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1.1 Scope
This Report provides tutorial information and an overview on the subject of digital terrestrial television broadcasting (DTTB). The document describes a system designed to transmit high quality audio and video services over a single 6, 7, or 8 MHz broadcasting channel and provides a tutorial on the technologies that support the Recommendations developed by Task Group 11/3 during the period 1992 through 1996. It also provides a summary of the state of development in systems specifications and plans for service implementation up to the end of 1995.
The majority of established broadcasters use terrestrially-based emission systems operating in the VHF/UHF frequency bands. The issue of delivery of high-definition television (HDTV) picture signals and associated sound services within a single 6, 7, or 8 MHz VHF/UHF channel resulted in a review of the application of digital coding techniques in terrestrial transmission.
The migration from a television service dependent primarily on the application of analogue technologies to one that is based on digital technologies has been evolving over the past thirty years. This television service migration is part of a natural outgrowth of the convergence of the television, telecommunications, and computer arts and sciences through the shared use of digital technology.
The input and output signals of television systems, at the camera and at the receiver, respectively, are inherently analogue. Thus, the question "why digital?" is a natural one.
While signal degradations in the analogue signal are cumulative and the characteristics of the degradations make them difficult to distinguish from the video signal, the ability to regenerate a digital pulse train exactly renders the digital signals theoretically immune to impairments from external sources. Digital bit streams can be interleaved within a single channel. This interleaving process allows for the emission, transmission, storage, or processing of ancillary signals along with the video and associated audio. Further, compression techniques based on redundancy reduction can be applied to digitized video and audio services allowing the possibility of transmitting one HDTV service or multiple standard services in an existing broadcasting channel.
The arrival of second and third generation component and composite digital video tape recorders, switchers, animated graphics and special effects machines and agreement on a serial digital signal interface by 1990, have hastened the move to implementation of the all-digital production facility. Digital production and use of digital tape recorders moved the broadcaster's practice on multi-generation editing from five generations of post-production editing using analogue technology to tens of generations using digital technology. The application of digital techniques has reduced camera set-up time from hours to near-instantaneous. Digital library systems made the location of recorded media transparent to the user. Computer control of the entire process penetrated deeply into the programme generation and distribution facility bringing with it precise control and function repeatability.[1]
The only domains in broadcasting left solely to the analogue world had been interplant transmission and final transmission to the consumer. These last barriers were overcome in the early 1990's with the application of digital compression technology, generally constructed on the application of discrete cosine transform (DCT) based encoders and the use of quadrature amplitude modulation (QAM) and related multi-level modulation techniques.[2]
By 1990, efforts within North America to find a means of transmitting a HDTV image within the existing 6 MHz bandwidth, UHF television channel focused on the use of digital data compression and modulation schemes to meet system requirements. Practical feasibility demonstrations of different systems in North America were quickly followed by similar demonstrations in Europe and in the Asia-Pacific region.
By mid-1991, reports of work being done in the United States, in the Nordic countries, in the United Kingdom, France, Italy, Japan, and in other parts of the world, showed that bit-rate reduction schemes on the order of 60:1 could be successfully applied to both HDTV source images and conventional television source images. The results of this implied that HDTV images could be transmitted in a relatively narrow-band channel in the range of 15-25 megabits per second (Mbits/s) and that conventional television services could be offered at rates ranging from 1.5 Mbits/s to 12 Mbits/s depending upon the service quality goals. Using standard, proven modulation techniques it would be possible to transmit either a single HDTV programme or multiple conventional television programmes within the existing 6,7, and 8 MHz bandwidth channels provided for in the VHF and UHF television bands.
In the period between 1991 and 1995, the development of related standards with common system elements for digital satellite, cable and terrestrial broadcasting had been undertaken worldwide. ITU Recommendations developed by Task Group 11/3 addressed the common elements of the digital terrestrial television broadcasting system. Specifications for digital satellite and cable broadcasting services were then in the final stages of approval in several areas of the world and are reflected in ITU Recommendations and regional standards. Broadcast services compliant with these standards were also in operation in several parts of the world. Specifications for digital terrestrial television broadcasting having common system elements with those for satellite and cable were also in an advanced state, and these were expected to be completed in 1996.
By 1996, plans for the introduction of digital terrestrial television broadcasting services were in an advanced state in a number of countries.
These advances in communication technology brought digital transmission of television services to a practical reality. The commonly held view is that the application of digital technology to television sciences provides higher picture quality and sound quality than conventional analogue terrestrial television transmission, and at the same time, increases the efficiency of the use of the spectrum by allowing multiple programme services to be broadcast in current single-programme channels.
For digital television services to be successful, there must be a consensus on standards in the areas of source and channel coding, modulation methods, content identification, and error protection and correction. In addition, it is important to consider harmonization with other media.
References
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