Digital Video Broadcasting – Terrestrial (DVB-T) is clearly a superior technology for terrestrial television transmissions providing many facilities for a more efficient use of the available radio frequency spectrum which is a clear advancement over the earlier analogue transmissions.
DVB-T is the standard issued by DVB consortium for broadcasting digital terrestrial television. DVB-T system operates with the transmission of compressed digital audio, video and other data using a MPEG transport stream. Under DVB-T MPEG coded data are transmitted using OFDM modulation. Presently DVB-T standard is being used in many of the European countries, Russia, Australia, and India, North African countries, Middle Eastern countries, Taiwan and in a number of other nations.
The standard for DVB-T was first published in the year 1997 and by 2008 the technology has been adopted in more than 35 countries. DVB-T is one of the three DVB systems, the others being the DVB-C for cable and DVB-S for satellite. DVB-T is the most sophisticated one among the three systems available. The system is based on Coded Orthogonal Frequency Division Multiplexing (COFDM) and QPSK, 16 QAM and 64 QAM Modulation for its functioning. DVB-T working on these latest technologies can be regarded as the most sophisticated and flexible digital terrestrial system working as of today. The specialty of DVB-T is its ability to allow service providers to improve upon the traditional analogue coverage with a fraction of power. While earlier digital systems were unable to extend the terrestrial television to the mobile domain, DVB-T has extended the application to mobile technology. One of the superior features of DVB technology is that it enables the point to multipoint transmission of large volume of data at high rates. At the same time during such transmission, the data are secured by protecting the data against all types of transmission errors. The data that are being transmitted may be audio and/or video. In many of the applications the data will be represented in the form of files or other forms of generic information.
In the case DVB-T, 2K-Mode and 8K-Mode are the two different types of carriers available. IN DVB – T, using OFDM technology the digital data stream is split into a larger number of slower digital streams. Each of the digital streams so split is enabled to digitally modulate a set of closely spaced adjacent carrier frequencies. DVB-T can use 6, 7, or 8 MHZ channel bandwidths for transmission. Under DVB-T technology it is possible to allow the network to implement a Single Frequency Network (SFN) where a number of operators can operate on the same frequency without interfering with each other’s frequencies. This is the advantage of using OFDM as the modulation. DVB-T uses error correction systems involving the use of convolutional coding and Reed Solomon with rates ranging from 1/2 to 7/8 depending on the requirements of each case. Thus DVB-T using a combination of the three options of modulations (QPSK, 16 QAM and 64QAM), five different forward error correction rates, four guard interval options, choice between 2K and 8K carriers and the ability to operate in 6, 7, or 8 MHz channel bandwidths, DVB-T meets the requirements of the network operator to find the viable balance between robustness and capacity.
Different Types of DVB-T
Another type of DVB-T is the ‘hierarchical modulation’ where two completely different data streams can be modulated into a single DVB-T signal. Under this technique, a “High Priority” (HP) stream and a “Low Priority” (LP) stream can be used which enable the DVB-T broadcasters to target two different types of receivers offering different types of services. DVB-H is the new standard developed to address the requirements of broadcasting to handhelds.
Several variations of DVB systems for terrestrial television are used in different countries. For example ATSC is used in the USA and Canada, ISDB-T is used in Japan and Brazil and DMB-T is being used in China.
Use of DVB-T in Australia
Australian Terrestrial Television Broadcasting (DTTB) Selection Panel has recommended the use of DVB-T as the standard for use in terrestrial over-the-air broadcasting throughout Australia in preference to the competing Advanced Television Systems Committee (ATSC) standard. The COFDM feature of DVB-T has been found to be attractive for Australian broadcasters since it helps reducing the effect of “digital cliff-edge”. Digital cliff-edge implies “where at the edge of service areas or patches of bad reception a scenario of views on one side of the street with perfect pictures and on the other side with none at all”. (Christ, Berkom and Rowlings)
The adoption of DVB-T in the form of Digital Terrestrial Television Broadcasting (DTT) in Australia is supported by Digital Broadcasting Australia Limited (DBA) a private company with more than 59 member companies for the successful implementation of DVB-T in the country. This company provides a range of information and other services in connection with the implementation of DVB-T to the organizations who subscribe to is membership. The government has also provided its full support by making various major decisions for introduction of DVB-T in Australia including the enactment of the Broadcasting Services Amendment (Digital Television and Datacasting) Act 2000.
Technology being used in DVB-T
There are different technologies that are needed for video broadcasting digitally.
MPEG- 2 Coding
For a High Definition Television Signal (HDTV) there is the need for a higher data rate. Therefore in DVB-T the MPEG-2 coding is used to compress the data rate within the range of 2Mbps and 15 Mbps. Error protection options improve the data integrity. The coding becomes useful to compress large data.
Coded Orthogonal Frequency Division Multiplex modulation in DVM-T represents the multi-carrier modulation consisting of around 2000-8000 carriers with the three different modulation options. COFDM has the capability to provide a higher level of immunity for the modulated signal for protecting them against moving (Doppler) and static reflections (echoes). “A DVB-T system with around 2000 carriers would allow for a wider intercarrier spacing and thus a bigger Doppler shift before the transmitted data is adversely affected.” (Sun) DVB-T is equipped with guard intervals between the transport stream packets in order to protect the signals from echoes. DVB-T while operating with around 8000 carriers is best equipped with longer guard intervals and therefore is capable of offering better protection to signals against static reflections.
With DVB-T in use for more than a decade as a standard for video broadcasting, a latest development in the form of DVB-T2 is being tried out. This technology as an advancement to the earlier DVB-T has additional capabilities with more flexibility and a number of additional features to make the technology more compatible and future-proof.
Christ, Peter, DT BErkom and Adrian Rowlings. Digital Television in Australia – Success for DVB-T. 1998. Web.
Sun, Wai Hoong. Introduction to DVB-T Technology, Standards and R F Testing. 2007.