. (click to enlarge)

The Data-Centric model distinguishes itself from the Broadcast Server model by using the server only for asynchronous data transfer. In fact, the "server" is really nothing more than storage. As with the Broadcast Server model, the stored content is strongly encrypted. Unlike the Broadcast Server model, data is only served from the storage array upon the request of the projector. Thus, the Data-Centric model projector "pulls" data from storage, a major distinction from the Broadcast Server model server, which "pushes" data to the projector.

Since the storage array does not decrypt the strongly encrypted data, it is not part of the security system, so does not require physical protection. Further, the link between server and projector also does not require protection, since the data travelling across it retains the original strong encryption applied during mastering. Thus, link encryption is not needed. The system becomes simpler, while also inherently more secure.

Most importantly, the link between the server and the projector can now be a standard link from the SAN (Storage Area Network) industry. The storage can also be shared among different screen systems. In the block diagram above, Fibre Channel is shown. However, the iSCSI protocol over Ethernet could also be used. While very high speed links are becoming available, they are not required. 1Gb/sec is sufficient, since the transported data remains both encrypted and compressed. Industry specifications have set the maximum data rate for compressed images at 250Mb/sec, for both 2K and 4K image data.

The use of shared standards-based SAN networks can be a plus for those exhibitors who need the ability to quickly move shows from screen-to-screen. Since no physical movement of data is required, multiple screens can be scheduled from a single unit of shared storage, up to the bandwidth limitations of the SAN. New schedules can be loaded and placed in action within minutes.

Shared SAN storage may not be for everyone, though. The cost of a SANs network must be weighed against the benefits gained. However, the Data-Centric model concept of "pulling" data versus "pushing" data is not limited to a SANs implementation.

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