What is DVB-T?
- an overview, information or tutorial about the basics of what is DVB-T, Digital Video Broadcast - Terrestrial, the digital television format adopted around the globe.
DVB-T or Digital Video Broadcast - Terrestrial is the most widely used digital television standard in use around the globe for terrestrial television transmissions. It provides many facilities and enables a far more efficient use of the available radio frequency spectrum than the previous analogue transmissions.
The DVB-T standard was first published in 1997 and since then it has become the most widely used format for broadcast digital in the world. By 2008, it was the standard that was adopted in more than 35 countries and over 60 million receivers deployed and in use.
Major milestones in DVB-T development and deployment
- December 1994: MPEG-2 ISO 13818-1 systems definition available
- July 1995: demonstrations by BBC of digital terrestrial broadcasting to several UK government officials.
- January 1996: The 4:2:2 video format standardised
- February 1996: QAM-COFDM transmission system agreed for DVB-T
- 9th April 1996: Implementation of the first phases of a digital terrestrial television pilot-service from Crystal Palace and Pontop Pike by BBC in the UK
- 24th December 1996: U.S. Government adopts DTV as first step towards a U.S. digital terrestrial network
- March 1997: First publication of the DVB-T standard
- December 1997: Over 200 DVB Satellite TV channels live using DVB-T
- November 1998: Transmission of DVB-T starts in the UK
DVB-T makes use of many modern technologies to enable it to deliver high quality video in a broadcast environment.
The DVB-T transmission is capable of carrying a very significant level of data. Normally several television broadcasts may be carried on a single transmission and in addition to this several audio channels may be carried as well. As a result each transmission is called a multiplex.
One of the key elements of the radio or air interface is the choice of the modulation scheme for DVB-T. In line with many other forms of transmission these days, DVB-T uses OFDM, Orthogonal Frequency Division Multiplex.
Note on OFDM:
Orthogonal Frequency Division Multiplex (OFDM) is a form of transmission that uses a large number of close spaced carriers that are modulated with low rate data. Normally these signals would be expected to interfere with each other, but by making the signals orthogonal to each other there is no mutual interference. The data to be transmitted is split across all the carriers to give resilience against selective fading from multi-path effects..
Click on the link for an OFDM tutorial
In order that the DVB-T network is able to meet the requirements of the operator, it is possible to vary a number of the characteristics:
- 3 modulation options (QPSK, 16QAM, 64QAM): There is a balance between the amount rate at which data can be transmitted and the signal to noise ratio that can be tolerated. The lower order modulation formats like QPSK do not transmit data as fast as the higher modulation formats such as 64QAM, but they can be received when signal strengths are lower.
- 5 different FEC (forward error correction) rates: Any radio system transmitting data will suffer errors. In order to correct these errors various forms of error correction are used. The rate at which this is done affects the rate at which the data can be transmitted. The higher the level of error correction that is applied, the greater the level of supporting error correction data that needs to be transmitted. In turn this reduces the data rate of the transmission. Accordingly it is necessary to match the forward error correction level to the requirements of the broadcast network. The error correction uses convolutional coding and Reed Solomon with rates of 1/2, 2/3, 3/4, 5/6, and 7/8 dependent upon the requirements.
- 4 Guard Interval options:
- 2k or 8k carriers: According to the transmission requirements the number of carriers within the OFDM signal can be varied. When fewer carriers are used, each carrier must carry a higher bandwidth for the same overall multiplex data rate. This has an impact on the resilience to reflections and the spacing between transmitters in a single frequency network. Although the systems are labelled 2k and 8k the actual numbers of carriers used are 1705 carriers for the 2k service and 6817 carriers for the 8k service.
- 6, 7 or 8MHz channel bandwidths: It is possible to tailor the bandwidth of the transmission to the bandwidth available and the channel separations. Three figures of bandwidth are available.
- Video at 50Hz or 60Hz: The refresh rate for the a screen can be varied. Traditionally for analogue televisions this was linked to the frequency used for the local mains supplies.
By altering the various parameters of the transmission it is possible for network operators to find the right balance between the robustness of the DVB-T transmission and its capacity.
DVB-T single frequency network
One of the advantages of using OFDM as the form of modulation is that it allows the network to implement what is termed a single frequency network. A single frequency network, or SFN is one where a number of transmitters operate on the same frequency without causing interference.
many forms of transmission, including the old analogue television broadcasts would interfere with one another. Therefore when planning a network, adjacent areas could not use the same channels and this greatly increased the amount of spectrum required to cover a country. By using OFDM an SFN can be implemented and this provides a significant degree of spectrum efficiency improvement.
A further advantage of using a system such as DVB-T that uses OFDM and allows the implementation of an SFN is that very small transmitters can be used to enhance local coverage. Small "gap fillers" may even be used to enhance indoor coverage for DVB-T.
DVB-T hierarchical modulation
Another facility that is allowed by DVB-T is known as Hierarchical Modulation. Using this technique, two completely separate data streams can be modulated onto a single DVB-T signal. A "High Priority" or HP stream is embedded within a "Low Priority" or LP stream. Using this principle DVB-T broadcasters are able to target two different types of receiver with two completely different services.
One example where this could be used is for a DVB-H mobile TV service optimised for more difficult reception conditions could be placed in the HP stream, with HDTV DVB-T services targeted to fixed antennas delivered in the LP stream.
DVB-T specification highlights
|Number of carriers in signal||2k, 8k|
|Modulation formats||QPSK, 16QAM, 64 QAM|
|Scattered pilots||8% of total|
|Continual pilots||2.6% of total|
|Error correction||Convolutional Coding + Reed Solomon
1/2, 2/3, 3/4, 5/6, 7/8
|Guard interval||1/4, 1/8, 1/16, 1/32|
DVB-T is now well established. Many countries, including the UK are moving towards a complete switch-over from analogue to digital, with a resultant digital dividend releasing a significant amount of bandwidth for other services. However as DVB-T has now been in use for ten years a new standard, which is a development of the original DVB-T standard known as DVB-T2 is being developed. This would have backwards compatibility, but allow additional services and flexibility as well as a number of features to future-proof it.
By Ian Poole
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