Technical Tutorial
The case for compatibility
Merritt Doyle
07/30/98
Satellite Communications
Copyright 1998 by PRIMEDIA Intertec, a PRIMEDIA Company. All rights reserved.
Direct broadcast satellite services deliver high-speed video and information services to homes and businesses. The key to their true global success is the compatibility of their compression and transmission technologies. The only way to ensure the necessary compatibility is to employ international standards.
Worldwide, the estimated market for high-power direct broadcast services is staggering: the potential subscriber base numbers several hundred million homes. In the U.S. market alone, direct broadcast systems already have attracted approximately 7 million subscribers. Estimates of the market's size keep growing. The Carmel Group predicts 17 million North American households will subscribe to direct broadcast services by 2001. Paul Kagan Associates pegs the potential U.S. market at 28 million homes.
The current advantages of direct broadcast satellites over their cable competition include digital picture and sound, greater channel capacity, and some exclusive sports programming. Direct broadcast offers crisper pictures and sound than the analog signals used in conventional broadcasting, at a price comparable to cable.
However, the playing field may level off in time, when digital offerings arrive from cable providers, various telcos and wireless cable (Multichannel Multipoint Distribution Service or Local Multipoint Distribution Service). Cable operators' digital offerings are beginning to include cable modems for high-speed Internet access. The telcos are announcing video and Internet service over standard telephone lines using very high bit rate digital subscriber line technology. Wireless cable will have similar offerings to those of direct broadcast satellite services.
In addition, British Digital Broadcasting is launching a new Digital Terrestrial Television service, and U.S. broadcasters are entering the race this year with over-the-air service in the high definition television (HDTV) format. Some broadcasters favor progressive scan and some believe interlaced scanning will lead to better quality; this split leads to competing HDTV standards like 720p and 1080i. The jury is still out on this issue.
Regardless of future competition, the success of satellite direct broadcasters seems assured, not only for the delivery of video and audio but also for the potential of interactive and data services. To remain competitive and cost-effective, direct broadcasters must rely on interchangeable equipment from cooperating equipment and service providers. The key to this is interoperability, and the keys to interoperability are digital compression and transmission technologies and standards.
Compression basics Transmitting full-motion video requires an enormous amount of bandwidth. Real-time audio requires a much smaller amount. Data compression has been standard for transmission of data for a long time, and modems normally compress data before transmission. Without digital compression, digital video broadcasting would be too costly to implement. Compression is absolutely essential for digital video broadcasting to be successful.
A video frame is composed of thousands of pixels. Much redundancy exists in video frames, which change 30 times per second. Large portions of a scene are unchanged from one frame to the next. Digital compression is an efficient way of representing these changes, rather than repeating the whole picture. The more similar the adjacent frames, the more compact the result. Today, an analog video picture consuming 90 megabits per second of bandwidth can be digitized, compressed and transmitted in 5 megabits per second or less, with exceptional video quality.
Statistical multiplexing techniques squeeze even more channels into a given amount of bandwidth. They adjust the bandwidth among several channels, giving less bandwidth to slow-moving scenes and more to rapidly changing scenes.
Digital video compression makes sense because it results in faster transmission, reduced operating costs and more effective bandwidth utilization. As a result, compression offers a manyfold improvement in capacity and economics. Without compression, broadcasters could pay more than $1 million a year for a satellite transponder to carry a single channel. But with compression, the broadcaster could pay the same amount to download as many as six to 10 channels.
MPEG -2 and DVB MPEG -2 is an international digital compression standard accepted by the Digital Video Broadcasting (DVB) standards organization, as well as by the Digital Audio-Visual Council and other standards organizations. MPEG -2 takes it name from the Motion Picture Experts Group, an International Organization for Standardization committee.
MPEG -2 uses Discrete Cosine Transform algorithms that, like fast Fourier transforms, essentially decompose data into sets of wave frequencies. The compression process retains only certain principal frequencies and discards less important ones. Some detail may be lost in the process, but the crispness of the result still significantly exceeds that of any analog video transmission. MPEG -2 allows direct broadcast systems to feed as many as 10 channels of digitized, compressed television programming into the satellite capacity that would normally carry just one analog channel. MPEG -2 has been adopted worldwide, and is used by most North American direct broadcast equipment providers.
The DVB Project, begun in Europe in 1990, is composed of more than 170 organizations from 21 countries. DVB is urging a single, worldwide standard format for digital video transmission (one centered on MPEG -2). Through this and other standards, DVB seeks to establish the technical framework for systems used for digital broadcasting worldwide. The group would like to avoid a digital repeat of the diversity that has plagued analog broadcasting. The analog world is divided among an alphabet soup of formats: NTSC, PAL, SECAM and MAC.
While DVB standards are widely adopted in the international marketplace, the North American market has been built primarily on proprietary standards. The situation is somewhat complicated in the United States with the adoption of the HDTV digital broadcast standard endorsed by the Federal Communications Commission.
Common ground MPEG -2 dictates a variety of specifications for two layers of the generic, multilayer compression system for digital video illustrated in Figure 1. One is the compression encoding layer consisting of the service components (i.e., digitized and compressed video and audio plus ancillary data formed into variable length packets). The other is the system or transport layer, which consists of a multiplex of fixed-length, 188-byte packets.
The third and fourth layers (the scrambling layer, where the payloads of secured packets are encrypted, and the transmission layer, containing the modulated RF signal with appropriate error-correction coding, etc.) are not addressed directly by MPEG -2 specifications.
In Europe, the DVB standard goes several steps further in the form of Implementation Guidelines, the documents that define rules of operation. This has resulted in a highly interoperable and very robust standard, as DVB codifies the four layers in Figure 1. The DVB standards in Europe have made cable or direct broadcast satellite deployment straightforward (Table 1 presents the DVB family of standards).
In the United States, the industry has made a variety of decisions and selections regarding video compression and transport, at least as it affects direct broadcast applications. The HDTV system codifies three of the four layers, omitting the scrambling layer.
The transmission layer, as it related to satellites, requires many steps with different parameters. It goes beyond Quadrature Phase Shift Keying modulation.
The MPEG -2 standard is like a toolbox from which selections can be made for specific purposes. For example, the system designer and operator can choose from a spectrum of functionality and picture quality. A controversial example of such a choice relates to picture quality and the use of "B" frames and designed picture definition. The controversy revolves around the cost of memory to implement MPEG -2 Main Level, Main Profile operations vs. Main Level, Simple Profile operations. Vendor cooperation and agreement on these and other parameters are necessary to ensure seamless interoperability at the compression layer.
different parameters for all the functions involved. To ensure interoperability, some combination of standardized parameters and adaptive operation of system elements is essential.
Simple switching Direct broadcasters need a critical mass of consumers to break even on their investments. Regional or national protocols and proprietary standards limit the number of consumers that can buy a given technology, effectively slowing worldwide deployment of new digital services.
Consumers who are asked to pay several hundred dollars for a set-top box, plus more for the installation, want to know that the box will work with a new provider if they decide to switch direct broadcast service providers. Today, a DirecTV customer who wants to switch to EchoStar will have to purchase an EchoStar set-top box (and shelve the already purchased box for DirecTV).
DVB-compliant set-tops would interoperate with any other DVB-based service. The consumer would just have one service shut off and the other turned on, as he or she does with telephone services today. Adherence to the MPEG -2 and DVB standards, and the interoperability that implies, will fuel expansion of the direct broadcast market. The program provider could purchase set-top boxes from a variety of vendors, and they would be interoperable.
The chip chase The cost of set-top boxes must be reduced to fuel further expansion of direct broadcast services. In some countries, subsidies are being given to reduce the price of the set-top box to the consumer. One vital factor in reducing costs is better efficiency in the integrated circuits, or microchips, within set-top boxes. Dozens of chip makers are designing decoders based on the MPEG -2 standard. Recent developments have reduced the number of chips in the set-top box, while increasing their power. A set-top box on a single chip is the eventual dream, and much progress is being made toward this goal.
As an emerging worldwide standard, DVB is growing rapidly in Europe, while proprietary systems have a sizable U.S. customer base. A single, worldwide, nonproprietary standard is desirable for several reasons. It would drive down the price of set-top units because each manufacturer would build to the same standard, eliminating proprietary licensing fees. The resulting high volume would lower the price of the microchips.
Conditional access Conditional access is the next area of standards that need to be pursued, but DVB is attacking this with the DVB Common Interface standards.
A number of interoperability tests on DVB-compliant equipment have been performed in recent years at the National Association of Broadcasters convention in Las Vegas and at Intelsat headquarters in Washington. TV/COM, a leading supplier of MPEG -2, DVB compliant video encoding and decoding systems, as well as Tiernan and Scientific Atlanta, were among many manufacturers that demonstrated interoperability of equipment at the Intelsat Forums. The ready participation of the leading manufacturers in these tests shows that the industry is realizing the need and importance of interoperability. The movement is definitely toward greater interoperability of equipment.
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