Dateline Asia
THE LONG AND WINDING ROAD
The Future of High Definition Television in Asiaby Mark Long
copyright 1999 MLE INC. All Rights Reserved
(This article originally appeared in the July/August 1999 issue of Cable & Satellite Asia.)
For an innovation, HDTV is a pretty old concept. It was twenty years ago when broadcast engineers first proposed the worldwide transition from the analogue-based PAL and NTSC TV standards developed during the 1940s to a new high definition television (HDTV) standard with improved resolution capabilities. The earliest HDTV proposals provided for a picture display containing approximately twice the number of active lines and an aspect ratio (picture width to picture height) of 16:9 as opposed to the 4:3 aspect ratio used by conventional analogue TV transmission systems so that film-based materials could displayed on the TV screen in a wide-screen cinemascopic format. Also included were options for the delivery of high-fidelity stereo audio in a surround sound format.
It wasn't until last year, however, that HDTV became a regular part of the American entertainment universe. What exactly is HDTV, why has it taken so long for HDTV to get off the drawing boards in America, when can we expect this technology to arrive here in Asia and how will the region's transition to this new technology be implemented? These are the questions that this article sets out to answer.
In today's world, HDTV is typically regarded as a technology that was developed in the West. What may come as a surprise to some is that the roots of HDTV can actually be traced right back here to Asia. NHK Japan was the first broadcaster in the world to pioneer in the development of a complete end-to-end HDTV delivery system. During the early 1980s, this writer was invited to Japan to observe as NHK conducted a series of experimental HDTV broadcasts of its "MUSE" HDTV system through the Japanese direct broadcast satellite called "Yuri." In 1985, I also stood with Al Gore, Jr.--then the newly elected Senator from Tennessee--as NHK engineers demonstrated their MUSE system to various U.S. government officials. Although their presentation of HDTV's potential was breathtaking it ultimately failed in its attempt to convince the U.S. government to adopt the MUSE technology as the basis for a new broadcasting standard for the U.S.
The primary drawback of MUSE was the large amount of bandwidth that it required--the equivalent bandwidth of two or more satellite transponders--to broadcast a single HDTV service using conventional analogue transmission techniques. At the end of the day, the FCC decided to wait until a more spectrum-efficient technology became available. The development of digital compression technologies during the early 1990s proved to be the key to making HDTV a reality for TV viewers around the world.
During the past four years, the U.S. Federal Communications Commission has issued a series of rule-makings that formally mandated the switch to a new digital TV (DTV) standard that includes HDTV as one of its integral subcomponents. The U.S. DTV standard has also been formally adopted by the telecommunication authorities of Canada, South Korea, and Taiwan and currently is being considered for adoption in Argentina, Australia, Brazil, China, Mexico and Singapore.
America's national broadcasters, which already are transmitting some network programs in an HDTV format, have recently announced plans to increase the number of hours in which they transmit HDTV signals beginning with the forthcoming fall season. Moreover, American cable networks such as HBO are moving ahead with plans to broadcast selected programming in an HDTV format to U.S. cable and satellite TV subscribers.
The Digital Terrestrial TV Broadcasting Standard
On May 30, 1997, The International Telecommunication Union approved a new global standard for digital terrestrial television broadcasting. The new DTTB standard is based on an all-digital architecture that simultaneously accommodates the delivery of HDTV and conventional standard-definition TV within the terrestrial broadcasting environment. DTTB also is fully interoperable with various other digital delivery platforms, such as cable, SMATV, MMDS and satellite TV.
The new DTTB standard is based on a High Definition Common Image Format (HD-CIF) that uses a single matrix of samples (1920 pixels by 1080 lines). What's more, HD-CIF is not sensitive to the field and frame rates contained in the originating signal, a factor that in the past made the worldwide exchange of PAL and NTSC video source materials an expensive and time consuming task. HD-CIF also supports multi-program transmissions in existing channels through the use of digital video compression technology.
In most parts of the world, adjacent TV channels are left unoccupied to prevent interference between broadcast TV stations operating within the same general coverage area. New DTTB-based services, however, will be able to occupy these unused channels without causing interference to existing analogue TV stations. Those national telecommunication authorities that comply to the above-mentioned system therefore will not need to assign additional spectrum resources before introducing DTV services.
In Asia, each country will need to reallocate frequencies for DTTB on an individual basis. This is a process potentially fraught with peril. In Thailand, for example--where the new constitution mandates the return of all frequency resources "to the people"--the Armed Forces already are resisting any changes to their control of certain frequency allocations.
For this reason, the ITU did not mandate the phasing out of existing analogue TV transmissions within a ten-year time frame as proposed in the U.S. The transition will obviously take longer here and will be dictated by national rather than regional policy. Eventually, however, the old TV channel frequencies will become available for reassignment to other communications services, an important development here in Asia where the wireless transmission of telecommunication signals is the dominant mode of delivery.
Looking Through A Glass Onion
Once DTTB finally comes of age here in Asia it will use the same MPEG-2 compression standard that today's digital DTH satellite services already employ. Because of its highly flexible family of compression tools, MPEG-2 can readily be compared to an onion. When we peel back MPEG's onion, a complex structure of inter-related Layers, Levels and Profiles become evident.
MPEG-2's layered structure permits the simultaneous transmission of standard-definition (SDTV) and high-definition (HDTV) signals. The low-resolution "base layer" delivers SDTV signals with the 4:3 aspect ratio that conventional TV sets are capable of displaying. The higher-resolution "enhancement layer" merely contains the additional information required to display the same TV program at higher resolution and with a 16:9 cinemascopic aspect ratio that the new HDTV screens are capable of displaying.
One step up from the base and enhancement layers are MPEG-2's High and High-1440 Levels, which support high definition TV (HDTV) transmissions using a 1920 x 1080 sample matrix as well as a higher resolution 960 x 576 sample matrix that can be viewed as a sort of half-way house between SDTV and HDTV. A lower resolution sample matrix called the Main Level, which does not incorporate an enhancement layer, also is available for the transmission of digitally-compressed SDTV signals.
On step down from the base and enhancement layers are the five groups of compression tools called the Profiles: Simple, Main, SNR Scaleable, Spatial Scaleable and High. The Main Profile may use up to 1920 pixels per line at High Level or up to 720 pixels per line at Main Level. In America, broadcasters currently transmit HDTV signal using MPEG-2's Main Profile, High Level (MP@HL). Here in Asia, each national telecommunication authority will select from amongst the various compression tools that MPEG-2 offers. It is conceivable that some of the less developed nations may elect not to offer HDTV at all, preferring to conserve scarce spectrum resources by limiting terrestrial broadcasts to standard definition signals in a digital format. This approach certainly will be considered in countries which have an insufficient base of high-income households that could afford to purchase a new HDTV set. In this case, regional satellite TV programmers will greatly benefit from MPEG-2's layered structure which permits SDTV and HDTV signals to be broadcast simultaneously within the same digital bitstream.
Because of the mutual compatibility between today's digital DTH delivery system and tomorrow's HDTV standard, satellite broadcasters also will not have to wait for national telecommunication authorities to set standards before the satellite delivery of HDTV signals can begin. Satellite-based HDTV transmissions will be implemented first in those locations in Asia where the programmers perceive there to be a potential subscriber base with the wherewithal for purchasing a new HDTV set. Since the purchase of high-cost luxury item is cyclical and directly relates to the fortunes of the local economy we can expect broadcasters to wait for favorable economic forecasts before introducing HDTV services anywhere in Asia.
Competing delivery platforms such as cable, SMATV and MMDS will have to wait until their respective national telecommunication authorities establish the requisite regulations before HDTV services can be introduced. The operators of these systems will not be eager to make a major investment in the set-top boxes that will required to convert HDTV satellite signals to a signal that is compatible with each subscriber's set-top box without knowing whether or not the hardware will be compatible with forthcoming national standards. In particular, system operators will want to pass terrestrial broadcasts directly down the cable to those subscribers equipped with HDTV sets without having to modify the incoming terrestrial TV signal in any way.
The Competing Terrestrial Standards
The type of equipment that cable, SMATV, and MMDS systems will require to do this will depend on which of two competing systems is selected for use in their respective countries. Based in Western Europe, the Digital Video Broadcasting (DVB) Group has established several specifications governing the use of the MPEG-2 digital compression standard for HDTV broadcasting purposes. The DVB specifications provide for the cross-platform portability of digital video, audio, and data signals so that any DVB-compliant bitstream can be transported from one operating environment to any other without requiring any baseband interfacing. (In this context, "baseband" means a frequency band containing information, either prior to the modulation of the information onto a radio frequency carrier or following demodulation at the receive end.)
DVB-C is a cable broadcasting system for television, sound, and data services using standard cable TV distribution frequencies and bandwidths. DVB-C uses quadrature amplitude modulation (QAM), a form of amplitude-shift keying where the amplitude and the phase of a series of baseband pulses are modulated to represent the message. QAM is used for cable distribution systems because it is more spectrum-efficient than QPSK in bandwidth-constrained cable and SMATV environments. A single 8-MHz-wide European cable TV channel, for example, can accommodate a payload capacity of 38.5 Mbit/s if 64-QAM is used as the modulation scheme. The "64" in 64-QAM refers to the number of discrete signal-state values of vector magnitude that the QAM signal supports. Other levels of QAM, such as 16-QAM, 32-QAM, and 128-QAM, also may be employed.
DVB-CS is the DVB specification adapted from DVB-C (cable TV) and DVB-S (satellite) that is applicable to SMATV installations. The DVB-CS standard codifies several different methods for adapting digital signals for distribution through SMATV systems. Each of these methods is tailored to conform to the technical characteristics of bandwidth-limited SMATV channels.
In the U.S., the DTV digital modulation subsystem for terrestrial broadcast applications is based on 8-VSB (vestigial sideband) transmission technology, which ensures a broad coverage area, reduces interference with existing analog broadcasts and provides immunity from interference into the digital signal. Terrestrial broadcasters using the American system will transmit at a maximum bit rate of 19.28 Mbit/sec, which can support a single HDTV program or as many as five Standard Definition Television (SDTV) programs with a visual quality that is superior to analogue NTSC TV signals.
For cable TV distribution, DTV signals transmit at a higher data rate mode (38.56 Mbit/sec) that uses 16-VSB to permit the transmission of two HDTV signals or multiple SDTV signals over a single 6-MHz-wide cable TV channel. The higher bit rate is possible because the cable environment is more robust than the terrestrial TV environment.
The Big Squeeze
If we assume the use of 8 bits for the video luminance component and 4 bits for each of the two color difference components (Cr & Cb), the transmission of 60 pictures per second will require an uncompressed data rate of almost 2 Gbits/sec for the active video only:
1,080 lines x 1,920 pixels x 60 frames per sec x 16 bits (8 luminance & 8 chrominance) = 1,990 Mbits/sec
A compression ratio in the order of 50:1 therefore will be required in countries such as South Korea, Japan, Myanmar, or the Philippines to transmit an HDTV signal within the 6-MHz-wide bandwidth of a single terrestrial or cable TV channel. For those other Asian countries which specify up to an 8-MHz-wide bandwidth for a single terrestrial or cable TV channel and which transmit at 50 frames per second, the required compression ratio will be substantially less.
Like MPEG-2, the DTTB standard will feature a packetised data transport structure that permits the transmission of virtually any combination of video, audio and data packets as well as the optional selection of progressive rather than interlace video scanning for computer interoperability. This provides terrestrial TV broadcasters and cable TV operators with tremendous flexibility to provide a wide variety of video, audio, voice, data and multimedia services. Many of these services can be provided concurrently with a full HDTV program service, while others may be provided in place of an HDTV program service at different times of the day.
For example, a national Asian TV channel station could broadcast HDTV programs during the evening "prime time" viewing hours. During other portions of their schedule, the station may elect to deliver as many as five SDTV programs simultaneously, some of which could provide local schools and individual with educational TV services.
DTTB also will support the delivery of ancillary data services, such as weather forecasts or stock quotes that would be available only to those viewers who wished to subscribe to them. Broadcasters also may elect to transmit CD-quality audio services as part of its 19.28 Mbits/sec digital bitstream.
Each transport packet will have a fixed length and contain a data "payload" preceded by a transport header that identifies the contents of each packet and the nature of the data that it carries. The transport header field contains both a fixed-length link layer and a variable-length adaptation layer. These fixed and variable length components provide the required flexibility for the allocation of channel capacity among various video, audio and auxiliary data services. The entire channel capacity also can be reallocated in bursts for data delivery as may be required to authorize a universe of decoders just prior to the airing of a pay-per-view event.