Digital television

Digital television appeared as a natural evolution of analog television. Previously, the phases that constituted the production of a TV show (shooting the scenes, editing, finalizing and storing videos), broadcasting (generating the video composite, modulation, amplification, radio transmitting) and reception (the capture of the signal by the antenna, the demodulation of the television set receiver and the presentation of the image and sound to the viewer) of the signal by the user were all analog, i.e. the signals that represented the image and the sound generated in the studio were all analog, as well as the signals transmitted tothe TV receiver (Carvalho, 2006).

Nowadays, the information is generated digitally in the studio. These signals are converted into analog signals and transmitted to analog television receivers. With digital television, all of the processes are digital; thus the image, the sound and all the additional information are generated, transmitted and received as digital signals. This gives the best definition of image and sound: the image is wider than the original one (panoramic screen), with a higher degree of resolution (high resolution) and stereo sound (Graciosa, 2006, Zuffo, 2006). A digital television system is made up of a set of standards, as presented in Figure 1.1, which identifies the basic components: video and audio represent the services that are essential to the broadcasting of digital television; interactivity and the new services (e-commerce, Internet access) are added to the system by the middleware (Herbster et al., 2005). These new services, introduced by digital television, originated from data transmission with video and audio.

Introduction

The Advanced Television Systems Committee, Inc. (ATSC) was formed in 1982 by the member organizations of the Joint Committee on InterSociety Coordination (JCIC): the Electronic Industries Alliance (EIA), the Institute of Electrical and Electronic Engineers (IEEE), the National Association of Broadcasters (NAB), the National Cable Television Association (NCTA), and the Society of Motion Picture and Television Engineers (SMPTE). Currently, there are approximately 140 members representing the broadcast, broadcast equipment, motion picture, consumer electronics, computer, cable, satellite, and semiconductor industries. ATSC digital television standards include digital high-definition television (HDTV), standard-definition television (SDTV), data broadcasting, multichannel surround-sound audio, and satellite direct-to-home broadcasting (ATSC, 2008).

In 1987, the Federal Communications Commission (FCC) formed a group of television industry entrepreneurs, the Advisory Committee on Advanced Television Service, to help it with the development of new technologies for television, as well as with the planning of public policy for organizing the exploration of possible applications of the research.

At first, 23 different systems were proposed to the Advisory Committee. These systems ranged from improved systems, which worked within the parameters of the NTSC system, to enhanced systems, which added additional information to the signal, and to HDTV systems, which presented substantially higher resolution, a wider picture aspect ratio, and improved sound.

The Advanced Television Test Center, funded by the broadcasting and consumer electronics industries, conducted transmission performance testing and subjective tests using expert viewers. Cable Television Laboratories, a research and development consortium of cable television system operators, conducted an extensive series of cable transmission tests as well. The Advanced Television Evaluation Laboratory within the Canadian Communications Research Centre conducted assessment tests using non-expert viewers.

An overview of audio and video compression

MPEG-1 and MPEG-2, developed by the Moving Pictures Expert Group (MPEG) within the International Organization for Standardization, were the first international standards in the field of high-quality digital audio compression. MPEG-1 includes coding of stereophonic audio signals at high sampling rates aiming at transparent quality, whereas MPEG-2 also offers stereophonic audio coding at lower sampling rates. In addition, MPEG-2 introduces multichannel coding, with and without backwards compatibility to MPEG-1, to provide an improved acoustic image for audio-only applications and for enhanced television and video-conferencing systems. MPEG-2 audio coding without backwards compatibility, called MPEG-2 advanced audio coding (AAC), offers the highest compression rates. Typical application areas for MPEGbased digital audio are in the fields of audio production, program distribution and exchange, digital sound broadcasting, digital storage, and various multimedia applications.

Video compression is the process of converting digital video into a format that takes up less storage space or transmission bandwidth. Video compression (or video coding) is an essential technology for applications such as digital television (terrestrial, cable or satellite transmission), optical storage/reproduction, mobile television, video-conferencing, and Internet video streaming. Video compression standards make it possible for products from different manufacturers (e.g. encoders, decoders, and storage media) to interoperate. Anencoder converts video into a compressed format and a decoder converts compressed video back into an uncompressed format.

Recommendation H.264: Advanced Video Coding is a document published by the international standards bodies International Telecommunications Union–Telecommunication Standardization Sector (ITU-T) and International Organization for Standardization/International Electrotechnical Commission (ISO/IEC).

Introduction

Digital television signals are mainly transmitted over the air, which is also called terrestrial transmission, through cable systems, by geostationary satellites, using a microwave carrier, and over the Internet. For each transmission channel, a specific modulation scheme must be selected as the most appropriate, depending on the type of noise, power limitation, fading characteristics, transmission rate or cost.

Carrier waves are employed to allow efficient radiation of radio signals, because it is important to match the wavelength and the principal dimension of the transmitting and receiving antennas. The information is represented by an electrical signal, called a modulating signal, used to modify one or more parameters of the carrier. Appendix B presents a review of signal analysis and the use of the frequency spectrum by several services.

Modulation is the variation of one or more characteristics of the carrier waveform as a function of the modulating signal. The sinusoidal waveform is traditionally used as the carrier, and the modulation can be performed in three distinct ways:

• Amplitude modulation (AM), if amplitude is the carrier parameter that is varied. This is the oldest modulation scheme; its fatherhood was disputed by Lee DeForest and Howard Armstrong.

• Quadrature modulation (QUAM), if both the amplitude and the phase of the carrier are varied simultaneously. Single sideband (SSB) was the first QUAM scheme.

• Angle modulation, if either the phase (PM) or the frequency (FM) is the carrier parameter that changes. The first FM modulator was designed by Howard Armstrong in 1933.

The beginning of image broadcasting

The history of television owes a lot to engineers, mathematicians, and physicists, who have introduced an important vehicle for information transmission to civilization. Since the beginning of the nineteenth century, scientists have been investigating long-distance transmission of images, and it was with Alexander Bain's invention, in 1892, that the telegraphic transmission of an image (facsimile) was obtained.

In 1817, the Swedish chemist Jacob Berzelius discovered selenium, but it was not until 56 years later, in 1873, that the Englishman Willoughby Smith proved that this substance possessed the property of turning light into electricity. This property was used by Father Roberto Landell de Moura in his inventions (Alencar et al., 2004).

Julius Elster and Hanz Geitel invented the photoelectric cell in 1892. In Russia, Boris Rosing made similar discoveries. The system made use of moving mirrors in addition to the cathode ray tube.

In 1920, broadcasts were made with the help of John Logie Baird, from the United Kingdom, who used a mechanical system based on Paul Nipkow's invention. Nipkow's disk consisted of a device, invented in 1884 by Nipkow, that sent moving images using an electric conductor. To accomplish this, he used selenium's property to vary electrical conductivity proportionately to the intensity of light.

In 1924, Baird made the first distance broadcasts of object outlines, and, in the following year, of people's faces. In 1926, Baird gave the first demonstration to the scientific community at the Royal Institution in London, and then signed a contract with the BBC for experimental broadcasts.

Introduction

China has the world's largest population of electronics consumers. There are currently more than 400 million television sets in China, which represents around 30% of all television sets worldwide. Like other developing countries, China decided to develop its own standard in order to avoid having to pay licensing fees. Both the US and the European digital television systems, for example, depend on patented technology. Although paying to use that technology may not represent much in more developed countries, it is certainly an important issue for China. Adapting the digital television technology to the local reality is also an important reason for developing a national standard.

Efforts to develop China's digital television standard started in 1994. In 1995, the government created the high-definition television (HDTV) Technical Executive Experts Group (TEEG), with members from the research community. The first HDTV prototype was developed three years later. It included a high-definition encoder and decoder, as well as a multiplexer, a modulator, a demodulator, and a demultiplexer. This system was used to broadcast a live program celebrating the 50th National Anniversary Day in 1999. In 2001, the government started to accept proposals for a digital terrestrial television broadcasting standard. In order to evaluate the suitability of the proposals for the Chinese market, laboratory and field tests were performed, along with intellectual property analysis. Three of the submitted proposals were chosen:

• Advanced Digital Television Broadcasting – Terrestrial (ADTB-T): a single-carrier approach from HDTV TEEG.

• Digital Multimedia Broadcasting – Terrestrial (DMB-T): a multicarrier approach from Tsinghua University.

• Terrestrial Interactive Multiservice Infrastructure (TiMi): a multicarrier approach, proposed by the Academy of Broadcasting Science.

Introduction

Reliable information transmission and storage has been a constant challenge for engineers and researchers in telecommunications due to the ever increasing demands for higher processing speeds and higher reliability. Error-correcting codes (Shannon, 1948a, Peterson and Weldon, 1972) are nowadays found in most electronic digital processing equipment, from the audio compact disk (CD) (Wicker and Bhargava, 1994) to deepspace probes (Hagenauer, 1992), and more recently in digital television.

The purpose of this chapter is to focus on specific coding schemes employed for digital television, covering a minimum of background material on error-correcting codes. The term reliability in telecommunications refers to the level of signal immunity to noise and interference, and is not concerned with secrecy or authenticity of the data. Cyclic codes (Lin and Palais, 1986) include important subclasses of codes, e.g., the Bose–Chaudhury–Hocquenghem (BCH) codes, the Reed–Solomon codes, and practical low-density parity-check (LDPC) codes, and for this reason are the subject of the next section. In Section 4.5 another important class of codes, the convolutional codes, will be addressed.

Cyclic codes

Cyclic codes are among the most important codes for practical applications in engineering (Lin and Palais, 1986). Cyclic codes have been used as part of many communication protocols, in music CDs, in magnetic recording (Immink, 1991), etc. The preference for cyclic codes is a consequence of their mathematical structure based on discrete mathematics, which allows a considerable simplification in the implementation of encoders and decoders for such codes. A formal mathematical treatment of q-ary cyclic codes relies on polynomial rings (Peterson and Weldon, 1972), modulo xn – 1, with coefficients in the Galois field GF(q), in which n denotes the code block length (Berlekamp, 1968).

Introduction

Television is one of the most popular communication media in Brazil, where over 94.6% of households have at least one television set. This corresponds to more than 80 million television sets. Besides providing free access to entertainment and culture, broadcast television acts as an unifying factor of fundamental importance to a country of the size of Brazil.

After 50 years of existence, Brazilian television is undergoing significant changes with the arrival of the digital television (DTV) technology. DTV not only represents progress in terms of communications technology, but also a new concept in terms of accessing media information. Within the same bandwidth currently used by analog television channels, DTV is able to transmit different types of programs that may include high-definition television (HDTV) programs, several standard-definition television (SDTV) programs, several audio programs or even several multimedia (lower-definition) programs. One of the major novelties of DTV is the advent of interactivity between users and broadcasters/content providers. With DTV, users are able to participate in polls, play games, surf the Web and send and receive emails. In summary, besides providing a better quality of image and sound, and a large variety of services, the technology of DTV transmission allows better utilization of the spectrum, which translates into more channels in the same geographic area.

Effective action to implement DTV in Brazil started in the late 1990s with the work of the Brazilian Commission of Communications, a body of the National Telecommunications Agency (Anatel).

Introduction

The Digital Video Broadcasting (DVB) Project was created in 1993 as a consortium composed of more than 300 members, including equipment manufacturers, network operators, software developers, and regulatory agencies; it started its activities in Europe, but since then has become worldwide (Reimers, 2004). This project developed several technical solutions for coding and transmission of digital television.

For several reasons, but mainly because of market pressures, which demanded satellite, cable, and Satellite Master Antenna Television (SMATV) solutions, the development of a terrestrial transmission system was started in 1994/1995.

The word broadcasting is generally used to describe the transport of media content from a single point of origin to multiple receivers, regardless of the physical layer used for this transport. The specification for transport of Internet Protocol (IP) data in DVB networks, the interactivity solutions and return channels, the software environment, called Multimedia Home Platform (MHP), and the specifications that allow a point-multipoint distribution of different types of data to mobile devices are part of the solution.

DVB is focused on the private customer, who uses the offered services at home, but DVB broadcasting capabilities are also used for professional applications. In addition, DVB makes possible the reception of digital signals by portable and mobile devices.

The DVB standard has been adopted by the European Union and other countries, such as Australia, New Zealand, Malaysia, Hong Kong, Singapore, India, and South Africa. The United Kingdom is the country where the adoption of DVB first consolidated (Reimers, 2006).

Introduction

The Integrated Services Digital Broadcasting for Terrestrial Television Broadcasting (ISDB-T) was conceived in Japan to broadcast digital television signals. It allows high-definition television (HDTV) to be accessible for users of the mobile and wireless receivers, with high or low image definition.

The ISDB-T standard can be analyzed as the transformation of a binary signal, which is the digital signal generated by the television operators formatted by MPEG-2, into electromagnetic waves. These waves travel in the air until they reach the ISDB-T receivers, which have the function of executing the reverse process i.e. turning electromagnetic waves into digital signals that the television decoders will understand: the video and audio signals.

The transformation process is complex and attempts to guarantee a perfect signal at the receiver, which implies high quality of sound and image.

ISDB transmission

The ISDB transmission can be divided into two main phases, called channel encoding and modulation, as shown in Figure 8.1.

The first block of the system is called the channel encoder and has the function of encoding the bits to minimize the destructive effects of the communication channel. The input of this block is fed by an MPEG-2 stream. The first alteration suffered by the signal is structural, i.e. the MPEG-2 stream is rearranged in groups of predefined sizes that remain unchanged until they reach the transmission subsystem.

The method used to arrange the data accepts the implementation of special services, such as hierarchic transmission. The ISDB-T uses a multicarrier modulation method for data transmission, which has advantages over the single-carrier mode.

Introduction

Digital television signals are usually generated by cameras, when captured from reallife footage, or are produced by computer animation. In either case, there is a need for coding the source signal. Analog to digital conversion is necessary in the first case, in order to reduce the entropy of the information source. The original analog signal has an infinite number of levels, which implies an infinite storage space in the camera or studio equipment. For the second case, coding is also needed to compress the original signal for transmission.

Digital television typically generates audio and video signals, as well as the data for controlling the broadcasting and for interactivity. The audio and video signals, after being captured by the camera, are processed by a piece of equipment generically known as a source encoder. The source encoder turns the analog signal captured into a digital signal either for storage in a mass memory device, such as the hard disk (HD) or digital video disk (DVD), or for direct broadcasting to the studio.

The analysis of any communication system, including television, reveals that, roughly speaking, the systems may be represented as shown in Figure 2.1, the blocks of which are defined as follows:

• Source: this generates the information to be transmitted. The following are examples of output sources: a computer terminal, a microphone, a television camera, and a remote sensor in a telemetry system. The source is commonly modeled by a stochastic signal or by a random data generator to be transmitted.

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