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.

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.


1. Digitalization of broadcasting

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.

Figure 1: Sequence of Broadcasting Signals


2. Digital broadcasting in major regions

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

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.

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)