Interoperability in Digital Cinema
by Michael Karagosian
©2004 Karagosian MacCalla Partners, all rights reserved worldwide
Published in the March 2004 issue of INS Asia Magazine
Interoperability is the driving factor for standards and competition when introducing a new technology. When considering digital cinema, interoperability takes on special importance with cinema owners. Cinema owners recognize that it is unlikely that all screens in a complex, and indeed, all screens within a circuit, will be changed out at one time. In fact, the process of rolling out digital cinema is expected to take many years. If systems installed in year five are to work the with cinema infrastructure built in year one, then interoperability at the system level is imperative for a successful rollout. This article explores the concept behind interoperability in digital cinema, and how this affects development of a key component, the Presentation System.
A successful strategy of interoperability must work at natural business boundaries. Manufacturers embrace interoperability when it extends beyond the boundary of their product, inviting healthy competition which in turn brings confidence to the marketplace. Manufacturers are most resistant to interoperability when it is asked for inside the boundary of their product, when it invites competition in unattractive ways. Thus, it is important to base system-level interoperability on services and traditional product boundaries.
Understanding the driving factors for interoperability, US exhibitors have presented the model for interoperability illustrated below.
Interoperability, as presented, outlines the four business domains that meet in the projection booth: Delivery, Presentation, Back Office, and Security. Of these, only Delivery and Presentation exist in film systems today.
The digital Delivery system must deliver a well defined digital cinema package that can work with all platforms. The Security system must deliver security keys such that the Presentation system can decrypt the desired content. The Back Office system gives the exhibitor the ability to automatically generate show schedules and show play lists, as well as monitor system status. These are topics which will be discussed in depth in later articles.
The Presentation system is the core of the digital cinema system. The Presentation system includes both projector and media block (a more general name for "server"). Note that the interoperability diagram does not identify the input to the projector as a significant point of system-level interoperability. Following traditional product boundaries, projector manufacturers prefer to include the media block in the presentation systems that they sell.
The Presentation System
The digital Presentation system is the equivalent of the film-system's platter and projector. When discussing the digital replacement for the platter, most engineers use the word "server". But few words have caused so much confusion among technologists. To a broadcaster, a server outputs a synchronous stream of content. To an information technologist, the server outputs either asynchronous or isochronous data. To facilitate meaningful conversations among experts of both camps, the term media block was coined in late 2002 to address the functions of storage and playout.
Two distinct media block architectures for digital cinema systems have been implemented. These are the Broadcast Model and the Datacentric Model.
The broadcast model gets it name by following the server architecture of the broadcast industry. In the broadcast industry, the server is the image rendering device. The output of the server is essentially what consumers view in their homes, after transmission over air, satellite, or cable.
A broadcast model server stores content, decompresses the image and audio data, pushes a synchronous audio stream to the cinema audio processor, and pushes a synchronous stream to the projector. Image data is transmitted to the projector using either the single-link SMPTE 292M or the dual-link SMPTE 372M interface, also known as single-link or dual-link HD-SDI (high definition serial data interface). The push-nature of the server requires it to be the synchronization point for image and audio. Most digital cinema servers protect their SMPTE 292M-style outputs by incorporating the Texas Instruments (TI) CineLinkô link encryption.
The block diagram below depicts the broadcast style server in a digital cinema system.
Broadcast-style digital cinema servers have been marketed by Avica Technology Corporation, EVS Broadcast Equipment SA, GDC Technology Limited, and QuVis, Inc.
While the common feature of these servers is their self-contained storage, this feature may not be a long-term advantage in the cinema. In the cinema environment, moving a show from one screen system to the next requires a transfer of all show-related files. With sustained high-speed hard drive transfer rates of 50MB/sec, moving 100GB of data from one screen system to the next will take over 30 minutes to complete the transfer, regardless of the speed of the network. If 4K images are used, the transfer times grow even higher. Compare this transfer time to the 15 minutes that exhibitors can achieve when manually transferring a platter from one screen to the next. As systems progress to higher image resolution and color bit depth, the large transfer time required of the broadcast model server becomes a major disadvantage of this approach.
The datacentric model borrows its approach from the IT industry. Signal processing is separated from storage, so that off-the-shelf IT storage solutions can be applied. These storage systems do not stream synchronous data, but instead transport data isochronously over a network when requested to do. The isochronous image and audio data is requested, or pulled, by the signal processing device, which then operates on the data, performing the functions of data decryption, image decompression, and fingerprinting.
While the input to the signal processing unit is isochronous in nature, the output of the signal processing unit is synchronous, usually an HD-SDI link that feeds the projector. The prime advantage of using an IT model is that large off-the-shelf redundant storage arrays with fast data transfer capabilities can be employed. These large arrays can feed multiple screen systems when high speed data transports are used.
The block diagram below depicts the datacentric server in a digital cinema system.
In 2000, QUALCOMM Incorporated introduced the first datacentric digital cinema system to the marketplace. Their system was first to employ a SAN (Storage Area Network) storage array, sending isochronous data over a FibreChannel network to their decoding module located at or within the projector.
There are several advantages to the datacentric technique. Network data transported to the decoding module retains its original encryption and compression, minimizing the bandwidth required of the link to the projector. By mounting or integrating the module inside the secure projector housing, the original encryption is maintained right up to the projector. This feature makes this method the most secure available for delivering content to the projector, while also eliminating the need for link encryption.
The use of SAN centralized storage also has the distinct advantage of allowing movies and other show elements to be rescheduled on the fly, without requiring time-consuming data transfers between screen systems. This is possible because movie files remain in the central storage/server array, while the networked data streams are electronically switched to the desired projector according to the programmed schedule.
System-level interoperability brings a 'step-and-repeat' ability to the digital cinema system. This is important if manufacturers are to gain the confidence of exhibitors. It is also important to pay attention to the design of the Presentation system to match or exceed the operational performance of film systems. Both the Interoperability Model and the Datacentric model will have important roles in the rollout of digital cinema.