Data broadcasting: Making the most of the DTV environment
It has long been felt that data broadcasting holds a key to profitability for DTV stations - at least in the early years of implementation. The television industry is clearly entering a new era in its service to the consumer. The rollout of the digital television infrastructure opens up a new frontier in communication. Two worlds that were barely connected - television and the Internet - are now on the verge of combining into an entirely new service: interactive television. Thanks to television's ongoing transition from analog to digital, it is now possible to combine video, audio and data within the same signal. This combination leads to powerful new applications that hold considerable commercial potential.
Data broadcasting (datacasting) is one element of this new environment. With modern encoders, even a high definition (HD) broadcast requires only 16- to 18Mb/s of digital bandwidth out of the 19.38Mb/s available in a 6MHz broadcast band under the ATSC DTV standard. This leaves a significant amount of bandwidth available for arbitrary data.
There are basically two ways that broadcasters can use the excess bandwidth to generate additional revenue:
- To broadcast data that enhances the appeal of their TV programming and/or TV advertising to attract more advertising dollars.
- Lease it to other enterprises that want to distribute data to large numbers of users in the broadcaster's viewing area.
In practice, both of these approaches will likely be used to varying degrees by different broadcasters.
The consumer connection A key requirement in the success of datacasting applications targeted at consumers is that large numbers of consumers have a DTV receiver that can receive and use the broadcast data. Utilizing broadcast data requires not only standards for encoding the data in the broadcast stream, but also standards for applications in the receiver to operate on the data.
Datacast applications targeted at consumers can further be classified by the degree of coupling to the normal TV programming as follows:
- Tightly coupled data are intended to enhance the TV programming in real time. The viewer tunes to the TV program and receives the data enhancement along with it.
- Loosely coupled data are related to the program, but are not closely synchronized with it in time. For example, an educational program might send some supplementary reading materials or self-test quizzes into the broadcast.
- Non-coupled data are typically contained in separate "data-only" virtual channels. They may be data intended for real-time viewing, such as a 24-hour news headlines or a stock ticker service.
Data broadcast architecture A datacasting system must meet a number of requirements to properly support enterprise-to-enterprise data broadcasting. One key factor is its underlying system architecture. In particular, it should explicitly recognize and support the roles of data provider, broadcaster and data subscriber.
The example system shown in Figure 1 consists of three primary components:
- Data source server, which allows the data provider to specify the detailed scheduling for retrieval and broadcast of individual data items.
- Data hub server, which receives data items from the data source servers and turns them over to the MPEG-2 gateway according to the schedules obtained from the data source servers.
- Data receiver, which extracts the data from the broadcast stream and applies decryption, decompression and forward-error recovery as needed.
Industry standardization efforts The ATSC Specialist Group on Data Broadcasting was charged with investigating the transport protocol alternatives to add data to the suite of ATSC digital television standards. The Specialist Group subsequently prepared a standard to address issues relating to data broadcasting using the ATSC DTV system.
The foundation for data broadcasting is the same for video, audio and PSIP - the MPEG-2 standard for transport streams (also known as ISO/IEC 13818-1). Related work includes the following:
- ISO standardization of the Digital Storage Media Command Control (DSM-CC) framework.
- The Internet Engineering Task Force standardization of the Internet Protocol (IP).
- The ATSC specification of the data download protocol, addressable section encapsulation, data streaming and data piping protocols.
The service-specific areas and the applications are not standardized. The DTV data broadcasting standard, document A/90 - in conjunction with the other referenced standards - defines how data can be transported using three primary methods:
- Data piping, which describes how to put data into MPEG-2 transport stream packets. This approach supports private data transfer methods to devices that have service-specific hardware and/or software.
- Data streaming, which provides additional functionality, especially related to timing issues. The standard is designed to support synchronous data broadcast, where the data is only sent once (so much as the video or audio is sent once). The standard is based on packetized elementary stream (PES) packets as defined by MPEG-2.
- Addressable section encapsulation built using the DSM-CC framework.
The ATSC added specific information to customize the framework for the ATSC environment, especially in conjunction with the PSIP standard, while retaining maximum commonality with the Digital Video Broadcasting (DVB) standards. The ATSC's system for data broadcast is shown in Figure 2. These methods enable repeated transmission of the same data elements, thus enabling better availability or reliability of the data.
Data download Because receivers will have different capabilities as technology evolves, methods to enable the receivers to determine if they could support data services were also developed. This type of "data about the data" is referred to as control data or metadata. The control information describes where and when the data service is being transmitted and provides linkage information. The standard uses and builds upon the PSIP standard (document A/65). A data information table (DIT), which is similar in structure to an event information table (EIT), transmits the information for data-only services. For data that is closely related to audio or video, the EIT can also contain announcement information. Each data service is announced with key information about its data rate profile and receiver buffering requirements. Both opportunistic and fixed data rate allocations are defined in four profiles. These data facilitate receivers that only present services to the consumer that the receiver can actually deliver.
Looking ahead All of the elements are now in place to make DTV data broadcasting a reality. A number of commercial applications have been proposed, and more will emerge as broadcasters learn by doing. For now though, one thing is certain: Data broadcasting will play an integral part of the future television station.
Editor's note For additional information on data broadcasting consider the following resources:
- ATSC: "Amendment No. 1 to ATSC Standard: Program and System Information Protocol for Terrestrial Broadcast and Cable," Doc. A/67, ATSC, Washington, D.C., Dec. 17, 1999.
- ATSC: "ATSC Data Broadcast Standard," Advanced Television Systems Committee, Washington, D.C., Doc. A/90, July 26, 2000.
- ATSC: "Implementation of Data Broadcasting in a DTV Station," Advanced Television Systems Committee, Washington, D.C., Doc. IS/151, November 1999.
- Chernock, Richard: "Implementation Recommendations for Data Broadcast," NAB Broadcast Engineering Conference Proceedings, National Association of Broadcasters, Washington, D.C., pp. 315 - 322, April 2000.
- Thomas, Gomer: "ATSC Datacasting - Opportunities and Challenges," Proceedings of the 33rd SMPTE Advanced Motion Imaging Conference, SMPTE, White Plains, N.Y., pp. 307 - 314, February 1999.
- ATSC Standards are available online at www.atsc.org.
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