Digital television has revolutionised the television concept, using new digital models that allow for an optimised sound and image broadcasting, improving their quality. Additional access to the Information Society or interactive services can be included.
Digital television can be accessed through different access technologies: ground waves (DTTV), cable, satellite, ADSL and mobile devices.
Digital Terrestrial Television (DTTV)
In the case of DTTV, the transmission is made using ground Hertzian waves, that is to say, those that are broadcast through the atmosphere with no need of cable or satellite and are received through conventional UHF antennas.
There are different broadcasting standards in the world, which are described in Figure 1.
All systems have a similar digital transmission scheme, based on the MPEG transport stream standard, with MPEG-2 and H264 video encoding. The main differences lie in the way the transport stream becomes a broadcast signal, in the video format before encoding (or, alternatively, after decoding), and in the audio format, as well as in the modulation systems.
Satellite digital television
In Satellite Digital Television the signal is broadcasted to a wide geographic area through communications satellites, as opposed to terrestrial television, whose waves do not reach the atmosphere, or cable television, based on the broadcast through fibre optic networks and coaxial cable.
In the satellite television broadcasting there are two different stretches: the uplink, through which the information is sent from the broadcasting centre to the satellite, and the downlink, which broadcasts this information from the communications satellite towards the area it illuminates in the surface of the earth. To avoid interference between both links, each of them uses a different frequency band.
The main advantage of using satellite television systems is how easy it is to reach big coverage areas. It also facilitates the signal reception in remote or isolated locations, as opposed to other systems such as ground-wave or cable television. However, satellite communications are also characterised by introducing important delays in the transmission of the signal, due to the distance it must cover.
The technical regulations on satellite broadcasting, DVB-S, are defined in the European Standard EN 300 421. Since the publication of the first version of the DVB-S specification, technology has kept developing, which lead to the publication of a new set of regulations known as DVB-S2. Both DVB-S and DVB-S2 use QPSK modulation. The main advantages of DVB-S2 are a 30% higher efficiency than with DVB-S, a wider range of applications for both domestic and professional use, techniques such as encoding adaptation to maximise the value in use of the satellite resources and backward compatibility towards the previous generation, DVB-S.
Digital cable television
In Digital Cable Television the signal is distributed through cable networks (fibre optic and coaxial cable).
Other services can be provided over these networks together with the Digital Television signal, such as land line phone service and Internet connection.
The standard used in Europe for Digital Cable Television broadcasting is the DVB-C (Digital Video Broadcasting – Cable).
DVB-C uses a QAM modulation, the signal is strong against noise, the broadcast is immune to interference and the delays are minimal.
In the Internet Protocol Television the signal is distributed through network protocols to the final user, normally through high-speed ADSL data connections.
As the main distinguishing factor against the rest of Digital Television access technologies, it is worth pointing out the strong interactive component of these types of services, which are mainly on demand (VoD), that is to say, the final user can choose the content that wishes to watch and decide when to do it.
Mobile Digital Television is the television broadcasting service with digital technology whose signal is received in mobile or portable devices or equipment (mobile phone, laptop, PDA, etc).
In this environment we can consider 2 broadcasting modes:
- Unicast.- An exclusive channel is established between the emitter or base station and the mobile device.
- Broadcast.- A broadcasting service where the signal is sent from the emitter or base station to all the mobile devices, for an unlimited number of users.
Among the different television broadcasting technologies over mobile we can find the following:
- Television through DVB-H, associated to a television broadcasting network.
- Television through DVB-H over IP, associated to a television broadcasting network.
- Television through UMTS, LTE, …., associated to a mobile phone network.