Technical aspects of digital television systems
There are basically two standards of Digital Terrestrial Television, an American (developed by the ATSC) and one European (developed by ETSI).
Technical review to the American system
This system is based on an 8-VSB modulation. It is a vestigial sideband system (like an analog system) based on an 8-QAM modulation that extends to 64-QAM with Trellis coding.
A third digital terrestrial television standard, Japanese, known as ISDB (Integrated Services Digital Broadcasing), maybe minor or previous deployment.
The European System of Digital Terrestrial Television Broadcasting
The European system is based on the specifications of DVB-T (Digital Video Broadcast-Terrestrial), carried out by ETSI (European Telecommunications Standards Institute), and included in the document ETS 300 744, March 1997. This standard has been adopted by European countries and by countries from outside, like Australia, Brazil or India.
The following describes briefly the most relevant technical aspects of DVB-T, in its application to systems of digital terrestrial television broadcasting.
As stated in the rule itself, the DVB Project (Digital Video Broadcast) is a consortium of public and private organizations in order to establish the framework for the introduction of digital television services based on MPEG-2. This is proposed to address real needs in this area taking into account the situation and state of the markets and the economic circumstances of both the consumer electronics industry as television broadcasting. The system defines the modulation schemes and channel coding for terrestrial broadcasting services LDTV (Limited Definition Television), SDTV (Standard Definition Television), EDTV (Enhanced Definition Television) and HDTV (High Definition Television).
The development of DVB-T was based on a set of user requirements produced by the Commercial Module (Commercial Module) of the DVB Project. DVB members contributed to the technical development of the DVB-T through DTTV-SA (Digital Terrestrial Television-System Aspects), Working Group Technical Module (Technical Module). European projects like SPECTER, HD-DIVINE, HDTVT, DTTB, and other organizations developed hardware and system results, which were reported to DTTV-SA.
One of the main features of DVB-T is the use of MPEG-2 packets, which implies that it is transportable any information which is digitized (video, audio, data, multimedia, etc ...). In addition, the specifications are included in a set of return channels for users to interact with digital services received (see section on user receivers).
The system is defined as a functional block that performs the task of adapting the television signal base-band output. MPEG-2 transport to the characteristics of terrestrial transmission channel. In the figure below you can see the system blocks
System Protection Schemes
As you can see, the system consists of a large number of blocks relating to protection against errors, leading to the modulation signal for transmission over the air interface. The diffuser system input signal is MPEG-2 packets, so that the output in the receiving equipment, you also have this format. Without going into details are described briefly and interlaced encoding schemes before modulation:
* The coding system uses outer Reed-Solomon (RS (204.108)) to protect against errors and outer convolutional interlaced to disperse the packets, and thus protect the transmission of burst errors (a large number of consecutive errors, package makes the unrecoverable).
* It uses internal convolutional coding (punctured Convolutional Code), as well as internal interlaced.
Modulation scheme. Networks
Following the above schemes to protect against adverse propagation conditions, is passed to the description of the modulation scheme employed. We concentrate more on it, it can assume a key part in implementing these services. Indeed, as is well known, one of the most expensive, and therefore of greater requirement on a system optimization is via radio frequency, spectrum. Because of this lack of band, try to use all available technology at the lowest possible cost to optimize the spectrum band to use. Here is already one of the key points in the discussion of the appropriateness of using television systems via radio broadcast in front of the cable cast.
In principle, the main argument for the defense of cable television, from a technological standpoint, the big band is available in its transmission, especially in fiber optic systems. This huge capacity inherent in cable systems will be his main defense argument against their lack of mobility (not necessarily of ubiquity) of television receiving equipment. Indeed, it is unthinkable to drag a fiber optic cable when you want to watch TV from inside a vehicle on the streets of a big city, although it is arguable whether or need for this type of service, when going driving for example.
Thus, it is seen as a new generation system of this type has to present a coding and modulation scheme is very robust and consistent in order to provide the desired services with the desired qualities, knowing the amount of information that may require the transmission of television, especially high quality, and the narrow spectrum that may be available.
The modulation scheme employed is the COFDM (Coded Orthogonal Frequency Division Multiplexing). The COFDM modulation scheme is particularly suited to the needs of terrestrial broadcast channels, mainly for the following reasons:
- Can withstand high levels of multipath (found mainly in large urban centers, market potential of digital terrestrial television), with high dispersion of delays between the signals received. This also results in single frequency networks (SFN: Single Frequency Networks), where we can speak of "artificial multipath". In reality, the legislation supports DVB-T networks use both Multifrequency (MFN: Multi-Frequency Networks), in which planning is similar to that of existing analogue systems.
- COFDM also supports co-channel narrowband interference, such as that produced other terrestrial analogue services. It is also important to anticipate that there will be a time of transition in which contain a number of television broadcasting services, including analog, to a total deployment of digital systems, both terrestrial and satellite, in addition to cable services. Therefore the technical planning, in point of frequency planning and electromagnetic compatibility has to take into account this fact.
In COFDM modulated data in a large number of carriers at low speed, using FDM techniques. The reason for using multiple carriers actually come from the fact that high levels of multipath. As discussed, cities and urban centers could, in a first approximation, the main market for these networks. The reason is that it is in these large groups of buildings and structures where radio systems could charge via advantage over cable systems, which at first glance appear as its main competitors, due to the great difficulty, especially economic and logistics involved wiring a city.
Multipath phenomena are also particularly augmented by the widespread use of the known set-top TV antennas. The basic idea is that if delays are expected higher the signal, the multipath effects, it has to have a symbol duration much larger than those tolerable delay for them, which seems more appropriate to employ many low-speed modulated carriers that one can speed alone. This effect is also noticeable in the frequency domain, seeing as multipath causes selectivity in frequency, avoidable (carrier to carrier, within a narrow band channel), with narrow bandwidths.
However, it is conceivable that although the symbol period has become much larger than the largest of delays for multipath, there is still interference between symbols (ISI), as shown in figure (right). To avoid this small fraction of time in which there is interference between symbols, what is done is to insert a guard time.
Technical Issues in the Implementation of DTT Network
Transmitter Installation
In principle, the digital television transmitters used the current locations of analog television transmitters, which could be reused much of the infrastructure currently available. In some situations require a new antenna, if available outside antenna to be employed should be noted that digital signals should be combined with high power current analogue signals (at least during the transition analog-> digital) either the whole should be passed by a multichannel amplifier, which would raise problems of filtering and non linearities.
Primary Distribution
It requires a primary distribution network to transport packets from MPEG-2 television studios to re-multiplexers centers (regional variations in programming) and to centers transmitters.
It considered several possibilities, among which include fiber optic networks, PDH (Digital Hierarchy) or SDH (Synchronous Digital Hierarchy), ATM or satellite. A complete network probably consist of a combination of the possibilities discussed.
Equipment User Receivers
Probably one of the most critical requirements for adopting a new standard is the availability of equipment that support it. Indeed, a key factor in the successful implementation of a DVB-T system is the system that is attractive and new services and benefits it offers over the earlier analog systems, which is largely marked by the possibility of simple receivers have on the one hand, and versatile and offer a variety of other services.
Among the advantages over existing analog systems emphasize:
- Better use of bandwidth, which leads to the possibility of offering more channels and / or better quality.
- Related to the use of spectrum, there is the possibility of conditional access (Conditional Access), which is reflected in new forms such as subscriptions, Pay-Per-View, etc, based on user interactivity.
- Best of both image quality and audio.
- Ability to allocate part of the spectrum data or images, allowing the user access to other information (such as player statistics at a sporting event).
On the subject, it should discuss the enormous additional specification work has been done in England (as a pioneer and probably reference) in order to maximize interoperability while maintaining the compatibility with DVB. Much of this work was intended to specify an API (Application Programming Interface) for interactive services
Set Top Box
The STB is the receiving terminal to be installed in homes for the reception of DTT. In this respect are fundamental Digitag forums (Digital Terrestrial Action Group) and VALIDATE. Digitag evaluates the characteristics required to meet the user's receiver. VALIDATE is the working group that validates all the experiences of Digital Terrestrial Television, as to the compatibility of equipment from different manufacturers.
STB designs continue to add new functionality and finding ways to reduce costs. One achievement is the development of an advanced platform that enables streaming video on demand (DVD) and other applications via DVB terrestrial, and represents a breakthrough in the convergence of the recipients of the homes. The development of advanced STBs, enables low cost solutions and ease of use of DVD, digital interactive TV (with teletext more advanced functions), and MPEG-2 applications such as PPV (Pay Per View or Pay per view) and video on demand providing new levels of interaction.
Multimedia Home Platform
In 1997, the DVB Project extended its reach to the Multimedia Home Platform (MHP), which will consist of the terminal access from home (STB, TV, PC), peripherals and digital home network.
This platform will enable users to access interactive services and Internet (e-mail, chat). The platform will increase the capacity of the STB allowing to provide interactive services. The platform is a software solution that makes television more useful, fun and service to households. In addition, it will create new economic opportunities for network operators and content providers, hardware and software. There are considerable possibilities for the devices, from advanced STBs to the integrated high definition televisions. A crucial role in integrating the API (Application Programming Interface).
DVB standards offer great opportunities for manufacturers of receivers. It is likely that the initial products differ substantially. The possibilities are enormous users as they choose to receive a combination of enhanced content, high quality images and new services. DVB specifications you manage multiple methods of transmission. One possibility for users is receiving combined terrestrial / satellite, although it is unlikely at first.
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