DVB-T is an abbreviation for Digital Video Brodcasting Terrestrial
DVB-T is a technical standard, developed by the DVB Project, that specifies the framing structure, channel coding and modulation for digital terrestrial television (DTT) broadcasting. The first version of the standard was published in March 1997 and in the twelve years since then it has become the most widely adopted DTT system in the world. It is a flexible system that allows networks to be designed for the delivery of a wide range of services, from HDTV to multichannel SDTV, fixed, portable, mobile, and even handheld reception. The DVB Project has now created a next generation terrestrial specification, DVB-T2, designed to meet the needs of countries after they have completed Analogue Switch-Off (ASO).
DVB-T, in common with almost all modern terrestrial transmission systems, uses OFDM (orthogonal frequency division multiplex) modulation. This type of modulation, which uses a large number of sub-carriers, delivers a robust signal that has the ability to deal with very severe channel conditions. DVB-T has technical characteristics that make it a very flexible system:
• 3 modulation options (QPSK, 16QAM, 64QAM)
• 5 different FEC (forward error correction) rates
• 4 Guard Interval options
• Choice of 2k or 8k carriers
• Can operate in 6, 7 or 8MHz channel bandwidths (with video at 50Hz or 60Hz)
Using different combinations of the above parameters a DVB-T network can be designed to match the requirements of the network operator, finding the right balance between robustness and capacity. Networks can be designed to deliver a whole range of services: SDTV, radio, interactive services, HDTV and, using multi-protocol encapsulation, even IP datacasting.
Whilst not originally designed to target mobile receivers, DVB-T performance is such that mobile reception is not only possible, but forms the basis of some commercial services. The use of a diversity receiver with two antennas gives a typical improvement of 5 dB in the home and a 50% reduction in errors is expected in a car. The DVB-H system for mobile TV was built on the proven mobile performance of DVB-T.
The use of OFDM modulation with the appropriate “guard interval” allows DVB-T to provide a valuable tool for regulators and operators in the form of the “single frequency network” (SFN). An SFN is a network where a number of transmitters operate on the same RF frequency. An SFN can cover a country, such as in Spain, or be used to enhance in-door coverage using a simple “gap-filler”.
One final technical aspect of DVB-T worth mentioning is its capacity for Hierarchical Modulation. Using this technique, two completely separate data streams are modulated onto a single DVB-T signal. A “High Priority” (HP) stream is embedded within a “Low Priority” (LP) stream. Broadcasters can thus target two different types of receiver with two completely different services. For example, DVB-H mobile TV services optimised for more difficult reception conditions could be placed in the HP stream, with HDTV services targeted to fixed antennas delivered in the LP stream.