Technologies and Services
on Digital Broadcasting (1) |
Significance of Digital Broadcasting |
"Technologies and Services
of Digital Broadcasting" (in Japanese, ISBN4-339-01162-2) is published
by CORONA publishing co., Ltd. Copying, reprinting, translation,
or retransmission of this document is prohibited without the permission
of the authors and the publishers, CORONA publishing co., Ltd.
and NHK.
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This issue features the first of a new series of articles
on state-of-the-art digital broadcasting technology. Researchers
at NHK have just published a new book titled "Technologies
and Services on Digital Broadcasting", whose chief editor
is STRL's Director General Dr. Osamu Yamada. Although the
book is written in Japanese, it has lots of useful information
on digital broadcasting technology, from basic techniques
to application systems. The editors over the next several
issues will serialize the Japanese book into English articles.
This issue's article has two sections: an introduction to
digital broadcasting technology and a compression technique
for video signals. |
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1. Digitalization of broadcasting |
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The era of total digital broadcasting, from program production to
digital receivers, will soon be upon us as a result of advances in image
compression technology, higher integration and improved performance
in LSI devices, and the rapid progress in digital technologies including
the development of digital transmission systems.
Figure 1 outlines the sequence by which broadcast signals are transmitted.
This sequence begins with the relay of video signal by a camera on the
scene or in studio and continues through program production, editing,
storage, network operations, transmission, and reception. In recent
years, broadcast studios have introduced advanced digital technologies
and functionalities that will allow processing of broadcast signals
in the studio and over relay networks. The digitalization of broadcast
signals sent to households has also become a major concern in many countries.
Some examples of the many advantages of digital broadcast signals are
multichannel broadcasting capability, high image quality, sophisticated
functions, convergence of broadcasting with other media, and efficient
spectrum utilization.
The International Telecommunication Union Radiocommunication Sector
(ITU-R), which debates the merits of broadcast systems from an international
perspective, has been the scene of a longtime controversy on standardization,
particularly among its membership from Japan, the United States, and
Europe. An agreement, however, has finally been reached, and international
standards are now being formulated for satellite, terrestrial, and cable
systems for each of these regions. Intense competition has subsequently
evolved with each of these groups trying to promote its own system throughout
the world. In this regard, a number of countries including Singapore,
Hong Kong, and Brazil have already begun comparison trials of the three
systems for digital terrestrial television broadcasting, the broadcast
signal of greatest importance.
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Figure 1: Sequence of Broadcasting Signals |
2. Digital broadcasting in
major regions |
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The United States was first to broadcast multichannel digital television
signals by satellite in 1994, and since then this form of television
service has been expanding. In Europe, multichannel digital television
of a similar type has been broadcast since 1996.
In the field of digital terrestrial broadcasting, a television service
based on the DTV (Digital TV ) system and including high-definition
television (HDTV) has been operating in the United States since November
1998. In Europe, the British Broadcasting Corporation (BBC) began a
multichannel service for standard television slightly earlier in September
1998 using the European system, called Digital Video Broadcasting Terrestrial
(DVB-T).
In Japan, digital broadcasting standards for satellite, terrestrial,
and cable television (CATV) broadcasting have been studied since 1994
by the Digital Broadcasting Systems Committee of the Telecommunications
Technology Council, an advisory body of the former Ministry of Posts
and Telecommunications (MPT) and the present Ministry of Public Management,
Home Affairs, Posts and Telecommunications (MPHPT). As a result of these
studies, PerfecTV, which uses a Communication Satellite (CS), began
operation in June 1996. PerfecTV has since merged with the other CS
broadcasters to form SkyPerfecTV, which currently provides several hundred
channels of programming.
There has also been extensive development of digital broadcasting using
Broadcasting Satellites (BS) in Japan. On May 30, 1997, the Radio Regulatory
Council (likewise an MPHPT advisory body) submitted a report titled
"The Need for Digital Broadcasting with a Focus on HDTV via the Second
Launched BS-4 satellite to be Launched in 2000." In line with this report,
standardization work got under way at the Telecommunications Council
and the Association of Radio Industries and Businesses (ARIB), and a
report on transmission systems submitted in February 1998 was turned
into a ministerial ordinance. Then, in December 2000, a diverse array
of BS digital broadcasts began using four BS transponders. These broadcasts
included seven HDTV programs, three standard TV programs, dozens of
radio programs, and various data broadcast programs.
Japanese research into digital terrestrial broadcasting has been keeping
abreast of similar research in European and American development in
this area, and a report on digital terrestrial broadcasting was submitted
by the Telecommunications Technology Council (another MPHPT advisory
body) in 1999. Slightly earlier, in June 1998, a report of the Advisory
Committee on Digital Terrestrial Broadcasting, a private advisory body
of the MPT Broadcasting Bureau's Director-General, stated that digital
terrestrial broadcasting should begin in 2003 in Kanto, Chukyo, and
Kinki, the three largest metropolitan areas in Japan.
3. Features of digital broadcasting
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Compared with conventional analog systems, digital broadcasting is
distinguished by the following features.
(1) Robust versus noise
In analog broadcasting, a weakening of the received signal means degraded
picture quality in the form of noise on the television screen. As shown
in Fig. 2, a digital signal needs only to identify itself as a "1" or
"0" making digital broadcasting more robust to noise compared with analog
broadcasting.
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Figure 2: Digital signal and noise |
(2) Large band compression of video and audio signals
ITU-R recommends the same compression technique for both video and audio
signals, namely, MPEG-2. In the digital compression of video signals,
however, the manner in which a disturbance appears depends on the characteristics
of the picture. Recent MPEG-2 image-compression techniques have achieved
compression ratios of 1/20 for standard TV and 1/60 for HDTV. Research
on MPEG-2 continues with the aim of further improving compression ratios
while maintaining an adequate level of picture quality.
(3) Error correction techniques not possible with analog signals
Basically speaking, noise cannot be removed in analog broadcasts. In
digital broadcasts, however, it is possible to correct bit errors caused
by disturbances on the transmission path by using error correction techniques.
Here, only bit errors that are too large to be corrected will actually
be labeled as "errors." While extra parity bits that must be transmitted
for the sake of error correction may be viewed as disadvantageous from
the viewpoint of transmission power, the obtained correction effect
essentially outweighs the advantage of not sending these bits. Error
correction has become an indispensable technology for digital systems.
(4) Identical method of handling video, audio, data, and control
signals
Digital signals consist of signals of "0" and "1" bits that are transmitted
in groups called packets within which the type of digital signal is
indicated. As result, all signal types can be handled in the same way.
This characteristic makes it easier to add new services.
(5) High-performance data broadcasting
In data broadcasting provided by conventional analog channels, such
as teletext broadcasting that uses the vertical blanking period of the
TV signal, the transmission capacity is quite small, at about 11 kbps
per scanning line (1H). Satellite and terrestrial digital broadcasting,
on the other hand, are capable of delivering advanced data broadcasting
services having transmission rates of several Mbps. Considering, moreover,
that telephone lines or their like can be effectively used as uplinks,
many digital data broadcasting applications can be envisioned, such
as receiving immediate responses from viewers and providing easy access
to the Internet.
(6) Easy to scramble signals
In contrast to the difficulty of scrambling an analog broadcasting signal,
scrambling a digital signal can be scrambled so that only subscribers
can receive the content of a broadcast by simply adding a specific "0"
and "1" signal to the original digital signal.
(7) Low transmission power
Because digital signals are robust to noise as mentioned in item (1)
above, transmitter power can be lowered. Although actual transmission
power depends on bit rate and send/receive conditions, it can generally
be said that digital terrestrial television broadcasting can reach a
particular service area for a transmission power of about 1/10 that
of an equivalent analog broadcast.
(8) Simplified channel planning
Because low power transmission is possible, there is little affect on
adjacent channels or on identical channels in different areas. Channel
planning is therefore easier and more channels overall can be used.
(9) Modulation systems robust against ghosts and fading
Ghosts, which is a form of interference caused by buildings, are a major
problem in digital terrestrial broadcasting. Assuming that the maximum
bit rate is desired for a limited frequency bandwidth, it is impossible
to deal with ghosts by using the conventional single carrier modulation
system. Instead, multi-carrier orthogonal frequency division multiplexing
(OFDM) can be used to eliminate ghosts. OFDM can also be applied to
the mobile reception environment in general.
(10) Applicable to LSI technology
LSI devices are achieving higher levels of integration and higher speeds
each year. Since most tasks performed by digital broadcast receivers
consist of digital signal processing, smaller and cheaper receivers
can be expected.
(11) Sudden drop in service quality just beyond service area
In analog broadcasting, moving away from the transmitting antenna means
more noise on the television screen and gradual deterioration of the
picture as receive power weakens. In digital broadcasting, on the other
hand, the use of error correction techniques results in a steep curve
for the relationship between receive power and bit error rate on the
receiver side. As a consequence, exceeding the receive-power limit results
in complete loss of reception as opposed to a gradual deterioration
in picture quality.
(12) New frequencies needed for digital broadcasting
At present, a large range of frequencies are being used for analog broadcasts
in Japan's terrestrial broadcasting system and few frequencies are available
for digital broadcasts. For digital terrestrial broadcasting, it will
therefore be necessary to appropriate some of the frequencies currently
being used for analog broadcasting or to allocate new frequencies for
digital broadcasting. It has been decided to reserve low frequencies
in the UHF band for digital broadcasting in Japan.
(13) Users must purchase new receivers
Since conventional analog receivers cannot, of course, be used for receiving
digital broadcasts, users will have to purchase receivers especially
designed for digital broadcasts.
(14) Facility investment required by broadcast stations
Broadcasters will have to invest in various types of equipment including
video/audio encoding devices, program-production equipment for data
broadcasting, operating equipment, and transmission equipment.
Although items (11) to (14) above represent disadvantages regarding
the digitalization of broadcast signals, there is a huge movement throughout
the world to realize high-quality, multichannel, high-performance broadcast
systems that will eventually overcome these shortcomings.
(Dr. Osamu Yamada)
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