Terrestrial digital TV (2)
The passage of the classic television to digital television offers many advantages for the viewer but also for broadcasters.
Thus the viewer has, among other things, a greater choice of programmes, a better interactivity but also a better visual and sound quality.
Action to transform several separate digital streams into one in order to pass it more easily.
Improvements of the audiovisual offer
Two main advantages are highlighted; the improvement of the Visual and sound quality and the increase in the number of programs that can be disseminated.
The improvement of the Visual and sound quality is due to the new techniques that are used to transmit and compress programs.
Thus the phenomena of 'snow', of colors washed out or drool, ripping of images will no longer exist with TNT. Digital is based on the principle \all or nothing\; either the quality of the image is perfect, or there is no picture.
The increase in the number of channels allows the emergence of new audiovisual actors.
National regulations (the CSA in France) organizations can able the appearance of local channels or chains of association.
Two different types of chain can be broadcast : pay-per-view channels and free channels. In some countries as the France it was decided that the compression technique would not be the same for the free channels for subscription channels.
Pay channels will increased visual quality and more accessible service.
The new processes used, compression and multiplexing, increase the number of channels that can be broadcast while decreasing the size of the frequency spectrum. Decrease the size of the spectrum is something
important because it is widely used in many areas. For example, mobile telephony, broadcasting, and military applications...
So while increasing audiovisual supply, will improve the use of frequencies. This step can be crossed as analog television will no longer
used. To all television programming centres to be updated for digital television.
In France it has been planned that analogue and digital television would work simultaneously for about 15 years. In the border areas, the TNT is difficult to implement.
Indeed the operation of DTT frequencies may interfere with border countries. Where sometimes the difficulties for implementing implement the digital television broadcast network.
The transition to digital technology is more save money and a new source of revenue for broadcasters and creators of programs.
Principle of television broadcasting
The broadcast must ensure the flow in the best conditions and flow of the data stream. Television broadcasting is based on the principle of \flood\.
We will see here the main processes used for the broadcast.
The architecture of the broadcast network
90, was codified by the ITU - R 4 three stages in the development and transmission of an audiovisual program.
Two of these steps concern the professional environment and the third concerns the general public :
the service of contribution : it ensures the formatting of the signals. It is at this point that video or sound alterations are made.
the primary distribution service : it is the link between the audiovisual production centres and the heads of broadcast networks.
the secondary distribution service : it distributes the audiovisual data since the heads of networks to the Viewer.
The DVB (Digital Video Broadcast) group
The DVB group grew out of a common desire not to see the birth of many digital TV standards incompatible between them as was the case in analog;
the DVB group has as objective to establish the digital television standards.
More than 150 European, American, Japanese companies have adhered to the DVB.
ITU - R : International Union of telecommunications - radio communications
The DVB is a consortium which defines standards of coding and transmission of programs mainly.
The standards proposed by this working group must be :
-Open : the standards are accessible to all.
-inter operable : according to the standard equipment, adapts without problem to the network.
-flexible : data transport is based on the MPEG standard. It also carries video, audio, multitrack, multimedia data, etc.
Groups of the DVB work are summarized in the following table.
|DVB-S||broadcast on satellite.|
|DVB - C||broadcast on all cable networks.|
|DVB - T||broadcast on a terrestrial network.|
|DVB-MC/S||multipoint distribution system by micro-wave.|
|DVB - If||information system service, which allows the user to navigate the world of DVB.|
|DVB - CA||a common scrambling system.|
The principle of modulation is to change the frequency of a signal; changing the frequency of a signal is essential to transmit it.
Indeed, the signals generated by a television program are not to the emission frequency (480 MHz for example).
The modulation has two types of signals : the reference signals (called carrier frequencies) and modulating signals.
Two steps summarize the modulation :
first defined the precise frequencies for transmitting audiovisual signals; these frequencies are called carrier frequency.
are \mixed\ then references with the modulating signals signals. We then get the audiovisual signals at the desired frequency.
The modulation system chosen by the DVB combines several technologies of modulations. This allows to resolve two types of problem that has defined the DVB consortium.
The signal is issued must be able to :
-be treated mobile,
-have a good resistance to electromagnetic noise
-have a good immunity to the echoes due to natural or urban obstacles
These features make the system of modulation is complex. For this reason they will not be considered.
However it may be noted that the types of modulation are COFDM 5 (immunity to noise and echoes) and 6 QPSK or QAM 7.