How Digital Cinema Works

Presentation System: The Broadcast Server Model

There are two models for the secure storage and playout of digital cinema content: the Broadcast Server model, and the Data-Centric model. Within the realm of the Presentation system, there is no need find a winning model -- the marketplace is very efficient for determining winners and losers. (Which leads us back to the importance of interoperabilty as described in Part 1.)

The Broadcast Server model is based on broadcast-style servers, where image content leaves the server as a decompressed, real-time stream. The server must decrypt the content sent by the studio, decompress it, and stream it to the projector. Natually, link encryption is needed to make the link between server and projector secure.

Focusing on the image and audio paths, the block diagram below depicts the Broadcast Server model. (click to enlarge)

The Broadcast Server model has the advantage of being readily available. More than one manufacturer of broadcast server products has adapted their commercial MPEG2 product to the cinema market. This convenience, however, is compromised by the industry requirement of layered JPEG2000 compression. A single JPEG2000 file can contain both 2K and 4K image data, allowing a single file to be distributed that accomodates both resolutions. (2K images are only slightly higher in resolution that full 1920x1080 high definition television. 4K images have 4 times the resolution of 2K images.) The Broadcast Server model also has the problem of not being efficient. The broadcast server must be an integral part of the security system, since it decrypts the strong encryption used by the studios. The projector by default must also be in the secure boundary, and thus both server and projector must be physically secure. The overhead presented by this design will ultimately add to the overall system cost.

The Broadcast Server model requires a secure link between server and projector. Currently, encrypted SMPTE 292M links are used, providing a little over 1Gb/sec bandwidth. (SMPTE 292M was designed for broadcast HDTV applications.) Since uncompressed full-bandwidth image data must travel through this link, the bandwidth must support the maximum needed for digital cinema. Dual-link SMPTE 292M works well for 2K, 12-bit-per-color image data, but is cumbersome for higher resolution images. Thus, for the Broadcast Server model to migrate to a 4K world, a new low cost secure synchronous transport must be developed, which could prove quite challenging.

Next: Presentation Systems: The Data-Centric Model