DVB-T
DVB-T |
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Digital Video Broadcasting - Terrestrial (DVB-T) is a system for broadcasting of digital television via terrestrial transmitters. DVB-T is part of the family of standards of the DVB Project. As all other DVB standards, DVB-T is based on the transmission of data containers. The DVB-T system uses the same 8 MHz (or 7 or 6 MHz) radio-channels as used for analogue television.
These containers carry a flexible combination of MPEG-2 video, audio and data. The container can contain more than one television program as well as radio programs or data services. The data of the different programs are combined in a so-called multiplex. Each container carries Service Information (SI) which gives details about the programmes being broadcast. Each analogue television channel of 8 MHz can be used to transmit about 3-6 television programs.
To receive DVB-T an decoder or set-top-box is needed. The decoder receives the signal and decodes the compressed video to a signal suitable for an ordinary television.
The transmission is based on Coded Orthogonal Frequency Division Multiplex (COFDM). COFDM uses a large number of carriers. Each of these carriers is used to transmit only a portion of the total amount of data. The data is modulated on the carriers with QPSK or QAM. COFDM has the advantage that it is very robust against multipath reception and frequency selective fading. This robustness against multipath reception is obtained through the use of a 'guard interval'. This is a proportion of the time there is no data transmitted. This guard interval reduces the transmission capacity.
Because of this multipath immunity, it is possible to extend the coverage area with the use of an overlapping network of transmitter stations which use the same frequency, a so-called single frequency network (SFN). In the areas of overlap, the weaker of the two signals is considered as an echo due to multipath reception. However, the stations have to be synchronized and the echo has to fall within the guard time. Hence, if two stations are far apart, the time delay between the two signals can be large and the system will need a large guard interval.
There are two COFDM transmission modes possible in the DVB-T system. A 2k mode which uses 1705 carriers and a 8k mode which uses 6817 carriers. The 2k mode is suitable for single transmitter operation and for relatively small single frequency networks with limited transmission power. The 8k mode can be used both for single transmitter operation and for large area single frequency networks. The guard interval is selectable.
Portable and mobile reception of DVB-T signals is possible. It is even possible to mix the reception modes by using hierarchical transmissions, in which one of the modulated streams (so-called HP – High Priority stream), is given a higher protection against errors, to make is suitable for mobile reception; while the other one (so-called LP – Low Priority stream), has a lower protection. The higher protection mode will have a lower net bit rate available.
From DVB-T is also a variant developed, DVB-H, that is optimized for mobile reception on handheld portables.