Terrestrial digital TV (3)
Multiplexing is the process that allows to multiply the number of strings that can be sent by carrier frequency.
The carrier frequency can be likened to a \pipe\. It used to pass information between the issuer of a geographical area and the antenna of the Viewer.
On each carrier frequency so through audiovisual information related to each TV station.
Mono or multi-frequency
TNT can be broadcast as two types of networks. SFN (Single Freqency Network) or MFN (Multiple Frequency Network) networks.
Networks deployed in analog type MFN are, for this technical broadcasters prefer this mode;
It allows you to reuse the same unlike the SFN network implementation. In addition, the MFN allows to offer more flow than the networks iso frequency.
The main advantage the SFN is to reduce spectral space that is already very important.
In France it has been decided to develop a network of type MFN.
COFDM : Coded Orthogonal Frequency Division Multiplex
QPSK : Quaternary Phase Shift Keying or displacement of phase 4 modulation
QAM : Quadrature Amplitude Modulation.
Method of data compression
When you want a movie or a show viewers, several preliminary steps are required :
First, the cameras and microphones convert images and sounds into electrical signals to analog. These are then converted into digital signals.
comfort and control data is added. Comfort data provide the viewer a set of useful information such as Teletext by
example; control data to control the broadcast, transmission and reception of audiovisual data.
All of these digital information generate a substantial stream of data. Whatever the type of support we have (cable, satellite, wireless...) it will take to compress this flow
before transmitting. DTT using the wireless medium, the compression is even more important because the data rate is very limited.
In the case of the Digital Terrestrial Television, this step will be described by the standards of the MPEG group.
The analog TV standard
Analog video represents information as a continuous stream of analog data. Two main standards are used and are quite similar.
The format PAL/SECAM (Phase Alternating Line/sequentiel color with memory) is used in Europe for terrestrial analogue television.
It allows you to encode videos on 625 lines (576 are only actually displayed) with a flow rate of 25 frames per second in 4 : 3 format (i.e. There is a report width to height that is 4/3).
The NTSC (National Television Standards Committee) is used in the United States and the Japan. It uses a system of 525 lines 30 images/sec. Just like the PAL/SECAM, the
NTSC displays a 4 : 3 picture format. However, it shows a lower resolution : 640 x 480.
The MPEG group
MPEG for Motion Picture Expert Group, is a working group for standards for compression of moving images, recognized by the ETSI (European
Telecommunications standard Institute). The Group MPEG has standardized the standard of the same name and its successors; MPEG2, MPEG4, MPEG-7, MPEG-21... In Television
Digital terrestrial only the MPEG2 and MPEG 4 are used.
The purpose of these standards is to define the encodings that will limit the flow of an audiovisual source. It is worth noting that the MPEG standard applies only to moving images.
Other data, information, and audio are encoded separately and then collected at the user.
As the table shows, we can't pass a source audiovisual 'gross '.
Table : Standard flow
|Definition Standard||HDTV (high definition)||TNT|
|Total flow rate||216 Mbps||1152 Mbps||-.|
|Useful rate||166 Mps||885 Mbps||30 Mbps per multiplex|
This video compression standard is very used for DVD-Video and digital television broadcasting by cable or satellite.
The definition of image ranges from 576 x 420 points to 720 x 576 points, with a flow rate of 25 frames per second.
The MPEG-2 compression allows to include data other than the video she even. We can add some information as to the number of the channel, the title of the program, the language used...
MPEG-4 is based on the principle that a video sequence can be broken down into different objects. For example, figure 6, the image would consist of two objects :
the ball and the basket. Each object being him even decomposable into several objects...
This principle allows for more compact files while providing a quality of image which can be greater or equal to that of MPEG-2.
Principle of audio compression
Audio compression uses the property of the human ear to reduce the flow requires an audio signal : it removes the inaudible frequencies by the human ear.
Audio compression is MUSICAM. Standard which encodes and compresses the sound of DVD-Video. The signals generated can then be used by the standard Dolby Digital. This one manages his multi channels.
Characteristics of the various channels
As stated above, are two types of channels, pay-TV and free channels.
MPEG-2 has the advantage of being a technique proven and widespread in trade.
So it is faster and less expensive to develop decoders for MPEG-2 for 4.
The table summarizes the main features of the channels.
|TYPE OF STRING||FREE||FREE||PAID|
|IMAGE QUALITY||576 LINES OF 720 PIXELS||DVD (720 LINES OF 576 PIXELS)||HDTV (1080 LINES 1920 PIXELS)|
|SOUND QUALITY||STEREO||ITS MULTI CHANNEL||ITS MULTI CHANNEL|
|SERVICES||TELETEXT||PROGRAMS INTERACTIVITY, PORTABILITY||ELECTRONIC PROGRAM GUIDE.|
|NUMBER OF PROGRAMS||1||6||2-3|
Currently, the majority of networks use the digital medium. The fact that television is now using this support opens up new possibilities.
Two main applications are directly concerned by the deployment of a digital television network :
the world of mobile telephony and the Internet. Other applications are most unexpected, as the appearance of TNT decoder in a music player.
Convergence with mobile
Worldwide, mobile phone networks are currently being deployment or redeployment. They evolve into a network of\third generation\.
This means that it supports high flow rates and that they planned to multimedia.
A digital television network is ideal for mobile phone operators.
This represents new sources of revenue and a new attraction for their network generation.
Convergence with the Internet
The Internet is currently booming. Access high-speed multiply and offer new services to subscribers at the same time.
Some providers of access to the Internet (ISP) provide video or telephone service in addition to their basic service.
DTT and Internet networks can become complementary. Internet can extend access to programs of the TNT area.
Convergence with transport
TNT has been developed to be able to receive it in mobile. Automakers already had the idea to integrate DTT receivers in their
car passengers to have access to the television network.
This was not possible with analog television since she could not be capture correctly when the receiver was mobile.
By this presentation could appreciate the many technological changes that are made to the television. Based on open and interoperable standards, TNT has
many assets that do that today ' today many countries adopt it for the evolution of their television system.
Showing their interest in this system, many companies, multinationals or not, participate in the definition of standards governing the operation of the terrestrial digital broadcasting.
The appearance of a terrestrial broadcast network opens up many prospects for convergent applications.
It offers new services in different modes of use and therefore new sources of revenue for operators.
The proliferation of programs and improving their Visual quality however opens the debate on the content of the proposed programs;
TNT will you it in the sense of a social and human progress ?
DTT : Digital Terrestrial Television, is a process of digital broadcasting over-the-air television. First, television was broadcast in analog and proposed
of less good quality sound and audiovisual; stereo sound and bad image definition. DTT changes that thanks to new digital techniques.
A broadcast network architecture remains the same as in analog : emissions stations are distributed throughout the territory to cover.
That, through radio waves provide viewers the audiovisual signals. The antenna then the decoder then allow to receive and make these usable by television signals.
The arrival of the decoder is due to the arrival of new issuance of the audiovisual signal and compression techniques :
multiplexing, modulation and compression.
The modulation used techniques to broadcast audiovisual signals at specific frequencies and increase immunity to electromagnetic transmission problems : echo, electromagnetic noises, obstacles,...
Multiplexing to multiply the number of strings that can be issued.
Channels are actually cut into packets and transmitted over-the-air.
The decoder then allows re - assemble all the packages that form then the audiovisual show.
Flow that allows the broadcast by Hertzian channel is limited. The specification states that a broadcast TV may not exceed the rate of 4Mbit/s. We use techniques, MPEG-2 or the MUSICAM, allowing to reduce the flow that will require a show. Here again, the decoder will be used to make understandable signals by television.