Digital TV in yout pocket by Marco Fiore Edited by GROUP
Graphic project and layout
Ser vizi Reali alle Imprese
Coor dination Ufficio ADV&Marketing Elettronika Group Printing L Editrice - Industria Grafica Editoriale Original title La TV Digitale in Tasca Supported by
First printed in Italy on July 10th 2009 Free copy not for sale © Copyright 2009 - All rights reserved. Partial or total reproduction of the contents herein is proibithed. DVB & MHP are registered trademarks of the DVB Project. The author is greatly thankful to Carmelo Tidona for the huge help in the English translation.
SUMMARY
Official Note By Mario Sepi Forewords By Raffaele Fasano
1 2 3 4 5 6 7 8 9
Introduction to Digital Terrestrial TV Digitalization of audio/video signals The Transport Stream Terrestrial microwave links Broadcasting over the terrestrial channel Architecture of a DVB-T network High-added-value applications A quick glance to the future Frequently asked questions
PAGE PAGE PAGE PAGE PAGE PAGE PAGE PAGE PAGE PAGE PAGE
SUMMARY
6 7 9 15 23 31 37 47 53 63 67
OFFICIAL NOTE This is definitely an interesting period for television operators and the beginning of another era for telecommunication. The switch over from analogue to digital television marks an interesting period for more then one reasons. This is the genesis of more high quality television transmission that uses lesser transmission spectrum and of greater stimulus to innovative technology in the areas of ICT, telecommunication and television. However the predominantly good news for this switch over is the freeing up of the spectrum better known to us as the digital dividend. Such spectrum, once the complete switch over is made, shall be fully dedicated for the enhancement of economic development and further social growth. The digital dividend shall bring about a stronger broadband coverage for Internet and shall hence greater accessibility to the web from remote and rural places ensuring further the eInclusion goals of the Union. SMEs particularly peripheral ones shall also benefit tremendously from this accessibility. It is also estimated that the potential benefit of EU coordination of the digital dividend spectrum to be € 20 to €50 billion (over a 15 year period compared to EU countries acting alone). This book provides some outlooks into the switching over from analogue to digital so as to free this important and limited resource. It is practical and offers a number of solutions without being too presumption.
Mario Sepi President European Economic and Social Committee
6
FOREWORDS The world of television is undergoing one of the greatest technological changes in its history, in a scenario of continuous evolution which is gradually leading to the switch-off of traditional analogue platforms and the convergence with the world of telecommunications, especially for what concerns digital platforms for the distribution of audio/video contents. Television broadcasters are nowadays completely involved into a transformation process of the equipment, the technical skills, the ways of working and the operative business models. All this is made even more complex by the presence of several reference standards, both for what concerns the digital audio/video streams and the signals radiated by the antenna system, with an additional issue created by the usage of distribution techniques so far used exclusively by telephone and data-broadcasting providers. My direct experience with television broadcasters showed in the last few years an undeniable need to provide a valid technical service to these operators, who are often forced to face new kinds of problems without having the time to "digest" important technological innovations with which they have to confront on daily basis. This manual is born with the declared intent of leading the TV professionals in their journey trough Digital Television, helping them to find their way among technical regulations, acronyms, frequencies, digital networks, protocols and any another subject which may seem difficult at first sight only because it is new. Subject have been dealt with in a synthesized and structured way, frequently making use of illustrations and references to official regulations, in order to create a quick and effective reference source, to be always brought along with the laptop or the toolbox. The manual is completed by a list of frequently asked questions, selected according to the requests which were made to the Customer Care experts of Elettronika Group since the beginning of the Digital Television era. I strongly hope that this manual will be appreciated by the readers and that it will help making their daily job in contact with new technologies easier.
Raffaele Fasano General Manager Elettronika Group
7
Concern for man and his fate must always form the chief interest of all technical endeavors. Never forget this in the midst of your diagrams and equations. Albert Einstein
8
1Introduction to Digital Terrestrial TV
Introduction to Digital Terrestrial TV
The word "digital" is one of the most used in today's technological language, and generally refers to a physical entity which is sampled, quantized and represented by means of binary characters. Talking about broadcasting, "Digital TV", also referred to as DTV (Digital TeleVision) or DVB (Digital Video Broadcasting) means the broadcasting of audio, video and auxiliary information in digital format. Obviously, since these signals have to be used to modulate a carrier (in UHF and VHF bands) so that they can go through a broadcasting channel, the actual broadcasting in digital television uses also analogue signals, which are emitted by broadcasting antennas and picked up by receiving ones. In order to fully comprehend the DTV technique it is not enough to just analyze numeric modulation techniques, it also takes examining technologies for the encoding of audio and video signals, multiplexing techniques and distribution modes of the contents from the production studios to the broadcasting stations. The first stage of a digital television broadcasting chain is always the compression and digital encoding of the video sources and the relevant audio channels, in order to decrease as much as possible the band occupied by these signals, while keeping a high video and audio quality. The second step is the so called "multiplexing", a digital technique which allows to aggregate in real-time the data coming from the digital encoding of several television programs into a single digital stream containing the information of all the programs which will be distributed to the end users. A few years ago, the most fitting techniques for the distribution of digital programs to the end users were satellite, cable and SMATV (Satellite Master Antenna TV) centralized systems. Recently, thanks to the continuous technological innovations, new distribution techniques have been added: - wired telephone lines; - wireless telephone lines 2.5G type or higher generations; - optic fibers; - MMDS (Microwave Multipoint Distribution System); - DVB-T (Digital Video Broadcasting – Terrestrial). One of the most recent evolutions of Digital Television consist in the real-time broadcasting towards small mobile terminals (the so called "Mobile TV"), such as mobile phones and smartphones. The technological development of these application is already quite mature, but the Mobile TV market is still at an early stage, with pilot projects in several parts of the world. In this manual we will mainly focus on the DVB-T technique, which is the base of the current "digital revolution" of television.
10
The European reference of Digital Television is the DVB Project, an international consortium of companies and certification bodies which draws standard reference documents describing the technical procedures for creating and managing digital television contents and possible applications related to the television media. Also the ETSI (European Telecommunications Standards Institute) plays a fundamental role in defining the operative regulations which translate into parameters and figures the directives of the DVB Project.
The reference for the audio/video compression and the handling of the transport of compressed information is the Moving Picture Expert Group (MPEG), whose work is expressed by the official regulations issued by the organizations ISO and IEC. www.mpeg.org www.standardsinfo.net
One of the most frequently asked questions in this transition stage is "why is it necessary to move to a digital broadcasting technique?" There are several possible answers to this question, but the most immediate and effective remark is that by now anyone can see how all communication and working means are nowadays based on digital technologies, thus television broadcasting would be set apart if it did not adapt to this trend, and lose all benefits stemming from an extreme availability of contents and applications coming from the world of telecommunication and information technology. The most technical reasons, instead, can be understood starting from an analysis of DVB-T, the terrestrial digital television broadcasting technique adopted in Europe and in most of the world countries. DVB-T is a way to broadcast digital television contents over UHF and VHF carriers using the COFDM (Coded OFDM) modulation, an excellent technique which allows to achieve very good performances in terms of coverage, thanks to an exceptional resilience to noise, fading and effects derived from multiple paths, all of which are typical problems of the terrestrial broadcasting channel. In particular, a terrestrial broadcasting channel differs from a satellite link or a cable channel for its high tendency to multi-path propagation, that is the generation of
Introduction to Digital Terrestrial TV
www.dvb.org www.etsi.org
11
multiple paths. The reflections of the broadcast signal against hindrances such as buildings or mountains overlap asynchronously to the signal directly received from the transmitter. These reflections are obviously delayed compared to the signal on the direct path and generate an interference effect known as co-channel interference.
Introduction to Digital Terrestrial TV
One of the strengths of COFDM modulation, as better explained later, is a "smart" use of multiple paths to strengthen the direct signal instead of attenuating and disturbing it. Generally there is more than one TV program over an RF channel, so each television operator must create, in his production studio, a multiplex of television programs to be distributed to all broadcasting stations of his network. Not only proper television programs can be added to a DVB-T multiplex and broadcast to the end user: several additional services, such as the Electronic Program Guide (EPG), additional audio signals, FM radios broadcast to TV sets, interactive applications and Mobile TV services, can be broadcast at the same time. Another advantage of the DVB-T technique is the possibility to build Single Frequency Networks (SFN) by means of a precise synchronization of all the transmitters in the network using the GPS as common reference signal. The architecture of a DVB-T network can be examined simply by splitting it into its key elements: TV production studio, broadcasting stations and reception sites. Figure 1 shows the architecture of a typical DVB-T network. Single Program Transport Stream
Multi Program Transport Stream
Service Information
SI
Program Coding
Video Audio
A/V Encoder 1
Video Audio
A/V Encoder 2
Video Audio
A/V Encoder 3
Video Audio
A/V Encoder 4
Data
Data Encapsulator
Optional Conditional Access System (Encryption for Pay-TV)
ASI
ASI
ASI
MUX
ASI
CAS
ASI
Distribution Network
ASI
ASI
Fixed, portable and mobile receivers
Figure 1 – Architecture of a DVB-T network
12
ISO/IEC 13818-1
Using the MPEG2 encoding, the typical size of a DVB-T bouquet is four or five standard-definition (SD) programs. Using a more efficient compression technique, such as H.264, this number may be tripled, thus increasing the spectral efficiency of the transmission.
Introduction to Digital Terrestrial TV
The example shown consists in the creation of a bouquet of four TV programs to be sent over a digital channel along with additional information, which might be, for example, taken from an existing IP network. The four analogue audio/video sources feed four encoders which compress and encode the information and generate digital streams called Single Program Transport Stream (SPTS), which transport each digital audio/video program using a serial protocol called ASI (Asynchronous Serial Interface). Each SPTS enters a multiplexer (MUX) which task is to convey the information of all the programs and additional data of the bouquet into a single digital stream (called MPTS, Multi Program Transport Stream). As shown by the picture, the multiplexer also adds to the output stream some Service Information (SI) used by the receiver to correctly tune the channel and decode the audio/video programs inside it. Following the multiplexer there may be an encryption system (also known as CAS, Conditional Access System) which can be used to administer one or more Pay-TV programs within the broadcast bouquet. If, on the contrary, all programs are Free To Air (FTA), the CAS block is not used and the ASI stream generated by the multiplexer can be directly broadcast without any additional data. The ASI stream generated by the multiplexer (or by the encryption system, if any) is the output of the digital production studio and has to be supplied to a distribution network in order to be brought to the input of all the transmitters of the DVB-T network.
Presently most of the countries who adopted DVB-T are using the MPEG2 encoding, which is well established and is supported by a wide range of fixed and mobile receivers. Some countries are adopting the MPEG4 encoding, which allows to better use the radiofrequency channels but inevitably increases the cost for the end user, considering the higher prices of receivers able to decode the MPEG4 standard. The distribution can be arranged using satellite links, terrestrial microwave links, optic fiber links or connections made using several types of wired or wireless
13
Introduction to Digital Terrestrial TV
telecommunication networks. The different distribution methods use different types of equipment, and the different techniques can also be combined in order to obtain a hybrid distribution network, made up by different types of networks connected together trough specific network adapters. For example, optic fiber links and telecommunication network links are made using network adapters which transform the ASI signal into an optic beam or into electric signals which characteristics differ in terms of physical level and protocol (e.g. Ethernet or G.703). Both terrestrial and satellite links are made using a digital QPSK modulator which modulates the ASI Transport Stream onto an intermediate frequency (IF) carrier, usually at 70MHz. The IF carrier is then sent to an adequate frequency conversion section, which in turn feeds a satellite or terrestrial RF transmitter. The signal is sent to each DVB-T broadcasting station either directly or through one or more "bounces", depending on the distance between each station and the bouquet production studio. DVB-T transmitters radiate a UHF or VHF signal (in either MFN or SFN mode, as explained later) and produce a geographical coverage strictly dependent on the quality of the emitted signal and on the COFDM modulation parameters chosen for each broadcasting station. The radiated DVB-T signal can be received and demodulated by fixed receivers (set-top boxes, either external or built-in into the modern TV sets), mobile ones (e.g. installed on public transport vehicles), and special portable receivers (usually laptops, mobile phones or smartphones equipped with DVB-T receivers). One of the already mentioned advantages of Digital Television is that it allows interactivity between the end user and the broadcaster, thanks to software applications especially engineered for the DVB-T platform. Implementing real-time interactive applications requires the addition to the network of a return channel, which allows the end users to send several types of data to the content provider. The return channel can be implemented using wired or wireless telecommunication networks, but presently wired telephone networks are mostly used, as they are still the most common means of communication among end users. http://bit.ly/bO9r4d
14
2 Digitalizazion of audio/video signals
Digitalization of audio/video signals
Audio/video signal sources, such as microphones and cameras, generate analogue signals, that is signals having an unlimited range of values which change continuously over time. For digital processing, these signals have to be sampled at discrete levels at fixed time intervals, "quantized" and encoded. These processes are normally identified under the single label "digitalization", and represent the first stage in the broadcasting chain of the Digital Television, regardless of the reference standard. Figure 2 shows a block diagram to explain the sampling procedure of a standard definition (SD) video signal, starting from the R, G, B outputs of a professional camera. Fs Luminance 13.5MHz
5.75MHz
G B
Y
Matrix
R
Pb Pr
Anti-alias filter Anti-alias filter Anti-alias filter
2.75MHz
8/10
A D
8/10
A D
8/10
A D
Y Cb
270Mbit/s
Cr
Fs Chrominance 6.75MHz
Figure 2 – Sampling of an SD video source
The R, G, B components are processed to generate the Y ("luminance"), Pb and Pr ("color differences") signals, which are then filtered and sampled according to the sampling frequencies shown in the picture. The sampling frequency of the luminance is an integer multiple of the line frequency, and it is easily noticed that the two color differences are sampled at a frequency which is half of the one used for luminance (the so called 4:2:2 format). The reason for this is that human eye is more sensitive to luminance than to color details, thus color information may be transmitted with a lesser accuracy without any loss in the perception of the image. An 8-bit resolution for each of the three signals confers to the image a universally acknowledged "broadcast" quality, thus adding up the resulting bit-rates of the three digitalized components, Y, Cb and Cr, the resulting total bit-rate is 216Mbit/s. Actually, the most common version of the protocol requires a 10-bit quantization of the three signals (SDI), thus the total bit-rate becomes 270Mbit/s. http://bit.ly/9evXwL
16
For a high-definition (HD) video signal generated by a camera with a superior acquisition accuracy, the digitalization of the signal is made as per figure 3.
Fs Luminance 72MHz
30MHz
G B
Y
Matrix
R
Pb Pr
Anti-alias filter Anti-alias filter
8/10
A D
8/10
A D
8/10
A D
Y Cb
1.44Gbit/s
Cr
Fs Chrominance 36MHz
Figure 3 Sanpling of an HD video source
The numerical data in the picture refer to an interlaced scan (even and odd lines scanned alternately) and to a system working at 50Hz (European standard). In case of progressive scan (all lines showed with no alternation) all data in figure 3 are doubled. http://bit.ly/VKzcB
The result of the digitalization of the video signal, in both cases, leads to the definition of the elementary details of the image (the so called pixels) for each field to be broadcast and displayed. Thus, in order to broadcast in real time the information of each pixel to the end users, a bit-rate of 270Mbit/s for SD and 1.44Gbit/s for HD should be transmitted. This would require the use of a radiofrequency bandwidth much higher than the one used for analogue broadcasting, which leads to the need of using efficient compression algorithms to decrease the informational content so that it can be carried by the distribution and broadcasting networks as the one in picture 1.
Digitalization of audio/video signals
15MHz
Anti-alias filter
Audio signals are sampled directly, without the decomposition needed for video signals, typically using sampling frequencies of 32kHz, 44.1kHz or 48kHz and quantizing the amplitudes with resolutions from 16 to 24 bits. Even though audio signals take up a much lower band compared to video (typically 1536kbit/s for a broadcast-quality uncompressed stereophonic audio), they still need compression for an efficient use of the available bandwidth of the transfer links and of the UHF and VHF broadcasting channels.
17
Digitalization of audio/video signals
Video sources can be "compressed" because they usually include some time and space redundancy, which can be suppressed with a minimal loss of perceptive quality. In particular, adjacent areas of the same image have equal or very similar values of luminance and chrominance, thus the information of each area can be "simplified" and broadcast with a lesser content of information. Similarly, there are usually small differences between the image of a field and the ones in the previous and next fields. Thanks to this, it is possible to broadcast only the information about the shift of each group of pixels between each image and the temporally-adjacent ones. Audio sources can be compressed by using the different sensitivity of human ear to the different frequencies of the sound spectrum, and the reduced capability of perceiving some frequencies in presence of tones on different frequencies (sound "masking" effect). A mathematical "psycho-acoustic" model allows to reduce the information related to the audio components to which the human ear is less sensitive in the various situations corresponding to speech and different musical genres. As previously mentioned, the reference body for the audio/video compression is the Moving Picture Expert Group (MPEG) and the standard currently adopted by DVB-T is MPEG-2, a technique capable of compressing the 270Mbit/s of the SD video signal down to 4-5Mbit/s, while retaining an excellent video quality. This manual will not try to describe the technical details of MPEG-2 compression, but simply to give a basic explanation of the technique used by the encoders to compress images, with the stated aim of helping television operators to choose the parameters of the encoders in their production sites.
http://bit.ly/dVwr5 http://bit.ly/9OXXdJ
18
The MPEG-2 video compression is implemented trough profiles and levels, as synthesized in table 1.
Profiles Levels High High 1440
Low
Scalable
Simple 4:2:0
Main 4:2:0
Main+ 4:2:0
Next 4:2:2
max resolution
N. D.
1920x1152@50
N. D.
1920x1152@50
min resolution
N. D.
N. D.
N. D.
960x576@25
bit-rate
N. D.
80Mbit/s
N. D.
100/80/25Mbit/s
max resolution
N. D.
1440x1152@50
1440x1152@50
1440x1152@50
min resolution
N. D.
N. D.
720x576@25
720x576@25
bit-rate
N. D.
60Mbit/s
60/40/15Mbit/s
80/60/20Mbit/s
max resolution
720x576@25
720x576@25
720x576@25
720x576@25
min resolution
N. D.
N. D.
N. D.
352x288@30
bit-rate
15Mbit/s
15Mbit/s
15/10Mbit/s
20/15/4Mbit/s
max resolution
N. D.
352x288@25
352x288@25
N. D.
min resolution
N. D.
N. D.
N. D.
N. D.
bit-rate
N. D.
4Mbit/s
4/3Mbit/s
N. D.
Video Quality High HDTV quality
Consumer HDTV quality
ITU-R BT.601 quality
VHS quality
Table 1 – MPEG-2 Profiles and Levels
In order for video compression to be effective, both in space and time, each of the fields to be broadcast is decomposed in smaller portions. In particular, each image is split into macroblocks of 16x16 pixels and, for 4:2:0 (the MainProfile@MainLevel format, MP@ML, highlighted in blue in the table), each macroblock is composed by four 8x8 blocks with Y luminance and two blocks with Cb and Cr chrominance (the latter always describe 16x16 pixels, but with a lesser informational content). Using a mathematical process called DCT (Discrete Cosine Transform), the informational content of each block is decreased taking into consideration the similarities between each block of pixels and the adjacent ones. This process is called "spatial compression" because it reduces the details of the images where they are not essential for visual perception. The other process for bit-rate reduction is called "temporal compression" and is based on the encoding of the movements of each block in the transition between each image and the ones which immediately precede and follow it. Temporal compression is based on the GOP (Group of Pictures) technique, that is sequences of images based on the prediction of the movements of the blocks over time.
Digitalization of audio/video signals
Main
Not scalable
A GOP sequence is made up by three kinds of frame: - I-frame: intra-coded image, which can be reconstructed by the decoder without the need of other images;
19
- P-frame: inter-coded image, which can only be reconstructed with reference to an I-frame or P-frame, with prediction; - B-frame: inter-coded image, which can only be reconstructed with reference to two I-frames or P-frames, with bidirectional prediction.
Digitalization of audio/video signals
The most used image sequence in the MP@ML format is IBBPBBPBBPBB. The frames are created in this order in the encoding process, but are broadcast in a different order to simplify the reconstruction job of the receiver. This sequence is surely an optimal compromise between required bandwidth and resulting video quality, but is not mandatory, and it is possible to select different GOP sequences on different MPEG-2 encoders as needed. The stream created by the video compression has a variable bit-rate, depending on the density of the informational content of each image inside the video sequence. A variable bit-rate stream, however, is not suitable for a broadcasting system which typically needs to use a constant bandwidth over time. For this reason, MPEG-2 encoders are equipped with quite large video buffers which allow to store the variable video stream and generate an output stream with a nearly constant bit-rate. The MPEG audio compression is based on splitting the entire audio band in several sub-bands (usually 32). Within each sub-band, the human ear has a different sensitivity to sound details, thus, as some sounds are in practice "masked" by the simultaneous presence of others, they are not transmitted, or transmitted with a lower accuracy. The MPEG-2 audio compression derives from the previous MPEG-1 technique with the addition of support for new bit-rates, the increase of the maximum compression factor and the introduction of the multi-channel audio concept. The MPEG encoders used for DVB-T use both the MPEG-1 and MPEG-2 standards, officially supported by DVB-T receivers. The characteristics of the MPEG audio compression can be summarized as follows: sampling frequencies: 16kHz, 22.05kHz, 24kHz, 32kHz, 44.1kHz, 48kHz encoding layers: I, II and III encoding bit-rate: up to 448kbit/s Besides, the MPEG audio standard supports a very useful characteristic for broadcasting, the stereo redundancy control. In detail, the four supported audio profiles are: - mono: broadcasting of a single audio channel;
20
- dual: broadcasting of two independent audio channels (different informational contents); - stereo: broadcasting of two synchronized audio channels for the reproduction of the stereophonic signal; - joint-stereo: uses the interdependence of the Left and Right channels to decrease the total bit-rate of the audio section.
http://bit.ly/aqFoJS
The encoding techniques of the audio/video signals are constantly evolving thanks to the availability on the market of increasingly more capable and faster integrated circuits, as well as the constant trend of the broadcasting market to concentrate a larger number of programs in decreasing portions of bandwidth. There are many new techniques compared to MPEG-2, proposed by various entities (universities, research centers, companies), but the one technique that has already been adopted by many and promises a future expansion is MPEG-4, also belonging to the Moving Picture Expert Group.
Digitalization of audio/video signals
Aside from compressing and encoding the audio and video contents, the MPEG-2 encoder also has to transmit timing information allowing the receiver to synchronize audio and video while decoding. In fact, as previously explained, the audio and video streams are encoded separately, thus have to be re-aligned for the final reproduction into the DVB-T receiver. The synchronization task is performed by the PCR (Program Clock Reference), which consists of a "program clock" transmitted by the MPEG-2 encoder along with the audio and video data. The PCR is obtained by a 27MHz frequency reference present on the encoder and allows the receiver to reconstruct (through a digital PLL) a local 27MHz reference locked to the clock frequency of the encoder. The reconstructed clock in the receiver, along with two other synchronization signals (PTS, Presentation Time Stamp, and DTS, Decoding Time Stamp) present in the broadcast stream, allows a perfect reconstruction of the image, synchronized with the relevant audio channels.
MPEG-4 is a standard adopted today not only for digital television but also for telephone and for the broadcasting of movie clips on the web, and thus represents a pivotal item of the convergence between the worlds of broadcasting and telecommunications. ISO/IEC 14496-1
21
Audio and video streams are processed by the MPEG-4 encoder as "objects" which can be manipulated and modified in real time bi-directionally. The properties of these "objects" which remain constant over time allow the encoder to save a large quantity of information, the result being a compression efficiency much higher compared to MPEG-2.
Digitalization of audio/video signals
One of the most interesting sections of the MPEG-4 standard is MPEG-4 Part 10, also known as MPEG AVC or H.264. It is a very efficient video compression algorithm which is being used for several applications, all supported with remarkable performances. The H.264 encoding allows, for instance, to host more than one High Definition (HD) TV program over a single UHF channel, it is used for "Mobile TV" (digital television on mobile phones), it is used in mobile players such as iPod and PSP and it is largely used for broadcasting movie clips on the internet thanks to the minimal bandwidth requirements. Table 2 summarizes the distinctive features of MPEG-4 Part 10 compared to MPEG2, in order to better understand the reasons for its higher compression efficiency.
Spatial
Temporal
Visual Statistic
MPEG-2
H.264 / MPEG-4 Part 10
8x8 pixel blocks DCT
Variable size blocks, from 16x16 to 4x4 Integer Transform 4x4 Multiple frames Weighed motion vectors Intra-frame prediction I, B and P slices
Adjacent frames only Simple motion vector Inter-frame prediction only I, B and P frames Quantization and rounding of the DCT coefficient Generic RLE encoding
"In-loop de-blocking" filter (to prevent macro-pixeling phenomena) CABAC especially optimized for H.264
Table 2 – Comparison between MPEG-2 and MPEG-4 Part 10
It is easy to see from the table that the H.264 system has several approaches to assure higher performances compared to MPEG-2. In summary, H.264 allows to spare usually more than 50% of the bit-rate, with a clear economical advantage for television broadcasters, who can now broadcast more programs using the same radiofrequency band. Anyway, in spite of these obvious advantages, the most common standard is still MPEG-2, due to the wider diffusion of MPEG-2-based DVB-T receivers and the more expensive price of the DVB-T receiver supporting MPEG-4 as well.
22
3 The Transport Stream
The Transport Stream
In addition to the encoding of audio and video signals, the MPEG-2 standard also defines the mechanism, called "MPEG-2 System", for the aggregation of audio, video and auxiliary data into a single digital stream. This mechanism is also called "multiplexing" and takes place at different levels during the generation process of the data to be broadcast in DVB-T. The first level takes place in the encoder, being the mentioned aggregation of audio, video and auxiliary data concerning a single program. The second level takes place into the multiplexer which, aside from gathering the information of different programs, must also allocate service information concerning the programs being transmitted, the content transmission network, information used by the staff of the technical services and program navigation guides available to the end users. Another kind of optional information in the stream is linked to the possibility to allow a conditional access to given programs using the encryption technique. This further level of multiplexing has been defined by the DVB Project and, along with the standard rules inherited from the MPEG standard, it represents the standard for the generation of a Transport Stream (TS). ETSI EN 300 468
Each television program is considered an object described by elementary digital streams, called Elementary Streams (ES), with reciprocal connection. These elementary streams can be formed by MPEG-encoded audio or video data, or by private data associated to pertinent system descriptors. The first fundamental step in the generation of a TS is the so called "packetization", that is splitting the encoded audio data, the encoded video data and the auxiliary data into "packets" of a given length, accompanied by control information to be used by the receiver for decoding purposes. After this "packetization" process (which happens within each encoder) the ES streams are called Packetized Elementary Streams (PES).
PAGE
The PES have a structure with a non-constant packet length (it being related to the information transmitted at any given time), which is a problem for the transport networks as they need a fixed-length structure. For this reason it is necessary to have another fundamental step to change the PES packets into TS packets. Figure 4 shows the transition from the variable-length structure of a PES (which actually sometimes may have a fixed length) to the strictly fixed-length structure of a TS packet, equal to 188 or 204 bytes.
24
PES Packets Fixed or variable length
Fixed length (188 bytes) Transport Packets
packet header
adaptation field
payload
Figure 4 – Transition from PES structure to TS structure (example with 188-byte packets)
The Transport Stream
The initial four bytes of each TS packet are called "header" and include control information about the packet itself. The first of the four bytes, called sync byte, has a constant value (47 in hexadecimal format) because it is the primary synchronization reference for all DVB transport networks. Another fundamental field included in the four header bytes is the PID (Packet IDentifier), a sort of 13-bit label identifying the information carried by each packet. For example, at the output of an encoder, all packets carrying encoded video information have the same PID (e.g. 0A00 in hexadecimal format) and all packets carrying encoded audio information have the same PID, which is different from the video one (e.g. 0A10 in hexadecimal format). The PID plays a fundamental role also in the multiplexing process because, when aggregating several programs into a single TS, the multiplexer must assure that there are no PID conflicts, that is each PID is associated always to a unique kind of information in the TS. Immediately after the four header bytes there is the so called payload of the packet, the portion of data which are not control information but information used by the receiver to reconstruct the audio/video signal of all programs transported by the TS. We have already mentioned that the length of a TS packet may be either 188 or 204 bytes. Actually the initial length of a TS packet is always 188 bytes, of which the initial four are the header and the remaining 184 are the payload. The packets with a length of 204 bytes, used in transport networks with higher probability of data interference, have 16 additional bytes coming from a complex
PAGE
25
encoding mechanism for protection against errors, called "Reed Solomon". In the introduction chapter we mentioned the need for a multiplexer to add to the output stream also service information used by the receiver to correctly tune the channel and decode all the audio/video programs that it carries. These additional information can be split into Program Specific Information (PSI) and Service Information (SI), both organized as tables. PSI tables are defined by the MPEG standard (ISO/IEC 13818-1) and include four tables:
The Transport Stream
Program Association Table (PAT) - contains the list of all programs in the TS and points to the PIDs of the relevant PMT tables - enables the receiver to find and use the NIT table - associated tol PID 0x0000 - mandatory in the TS Program Map Table (PMT) - points to the individual PIDs of the respective programs and in particular to the packets containing the PCR - can contain copyright information - mandatory in the TS Network Information Table (NIT) - contains private data on the broadcasting system (frequency, site number, etc.) - contents defined by the DVB Project and not by MPEG - mandatory in the current TS, optional for additional TSs Conditional Access Table (CAT) - contains private data used for conditional access Aside from this information, the DVB Project decided to adopt additional tables in order to provide the receiver (IRD, Integrated Receiver Decoder, or STB, Set-Top Box) an automatic tuning mechanism even on bouquets broadcast with a large variety of TV programs. The final goal was making the user interface of home DVB-T receivers fast and intuitive by using a method as standardized as possible. For this reason the so called SI (Service Information) tables have been defined:
PAGE
Bouquet Association Table (BAT) - provides information on the bouquet, such as name of the provider, available languages, conditional access (if any), services, etc.
26
The Transport Stream
- services included in other TSs (even provided by other networks) - optional table Service Description Table (SDT) - provides information on the services offered by the network, such as Service Name, Service Provider Name, textual information concerning each service and a list of all services included in the current TS - mandatory table for the current TS Event Information Table (EIT) - contains the name of each event, its starting time, its duration, information on the current event and the following one, and optionally on future ones - each service has an associated EIT table - mandatory table for the current TS Running Status Table (RST) - indicates the current status of an event (whether it is on air or not) and is uploaded in real time. This information is used by set-top boxes to automatically tune on an even when it starts Time and Date Table (TDT) - contains the current date and time in UTC format Time Offset Table (TOT) - indicates the difference between the local time and the UTC time in order to show the correct time anywhere in the world Stuffing Table (ST) - used for interfacing operations between different networks, typically at production studios for cable TV (DVB-C) when some sections of other tables are overwritten Discontinuity Information Table (DIT) - used in discontinuity points of other SI tables to show changes in the contents or in the source of the broadcast information Selection Information Table (SIT) - used in the description of formally incomplete TSs ("partial TS") coming from digital interfaces between the set-top box and digital data storage systems The transmission of all of these tables is regularly made within the Transport Stream. Anyway, instead of using PES packets (as in the case of audio and video information), the tables are split into short portions called sections and inserted into the payload of the TS. Each processing performed on the Transport Stream needs a dynamic update of
PAGE
27
the PSI and SI tables to inform the receiver of the modifications made to the information stream broadcast at any time. The two main operations which are normally performed onto a Transport Stream are remultiplexing and PID filtering. In case of remultiplexing, a bouquet is generated starting from one or more preexisting bouquets, generated by different sources, selecting the programs and services to be sent to the final bouquet starting from all of those available in the input bouquets. A practical case is the reception from two different satellite bouquets (with DVB-S receivers) to generate a new DVB-T bouquet which includes some of the programs received from the two satellite streams. This case is shown in figure 5 and has three fundamental steps:
The Transport Stream
1) elimination of the unwanted PIDs from the two received streams; 2) aggregation of the desired information into a single stream; 3) update of the PSI and SI tables.
PID filter
Multiplexer
TV1 TV2 TV3
TV1 TV3 TV6
PID filter
TV4 TV5 TV6
Figure 5 – Remultiplexing Process
In case of PID filtering the processing is simpler because it only concerns the elimination of some contents starting from a bouquet containing several programs, in order to select the ones to be broadcast from a given station. A practical case is the reception of a stream very rich in information distributed on a Gigabit Ethernet network (over telecom networks) to generate a DVB-T bouquet including some of the programs coming from the stream. This case is shown in figure 6 and has two fundamental steps: 1) elimination of the unwanted PIDs from the received stream; 2) update of the PSI and SI tables.
PAGE
TV1 TV2 TV3 ... TV25
28
PID filter
SI update
Figure 6 – PID Filtering Process
TV1 TV2 TV8 TV14 TV19 TV21
A deeper look SPI (Synchronous Parallel Interface) – defined by 11 simultaneous signals: 8 data signals (bytes), 1 clock signal, 1 synchronization signal (Psync) and a validation signal (Dvalid). The bitrate is variable up to 108Mbit/s and the reference connector is a 25-pin to allow a differential electrical format. The electrical interface can be LVDS for quite short connections between d i f f e r e n t d e v i c e s o r LV T T L f o r connections within a single device.
ASI (Asynchronous Serial Interface) – defined by a single signal with a constant bit-rate of 270Mbit/s on a 75ohm unbalanced coaxial line. The standard connector is BNC. The 270Mbit/s bit-rate is a gross bit-rate equal to the sum of the payload net bitrate and the so called stuffing bytes used only for the purpose of obtaining a constant bit-rate. Stuffing bytes are discarded in the de-serialization process in each device. The ASI interface allows long connections between different devices.
The Transport Stream
A fundamental operation that must be made inside modulators working in broadcast mode towards the end users is the so called "PCR Restamping". The PCR, as previously mentioned, is a sampled version of the system clock of each audio/video program included in the TS, and is used by the receivers to correctly regenerate the analog audio and video signals to be sent to the TV screens. Each time an aggregation and/or filtering operation of elementary streams is performed onto a TS, the values of the various PCRs are altered and, if the alteration is beyond a given tolerance threshold, it may cause annoying problems of defective audio/video synchronization (lip sync) at the receiver of the end user. For this reason it is important, before the signal is sent in broadcast mode, to check that the values of the PCR are within the tolerance boundaries set by the regulations. Otherwise, the value of the PCR is recalculated and rewritten (a process called restamping) inside the modulator to ensure a correct synchronization at the stage of audio/video decoding, even after more or less complex manipulations of the reference Transport Stream.
PAGE
29
4 Terrestrial Microwave Links
As mentioned in the introductory chapter, one of the most used methods for the distribution of the Transport Stream to the terrestrial broadcasting sites is the use of microwave links. Terrestrial links are made using a digital QPSK modulator which modulates the Transport Stream onto an intermediate frequency (IF) carrier, usually at 70MHz. The modulated IF carrier is sent to a frequency-conversion stage which in turn feeds a microwave transmitter working at the frequency bands allowed in the proper geographical area. The transmitted signal is then sent to each DVB-T broadcasting site, either directly or through one or more "bounces", depending on the distance between each site and the production studio.
Terrestrial Microwave Links
http://bit.ly/aVeGxP
The QPSK modulator manufactured by Elettronika uses the DVB-S technique to achieve maximum performances with the same bandwidth usage and a working stability assuring broadcast quality even at very low-level reception conditions. The renowned robustness of the DVB-S modulation derives from the numerical mechanisms used for the automatic error correction on errors over bits caused by noise and interference on the transmission channel. ETSI EN 300 421
PAGE
In particular, the encoding is made by using a combination of a Reed Solomon RS(204,188) code and a "punctured" convolutional code. This combination allows the DVB-S receiver to correct a large number of wrong bits (after passing through the transmission channel), thus the receiver is able to demodulate and decode the signal even with a very low signal-to-noise ratio. The DVB-S modulator produces an IF output at the standard frequency of 70MHz, so it can be used to replace the analog IF modulator of traditional TV links simply by adjusting the output level. This way, the conversion of an old analog TV link in a new digital link is a fast and easy operation. The QPSK modulation requires that the microwave amplifier works in linear mode to avoid generating intermodulation
32
(1)
Terrestrial Microwave Links
which would decrease the signal quality. For this reason, the output power used must be about -3dB lower than the saturation point of the amplifier (it is called 3dB backoff), and, if possible, the limiter stages (if any) have to be replaced by Automatic Gain Control (AGC) circuits. Actually, acceptable performances of the radio link can be achieved even decreasing the backoff, because most of the intermodulation products is outside the useful band, thus does not excessively affect the demodulation at receiver side. A DVB-S modulator needs some very simple settings to work correctly on the assigned band. Usually, during the period of transition to the digital technique, television providers use the 20MHz-wide radiofrequency band already used for analog links, so the parameters of the digital modulator have to be selected in order to generate a signal with a bandwidth equal to or less than 20MHz. Some broadcasters already use 28MHz-wide channels allocated for digital radio links, which allows to work on a wider bandwidth and consequently with a Transport Stream with a higher informational content. The bandwidth and the net bit-rate that can be carried on a digital radio link can be determined by selecting two fundamental parameters of the modulator: the Symbol Rate and the Code Rate. The Symbol Rate is equal to half of the carried gross bit-rate (including the redundancy bits used for error correction), while the Code Rate is a parameter which shows the percentage of the overall bit-rate which is used by the redundancy bits in order to immunize the signal from interference over the channel. To know the net bit-rate ("payload") available on the radio link, knowing the Symbol Rate and the Code Rate set on the DVB-S modulator, the following formula can be used:
net_bit_rate = Symbol_Rate x 188 x 2 x Code_Rate 204
while to know the radiofrequency band occupied by the radio link for each Symbol Rate set on the DVB-S modulator, the following formula can be used:
(2)
RF_band
1.35 x Symbol_Rate
The Symbol Rate of the DVB-S Modulator by Elettronika can be set in a range from 1 to 30MS/s at steps of 0.5MS/s and the Code Rate can be chosen among the standard values of the DVB-S modulation. PAGE
33
Using the (1) and (2) formulas with some example Symbol Rate values we obtain the Table 3 below, which can be used as quick reference in planning radio link paths. Code Rate Symbol Rate [MS/s]
RF band at -3dB [MHz]
1/2
2/3
3/4
5/6
7/8
Terrestrial Microwave Links
Net bit-rate (payload) [Mbit/s] 1
1,35
0,92
1,23
1,38
1,54
1,61
2
2,7
1,84
2,46
2,76
3,07
3,23
5
6,75
4,61
6,14
6,91
7,68
8,06
10
13,5
9,22
12,29
13,82
15,36
16,13
15
20,25
13,82
18,43
20,74
23,04
24,19
20
27
18,43
24,58
27,65
30,72
32,25
25
33,75
23,04
30,72
34,56
38,40
40,32
30
40,5
27,65
36,86
41,47
46,08
48,38
Table 3 – Net bit-rate on digital radio links with DVB-S modulation
Table 3 can be used to choose the Symbol Rate and Code Rate depending on the net bit-rate of the Transport Stream that has to be sent through the radio link. While selecting the Code Rate it is important to consider that lowest values (left side of the yellow part of the table) allow a more robust reception (with lower minimum values of S/N), while highest values (right side) need higher values of S/N in reception to obtain a correct demodulation. It has already been mentioned that often television operators use radio links with 20MHz-wide bands, and table 3 allows to see that in this case the highest Symbol Rate that can be used is 15MS/s. A very frequent case in the distribution of a TS to DVB-T broadcasting sites has a TS with a net bit-rate of 24.13Mbit/s and, looking at table 3, it is clear that this TS can be carried with 15MS/s, selecting a Code Rate of 7/8.
PAGE
The receiver of the digital radio link is made with a DVB-S demodulator which regenerates the Transport Stream onto an ASI stream and, if needed, decodes a selected audio/video program within the TS. The devices which can demodulate and decode are called IRD (Integrated Receiver Decoder) and are very similar in their functions to a DVB-S set-top box for consumer applications. The performances of a digital radio link, compared to the ones of a traditional analog link, are greatly better, thanks to the renowned "threshold effect", typical of
34
The digital radio links manufactured by Elettronika, thanks to a sophisticated real-time processing algorithm, allow to implement, at a competitive price, radio link connections to be used for the distribution of the TS within DVB-T SFN (Single Frequency Networks) Networks. It will be better detailed later that DVB-T SFN networks need very accurate time/frequency synchronization and Transport Stream distribution settings, thus the conventional (digital) radio links are not sufficient to guarantee the required performances. The digital radio links by Elettronika can be equipped on request with an "SFN Transport" option which assures perfect operation even in very strict conditions, such as the transport in SFN networks.
Terrestrial Microwave Links
digital modulations. In fact, a natural phenomenon, due to the atmosphere transmission, which no technology can overcome, is the TV signal degradation when moving away from the transmitter or in presence of urban or geographical obstacles. The performances of a link are however strongly dependent on whether the link uses analog or digital technology. The quality of a television signal received from an analog path gets worse the more the distance between transmitter and receiver increases. The quality of a signal received from a digital path is perfect (that is the same as it was at the transmitter) as long as the receiver is able to receive the signal. There is thus a distance threshold within which there is a "zero degradation" of video quality. Over this threshold, the quality of the picture drops abruptly to zero, meaning that the image is totally absent. In practical terms, these considerations lead to a link budget about 20dB higher. Thus, if a given analog path over a radio link has an acceptability (sensitivity) threshold of -65dBm at receiver side, after the link is turned to digital, most likely there will be a minimum signal threshold of -85dBm, increasing the robustness of the link and the resulting picture quality.
PAGE
35
5 Broadcasting over the terrestrial channel
Once the transport stream has been distributed to all the broadcasting stations, the step that follows naturally is broadcasting the signal to the end users with the best efficiency, that is trying to meet all of the following conditions at the same time:
Broadcasting over the terrestrial channel
1) minimum power consumption; 2) maximum geographical coverage; 3) minimum RF bandwidth usage; 4) maximum immunity to noise and interferences; 5) maximum quality of the signal; 6) possibility to receive the signal correctly even when moving; 7) possibility to implement single frequency networks. In order to obtain all the above results in the best way, the DVB Project had to carefully face the design of a modulation standard dedicated to digital broadcasting on the terrestrial channel, DVB-T. Of all the standards for digital broadcasting for "classic" channels (cable, satellite, terrestrial), the one for terrestrial transmitter has been the latest in time. The reason for this is that terrestrial broadcasting is much more complex than the one over cable or satellite for what concerns the reception requirements, in particular the characteristics of the broadcasting channel and the possible technical solutions. The reception of a TV signal can be described at first glance as the sum of the useful signal (coming from the transmitter) and a white noise contribution with Gaussian statistic (the easiest from the mathematical point of view). This description is a bit too simple and can only be applied to lowly urbanized areas, with low levels of electromagnetic interference. A much better schematization for city areas is the Ricean channel model, which takes into consideration, aside from the noise, also the multi-path propagation. Basically it is supposed that the main signal received by the antenna is the one coming directly from the transmitter, but in addition to it there are echoes created by the reflection of the signal against obstacles such as mountains, hills, buildings and so on. The effect of propagation on multiple paths is well known for analog TV, resulting in double or triple contours of the pictures which make the program very unpleasant to watch. The DVB-T standard greatly improved the consequences of the multi-path propagation so to completely remove the "double image" effect typical of analogue TV, as shown in the picture below.
PAGE
Reception within a city presents even more problems in case one wants to receive
38
ETSI EN 300 744
The DVB-T transmitter receives the TS on an ASI stream and modulates a set of carriers (from about 2000 to about 8000) at an intermediate frequency (IF) equal to 36MHz, using the COFDM modulation scheme (implemented numerically through a mathematical function called IFFT). The IF signal is then sent to a frequency conversion stage, which translates it onto a UHF or VHF channel, and the result is amplified to the desired power level by means of a proper amplification chain. The output of the amplifier is sent to a cavity band-pass filter which shapes the radiated spectrum in order to prevent disturbing the adjacent channels. The resulting signal is sent to the antenna system, properly designed to cover the desired area. Different redundant configurations can be used at the broadcasting site to assure the maximum continuity of service even in case of failure of some components in the transmission systems. With the DVB-T standard it is also possible to implement combined systems to broadcast over different channels from a single site, in the same way used for analogue broadcasting, by means of a single antenna system. As seen for the DVB-S modulator, also for the DVB-T modulator tricky numerical systems are used for the automatic error correction on bits caused by noise or interferences over the broadcasting channel. The system used by the DVB-T standard is however more complex compared to the one adopted for the DVB-S, as on the terrestrial broadcasting channel the
Broadcasting over the terrestrial channel
the signal while moving, because the distribution of echoes and interfering noise is continuously changing over time, which is a very complex condition for a receiving device. The solution adopted by the DVB Project for the DVB-T standard is the COFDM (Coded Orthogonal Frequency Division Multiplexing) modulation, a Figure 7 – Multi-path propagation multi-carrier modulation using the same band used for analog TV broadcasting (8MHz in UHF and 7MHz in VHF).
PAGE
39
PAGE
Broadcasting over the terrestrial channel
sources of interferences are much more in comparison with the satellite channel. Also in this case, the Reed Solomon RS(204,188) code and a convolutional "punctured" code are used, but this time they are accompanied with a series of interleavers placed at different levels to "help" the two codes in effectively correcting errors even in presence of very deep fading on the channel. The result is that, also for the DVB-T terrestrial standard, the receiver is able to demodulate and decode the signal even in presence of a very low signal-to-noise ratio. The COFDM modulation requires that each of the carriers in the IF packet can be modulated with a QPSK, 16QAM or 64QAM scheme (also called constellation). The lowest-order modulation diagram (QPSK) is simpler, thus allowing wider coverage, but has a lower "spectral efficiency", thus only allows to broadcast a lower bit-rate over the UHF or VHF channel. The highest-order constellation (64QAM) allows the transmission of a higher bitrate thanks to its better spectral efficiency but, being more complex, needs a higher S/N ratio at receiver side to be correctly demodulated, resulting in allowing smaller coverage. The 16QAM constellation is halfway and is in fact a compromise between amount of carried information and coverage capability around the signal emission area.
http://bit.ly/dfyz7y http://bit.ly/a9Clb5
Another fundamental parameter of COFDM modulation (the main reason of the resistance against multiple paths) is the so called Guard Interval, which value can be selected by the user among 1/32, 1/16, 1/8 and 1/4. This parameter has a fundamental importance when planning SFN networks, because it gives an upper limit to the distance between the transmitters in the network. The guard interval is a fraction of the OFDM symbol (that is a time fraction of the broadcast signal) in which the transmitter does not broadcast useful information (i.e. audio and video) but only redundant data, with the specific purpose of allowing a partial overlapping of the echoes of the received signal without causing a disruptive interference. The higher the selected value of the guard interval, the higher the allowed overlapping without disturbing the received signal. On the other hand, this increased resistance to multi-path is paid by a reduced capability of broadcasting usable data, thus decreasing the maximum transmission bit-rate.
40
Mode
2k
Broadcasting over the terrestrial channel
In numerical terms, as shown in table 4 below, the guard interval can go from a minimum of 7ms to a maximum of 224ms. This means that the overlapping of signals following different paths are tolerated, with a difference in the path from a minimum of 2.1km to a maximum of 67.2km. The latter values are shown in the table as "Max TX Distance" because they refer to an SFN network in which the overlapping signals at the receiver side are generated by different transmitters tuned on the same frequency. 8k
Symbol duration Guard interval Total duration Max TX distance Table 4 – Guard interval
At a more exhaustive analysis, the DVB-T modulator is characterized by several network parameters which can be programmed by the network operator in order to choose the most fitting radiated signal: channel bandwidth (5MHz, 6MHz, 7MHz, 8MHz), IFFT mode (2k, 4k, 8k), constellation (QPSK, 16QAM, 64QAM), FEC rate (1/2, 2/3, 3/4, 5/6, 7/8) and Guard Interval (1/32, 1/16, 1/8, 1/4). Every combination of these parameters leads to a different behavior of the transmitter in terms of channel capacity (which is the maximum carried bit-rate) and signal robustness (measured by the minimum signal-to-noise ratio required by the receiver to correctly decode the audio/video programs). The correct combination of these parameters should be chosen carefully in order to implement the optimal compromise between total bit-rate and signal robustness. The situation can be better understood by looking at table 5, which refers to an 8MHz channel bandwidth and is an excerpt from the official recommendation ETSI EN 300 744.
PAGE
41
PAGE
Broadcasting over the terrestrial channel
(From ETSI EN 300 744)
C/N required for BER = 2e-4 after Viterbi (QEF after Reed-Solomon) [dB]
Available payload bit-rate [Mbit/s] Guard Interval
Constellation
FEC Rate
Ricean channel (fixed)
Rayleigh channel (portable)
1/4
1/8
1/16
1/32
QPSK
1/2
3,6
5,4
4,98
5,53
5,85
6,03
QPSK
2/3
5,7
8,4
6,64
7,73
7,81
8,04
QPSK
3/4
6,8
10,7
7,46
8,29
8,78
9,05
QPSK
5/6
8,0
13,1
8,29
9,22
9,76
10,05
QPSK
7/8
8,7
16,3
8,71
9,68
10,25
10,56
16QAM
1/2
9,6
11,2
9,95
11,06
11,71
12,06
16QAM
2/3
11,6
14,2
13,27
14,75
15,61
16,09
16QAM
3/4
13,0
16,7
14,93
16,59
17,56
18,10
16QAM
5/6
14,4
19,3
16,59
18,43
19,52
20,11
16QAM
7/8
15,0
22,8
17,42
19,35
20,49
21,11
64QAM
1/2
14,7
16,0
14,93
16,59
17,56
18,10
64QAM
2/3
17,1
19,3
19,91
22,12
23,42
24,13
64QAM
3/4
18,6
21,7
22,39
24,88
26,35
27,14
64QAM
5/6
20,0
25,3
24,88
27,65
29,27
30,16
64QAM
7/8
21,0
27,9
26,13
29,03
30,74
31,67
Table 5 – DVB-T Modulation parameters
With the help of Table 5 we can mention an example to better understand the problem. A UHF DVB-T transmitter is configured with a bandwidth of 8MHz, 8k IFFT mode, QPSK constellation, 2/3 FEC Rate and guard interval equal to 1/4. Looking at table 5 we see that this combination of values leads to a maximum total bit-rate of 6.64Mbit/s, fitting for the transport of a single MPEG-2 TV program with high video quality. Thus the channel capacity of this DVB-T mode is very low. The table, though, shows also that the minimum signal-to-noise ratio required by the receiver is about 5.7dB for stationary reception; this means that this DVB-T mode is very robust against noise because a DVB-T set-top box will display a perfect picture even with the signal at just 5.7dB over the noise level. Consider now another UHF DVB-T transmitter with the same bandwidth and IFFT mode but this time 64QAM constellation, 5/6 FEC rate and guard interval set to 1/32. The table shows that these values give a maximum bit-rate of 30.16Mbit/s, enough to transport five MPEG-2 TV programs with high video quality. Thus the channel capacity of this mode is much higher. We also see from the table that the minimum signal-to-noise ratio required by the receiver is 20dB for stationary reception; thus the higher bit-rate is unfortunately compensated by a lesser robustness of the
42
signal, as the level of the received signal must be at least 20dB higher than the noise “carpet” in order to obtain a perfect picture. So, if the two transmitters have the same output power, the first will have a larger coverage than the second. Thus, the DVB-T mode must be carefully chosen every time by keeping into consideration the best compromise between bit-rate and coverage.
Broadcasting over the terrestrial channel
For an effective transformation of an analogue PAL transmitter into a DVB-T one, the IF frequency (that is to say the center frequency of the COFDM packet) of the DVB-T modulator is set to 36.15MHz, in order to be able to replace the analogue IF modulator without changing the frequency of the local oscillator for the conversion. It is however more effective to replace the whole PAL exciter with a new DVB-T exciter with direct in-channel output, in this way achieving higher performances and a very fast intervention time by the technical staff. The COFDM modulation requires the power amplifier to work in linear mode in order not to generate intermodulation products which would decrease the quality of the emitted DVB-T signal. For this reason, after the transformation to DVB-T, an RMS power (Root Mean Square, average square power integrated over the signal band, measured with an adequate calibrated probe) at a level about -6dB lower than the peak power used for the analogue technique (that is a 6dB backoff) must be used. The 6dB value typically refers to an amplifier built with LDMOS technology, while using older technologies, such as bipolar transistors, the backoff value may increase even up to 10dB. At first glance it might seem that switching to DVB-T would be a drawback, since it would bring to "underuse" the potential of the broadcasting equipment. Actually, however, the apparent drawback given by the 6dB backoff is greatly compensated by the higher coverage capability (thanks to the already discussed threshold effect) of the DVB-T signal, so the final result is that, even with one quarter of the analogue power, the DVB-T signal has a much larger coverage. In order to broadcast the DVB-T signal on one's licensed frequency, there are some important limitations on the signal to be met, just like for the analogue TV signal. For DVB-T there are some emission masks to comply with, called non-critical mask and critical mask (see figure 8) within the regulation ETSI EN 300 744. Both masks set some maximum limits for the power of the out-of-band emissions, in order not to interfere with nearby channels. The non-critical mask is the less selective of the two and is usually required in the majority of cases. The critical mask is more selective out of the useful band and is required when the neighboring channels of the broadcast frequency contain
PAGE
43
PAGE
Broadcasting over the terrestrial channel
Power level measured in a 4kHz bandwidth, Where 0 dB corresponds to the total output power
low-power services which are disturbed by the out-of-band emissions of the DVB-T signal. In addition to the masks, an essential condition to respect "neighbors", the DVB-T transmitter has to be characterized also according to the quality of the broadcast signal, highlighting how much Figure 8 – DVB-T emissions masks it differs from its ideal Non critical mask characteristics due to distortions of different natures. For this evaluation, directly connected to the coverage capability of the signal, a parameter called MER is used. The MER (Modulation Error Ratio) is the most important parameter of a DVB-T broadcasting system, because it summarizes in a single Frequency relative to centre of DVB-T channel (MHz) numeric value the quality of a transmitter. The MER concerns the DVB-T constellation (QPSK, 16QAM or 64QAM) used by the COFDM modulator and shows how much the Critical mask constellation radiated by the DVB-T transmitter is similar to the ideal constellation that would be radiated if there weren't any distortion or intermodulation effect. Any power amplification (PA) will add distortions to the COFDM spectrum. The higher the distortion of the PA, the more the constellation differs from the ideal one, and the Frequency relative to centre of DVB-T channel (MHz) MER decreases. Thus the value of the MER should be maximized, and the best way to obtain a high MER value with a DVB-T transmitter is to use a high-
44
- DVB-T modulator with excellent performances; - power amplifier built with high-linearity technology; - availability of an accurate digital pre-correction engine; - working stability assured by a reliable control system; - commissioning made by a team of very experienced testers; - scrupulously designed antenna system.
Broadcasting over the terrestrial channel
quality digital pre-correction. A higher MER leads to a larger coverage, thus two DVB-T transmitters with the same output power but different values of MER will have different coverage performances. The other typical parameter to characterize the quality of a DVB-T transmitter is the height of the shoulders ("shoulders" of the spectrum), that is the value of the signal rejection at the edges of the usable band. Also in this case, a high shoulders value indicates a higher quality of the signal, and it derives from the linearity (MER) with which the signal is broadcast and the selectivity of the channel cavity filter connected immediately after the transmitter. The previous analysis shows clearly that a highperformance DVB-T transmitter can only be made if several fundamental factors are met with:
The ten year experience of Elettronika in the field of DVB-T, along with a certified Quality System, assure to the television operators high-level competence and professional attitude, allowing them to enter the digital world in a fast, effective and reliable way.
http://bit.ly/cN7YM1
PAGE
45
A deeper look A DVB-T Modulator allows to generate a set of COFDM-modulated intermediate frequency carriers as per regulation ETSI EN 300 744. Below, a brief description of each sub-block of the DVB-T modulator can be found.
PAGE
Broadcasting over the terrestrial channel
- Energy Dispersal: the input stream undergoes a randomization process, meaning that the energy of the base-band signal is evenly distributed inside its band by means of a pseudo-random sequence generator. - Reed-Solomon Encoder: an RS(204,188,8) encoding process, based on the Galois fields theory, is used to correct up to 8 wrong bytes over 204 total bytes, thus granting the OFDM modulation a much higher robustness compared to other numeric modulations used so far. - Outer Interleaver: this block uses a 12-depth byte-per-byte interleaving algorithm which allows to extend the correction capability of the previous Reed Solomon code. The Reed Solomon code itself is in fact weak against sequential bursts of errors, but the Outer Interleaver can change the bursts into sequences with evenly distributed errors, which can be easily restored by a Reed Solomon decoder. - Convolutional Encoder: this block uses a 1/2 rate convolutional code (which doubles the starting bit-rate), followed by a Puncturing stage which allows to decrease the initial 100% redundancy to several lower factors depending on the noise over the broadcast channel on which the DVB-T modulator will be used. - Inner Interleaver: this is a complex interleaving algorithm which works in two stages; the first stage works on single bits, the second on blocks of 1512 (2k mode) or 6048 (8k mode) OFDM symbols. The overall process differs depending on the diagram used, whether QPSK, 16QAM or 64QAM. - Mapper: this block allows to select the modulation scheme applied to each of the carriers composing the OFDM packet, and to apply either a hierarchical or non-hierarchical mode to the modulation. The supported modulation schemes are QPSK, 16QAM and 64QAM, and are chosen depending on the noise level of the selected broadcast channel. - OFDM: the symbols generated by the previous Mapper block are organized in frames; these are then distributed among the 6817 carriers of the 8k mode and the 1705 carriers of the 2k mode by means of a frequency synthesis which uses the FFT algorithm as fundamental part. The carriers which transport data are also accompanied by carriers transporting reference information for the receiver. - Guard Interval Inserter: the Guard Interval is a cyclical repetition of part of the usable signal which is added before the signal itself. The purpose of this addition is to make the DVB-T signal immune to multi-path problems, typical of the terrestrial broadcasting channels. Existing reflections, if any (within given restrictions) have in fact a constructing effect instead of disrupting the signal, as would normally happen in presence of multi-path effects with the analogue TV signal.
46
6 Architecture of a DVB-T network
PAGE
Architecture of a DVB-T network
The first question to raise when planning a new DVB-T network is the most obvious one: "how many programs can be broadcast in the bouquet?" There is no unique answer to this, though it is good to list some general guidelines which are needed to implement the bouquet properly. As shown in the previous chapter, the maximum bit-rate strongly depends on the desired coverage, as such in order to obtain a wide coverage even with a limited number of transmitters it s necessary to use a DVB-T mode with a relatively low bit-rate. If, on the other hand, a large number of transmitters is available in the area to be covered, DVB-T modes with higher bit-rate can be used, because it is possible to obtain a high enough signal in the whole area. For example, let s suppose to have already determined the correct compromise and found that the adequate DVB-T mode (supposing to operate in UHF) is: IFFT = 8k; constellation = 64QAM; FEC = 2/3; guard interval = 1/32. Table 5, in the previous chapter, shows that the maximum bit-rate for this DVB-T mode is 24.13Mbit/s. In order to know the maximum number of programs to be added to the bouquet, the bit-rate of each program must be known. It is possible to use two different methods: the first easy and fast but not very optimized is setting an equal bit-rate for all of the programs in the bouquet. The second more efficient and professional consists in giving each program the most fitting bit-rate depending on the average contents of the program itself. The simpler method is giving each program a bit-rate which allows to obtain a good quality regardless of the contents. For instance, giving a 4.5Mbit/s bit-rate to each program leads for sure to a satisfying PAL (standard definition) quality (in MPEG2), both for "slow" programs, such as news and documentaries, and "fast" ones such as musical or sport events. In this case the bouquet would have 5 programs with a surely good quality for the whole day programming. A method to optimize resources, instead, consists in considering that contents such as sport events need, to the same quality extents, a higher bit-rate than contents with less details and dynamism, such as news or talk shows. Thus, if a soccer match needs at least 4.5Mbit/s for a satisfying visual quality, a talk show or the news can be broadcast also at 2Mbit/s without any problem. To broadcast, for instance, a bouquet with 2 sport channels, 3 news channels and 2
48
miscellaneous channels, all of the 7 programs could fit into the same bouquet by giving 5Mbit/s to each sport channel, 2Mbit/s to each news channel and 4Mbit/s to each of the miscellaneous channels. Another method, for sure more efficient but also much more expensive, consists in using variable bit-rate encoders and a statistical multiplexer. The encoders and the multiplexer work analyzing in real time the complexity of the video contents of each program and are able to dynamically adapt the bit-rate assigned to each encoder. The result is the assignation of a band which is proportional to the complexity of each program, thus cutting the "waste" of bit-rate for statistically less detailed or very static contents. A higher initial investment allows to obtain a bouquet with more programs thanks to an optimized usage of the available band resources.
Another possible choice in planning a DVB-T network is whether to operate in MFN (Multi Frequency Network) or SFN (Single Frequency Network) mode. In an MFN network, as in analogue networks, the frequencies of any two adjacent transmitters (that is two transmitters having a common coverage area) must be different. SFN networks are the best way to implement DVB-T networks with a very high spectral efficiency. Typically they are used for small-medium DVB-T networks for regional or metropolitan areas. A nationwide DVB-T MFN network can be efficiently implemented by means of several regional SFN networks, each working on a different frequency. The basic principle of an SFN network is that all the transmitters in the network have to broadcast simultaneously the same data at exactly the same frequency. Thus the headend devices (that is the one generating the bouquet) in the production studio and all the transmitters in the network need a common time/frequency reference signal in order to synchronize the data (Transport Stream) and the output
Architecture of a DVB-T network
A more effective use of the band is possible by using MPEG4 encoders instead of MPEG2 ones. MPEG4 encoders, as shown previously, allow to save more than the 50% of the bit-rate on each program, thus, with the same total bit-rate, at least a double quantity of programs would be available on the bouquet. The MPEG4 encoding, at present more expensive than MPEG2, is currently used in some European and extra-European countries, especially in order to offer highdefinition contents to users. Their results are encouraging, technically speaking, but the higher cost of receivers compliant with the MPEG4 standard is delaying the transition from MPEG2 to MPEG4, so that almost all European DVB-T networks today are based on the MPEG2 standard.
PAGE
49
Architecture of a DVB-T network
frequencies (local oscillators). This synchronization is implemented by means of the GPS signal available simultaneously in the whole world. At the studio, a device called SFN Adapter, connected to a GPS receiver, has to be placed immediately after the multiplexer in order to add a synchronization signal (called MIP, Megaframe Initialization Packet) into the ASI TS. The TS is then distributed to each broadcasting station by means of a very accurate distribution network which takes care of the absolute integrity of the original TS. As previously noted, digital terrestrial links manufactured by Elettronika provide the "SFN Transport" feature expressly devoted to this very delicate application. Each DVB-T transmitter has to be equipped with a GPS receiver to be synchronized with every other transmitter in the network and to automatically compensate the propagation delay of the distribution GPS Receiver network, as seen in figure 9. GPS Receiver
TX DVB-T ASI
TS + MIP
GPS Receiver
ASI ASI
MPEG-2 Multiplexer
SFN Adapter
GPS Receiver TX DVB-T
Figure 9 – Synchronization of an SFN network using GPS
TX DVB-T
The result of this global synchronization is that the interference in areas covered by more than one transmitter is perfectly constructive. The additional requirement to achieve this condition is that the distance between each transmitter of the network and each adjacent one must be less than the distance covered by the signal during the value of the selected Guard Interval (see Table 4).
PAGE
Thus using the GPS network is fundamental for the synchronization of an SFN network, to such an extent that the GPS network is considered the very groundwork of an SFN. At present this consideration is true, but recently some very interesting studies are being performed on different synchronization networks, so to enable the implementation of a "GPS-free" SFN. ETSI TS 101 191 The pull toward these alternative methods depends
50
on the fact that the GPS signal cannot be received at some sites in which the band used by GPS has interferences from other (more or less legal) signals, as well as from the consideration that GPS is an American military system and many television operators would rather be independent from it.
Architecture of a DVB-T network
At the end of the design and planning stage of a DVB-T network, the following stage is installing the transmitters for an actual assessment of the coverage of the radiated signal. Aside from rare instances of exceptionally favorable geographical conditions, there are always some shadow areas, that are regions in which the received signal is too weak to be correctly decoded by the DVB-T receivers. The coverage of shadow areas is implemented by means of medium/low power rebounding devices called transposers when the frequency of the broadcast signal is different from that of the received one, or gap fillers when the two frequencies are the same. Obviously, for MFN networks both devices can be used (after checking the optimal solution for each place) to cover shadow areas, while for SFN networks gap fillers are the only possible solution. Gap fillers are usually placed on mountains or hills in such a position to allow them to receive the signal with a satisfying intensity, with the transmitting antenna designed with a suitable radiation diagram to cover the shadow area.
25k m
The typical problem of a gap filler installation, due to intrinsic 4 kW (ERP) 4 kW (ERP) CH 30 UHF CH 30 UHF structural reasons, is the unavoidable RF coupling between the signal radiated by the transmitting antenna and the one received from the receiving antenna. If the coupling level is 500 W (ERP) too high, the situation GAP FILLER CH 30 UHF m becomes dangerous, 40k because the signal coupling from the transmitting to the 15k receiving antenna can m 4 kW (ERP) easily lead to a CH 30 UHF Shadow area positive feedback loop which may damage the Receiving antenna pointing towards the nearest transmitter hardware of the device. If the gap filler is installed within Figure 10 – Application example of a Gap Filler in an SFN network an SFN network, as per the example in figure
PAGE
51
10, the problem worsens because, in addition to the feedback from the transmitting antenna, the receiving antenna also receives echoes coming from the other transmitters in the network. The solution to this structural problem is using a Digital Echo Canceller in the digital processing section of the gap filler.
PAGE
Architecture of a DVB-T network
In summary, the design of a DVB-T network is – in any case – dominated by considerations concerning the maximization of the coverage in the reference area, in order to obtain, with the minimum transmitted power, the maximum probability to receive and correctly decode the signal in the desired area. The problem of coverage maximization has become even more complex in recent times, after the official adoption of a standard for Mobile TV called DVB-H, which represents an improvement of the DVB-T standard in the field of mobile reception. The reference model of a DVB-H network is an Internet Service Provider (ISP) who sends video and audio contents to the network broadcast operator and to the radio operator of a mobile telephone network. Through these operations, the contents are broadcast to the end user owning a DVB-H-compatible telephone terminal. It is easy to understand that, in order to assure a continuous reception to a mobile terminal, even when receiving indoor, the requested coverage tends to be 100% of the territory and demands much higher field levels in comparison with the staticonly reception case. Elettronika offers to TV operators an accurate and extremely professional network planning service, with the help of simulation and network planning software, so to forecast with extreme accuracy the coverage on the territory of a given DVB-T (or DVB-H) network, preventing unpleasant surprises after the installation and commissioning of the transmitters.
52
7 High-added-value applications
ENCODER
ASI
ENCODER
ASI
ENCODER
ASI
ENCODER
ASI
MULTIPLEXER
High-added-value applications
One of the most evident advantages for television operators of the transition to Digital TV is the possibility to expand their offer beyond the usual limit of audio/video programs offered in the analogue programming. Digital TV, thanks to the deep interactions between hardware and software, allows to offer to the end users an interesting range of high-added-value applications, renewing in the clients the interest in the television medium and creating for the operators new business opportunities, which were impossible with the analogue technique. One of the most remunerating possibilities for operators is for sure the Conditional Access System (CAS), that is the encryption of one or more programs to protect them from unauthorized viewing, a model already in use on satellite platforms, known also as Pay TV. Users are required to pay a monthly or yearly subscription fee to obtain access to a specific channel (pay-per-channel) or alternately to pay a given sum for each individual program (pay-per-view).
ASI
SCRAMBLER
ASI
SERVER CAS + SMS
DISTRIBUTION NETWORK
TRANSMITTER
Figure 11 – Equipment for Conditional Access System
PAGE
Even though it is not possible to devise a completely safe encryption system, the DVB Project developed a "common" scrambling system, supported by all the manufacturers of conditional access systems. The specifications of this system are not publicly available, so to complicate the work of "pirates" trying to hack the encryption system. The procedure used for scrambling is shown in figure 11, starting from the equipment used in the broadcasting network. The "scrambler" is the device which encrypts the data, either at PES (Packetized Elementary Stream) or at TS (Transport Stream) level, taking care not to include the synchronization bytes in the encrypted data so not to prevent the locking by reception systems.
54
High-added-value applications
As shown in the picture, the scrambler is added just after the multiplexer (it can also be embedded into it), which allows to intervene on one or more programs of the bouquet simply by modifying the information contained in the TS system tables, in particular in the CAT (Conditional Access Table). The scrambler is connected – usually by means of a TCP/IP Ethernet connection to a CAS (Conditional Access System) and SMS (Subscriber Management System) server. The CAS section of the server is the one which generates the encryption codes of the programs, periodically updating them in order to immunize the system from hacking as much as possible. The SMS section of the server manages the end users, distributing to authorized users (owning a legitimate smart card) the access permission for the encrypted services. This is achieved by keeping a database of authorized users, updated in real time according to the payments arranged to the service provider. Very often, when the TV station has to handle different Pay-TV systems, with simultaneous checks of monthly subscription customers and pay-per-view subscribers, the customer management system becomes quite complex and the SMS server is helped by an additional dedicated server ("billing server") which automatically manages the different kinds of customers in a more complex database. It is possible, though, for smaller TV networks, to use encryption systems with a much simpler customer management, using scratch cards sold, for instance, in stores, containing a code to be sent to the provider by means of an SMS message sent by a cellular phone. In this way the investment needed to implement a conditional access system is minimized, while the system can later be upgraded to a more complex one by adding elements to the existing network. Technically, the descrambling system is based on two different kind of messages sent to the decoder within the network. The ECM (Entitlement Control Messages, generated by the CAS server) inform the receiver about the operations to be performed in order to decode the encrypted signal and the EMM (Entitlement Management Messages, generated by the SMS server) are used by the decoder to check in real time whether the user is allowed to access the selected program. There are different solutions on the market for the encryption of TV programs, and the DVB-T set-top boxes usually do not support all of the existing systems, but just part of them, listed into their technical specifications. If, on one hand, there is this kind of difference between available set-top box models, on the other hand they have a standard interface called Common Interface. This is a standardized hardware and software interface for decoding pay-radio and pay-TV signals. All decoders labeled "Common Interface" usually have one or two slots in which a CAM (Common Access Module) can be inserted. Each CAM is compatible with the most used Pay-
PAGE
55
TV systems, and holds one or two smart card readers to insert the subscription cards for Pay-TV services.
High-added-value applications
A characteristic of most DVB-T decoders on the market is that they are equipped with a telephone modem. Many of them are also labeled "MHP" and are separated by the simpler and cheaper decoders called "zappers". The presence of the modem and the MHP (Multimedia Home Platform) logo indicate the presence of another fundamental high-added-value application of Digital TV, interactivity. We have seen so far that, in Digital TV, audio and video signals are acquired, broadcast and provided as a sequence of numeric values, and we have described in general terms the processing technique of these signals in each section of the network going from the studio to the end user. However, so far we only took into consideration the distribution of the digital signals in a single direction, that is from the provider to the end user. As in the web, even in the world of Digital TV it is a growing desire for users to interact with the content provider, or even contribute in creating the broadcast information. In order to allow this interactivity level, it is necessary for users to have a return channel allowing them to send data from the place in which they watch the program towards the contents provider. The decoder used to access interactive services has an architecture resembling the one of a small computer, with an on-board processor provided with an operating system (typically a Java Virtual Machine) and enough system memory. The basic principle of interactive services is that this kind of decoder can also execute applications by downloading files from the radiofrequency channel and launching them on request of the end user. The service provider can thus add to the DVB-T bouquet some files (completed by a series of descriptive tables) composing one or more interactive applications which will be executed on receivers which are compatible with the interactivity platform used. The standard platform for interactivity in Europe is the MHP (Multimedia Home Platform), an open software platform based on Java applications, completely documented for application developers. www.mhp.org
PAGE
The definition "interactive systems" actually defines a whole range of offered services
56
Figure 12 - Examples of interactive applications
Booking holidays
High-added-value applications
which require different levels of interactivity between the user and the service provider. The minimum level of interactivity, called local interactivity, is entirely developed within the decoder owned by the user. For this kind of interactivity data are sent to and stored into the decoder and applications react to user inputs without the need of communication with the provider by means of the return channel. True interactive TV (also called iTV) requires a high level of interactivity with the service provider, such as voting for a participant to a TV show or buying goods directly on a shopping channel. For this kind of interactivity the user needs the return channel to send his data and, in case of goods purchasing, also to receive a payment notification by the provider once the requested sum has been paid. It is easy to understand that interactive TV bears resemblance to a computer connected to the internet, thus the applications that can be offered to the end user range from public utility services, to games, to direct communication with other users, and everything else the imagination of developers will be able to devise on a completely open system. The examples in figure 12 are only some concepts, the potential of the MHP platform is really limitless and for sure, after the switch-off of the analogue system, we will see the development of increasingly more interesting and useful applications.
Managing bank accounts Shopping Movies on-demand
PAGE
57
Typically, interactive applications are made and managed by specialized companies (software houses) which provide the digital stream to the TV provider, who in turn allots a fraction of the band (bit-rate) of the DVB-T bouquet to the interactive applications and makes available an ASI input of the multiplexer to inject the applications into the TS. The fraction of band required by the interactive applications is usually irrelevant compared to the one needed for audio/video programs. Usually 1-2Mbit/s are enough to add to the bouquet three or four interactive applications of medium complexity. Figure 13 shows a block diagram highlighting the network elements to be considered in order to add MHP interactive applications to a DVB-T bouquet.
PAGE
High-added-value applications
MHP Carousel Generator
MPEG-2 Multiplexer
DVB-T Transmitter
ASI
MHP Editor and Publisher
ASI
MHP Server MPEG-2 Encoder
Java-based Web-browser interface Pages creation Images conversion Animations Ready templates
Pages management Third parties content Preview Return channels management
Telephone line
Figure 13 – Equipment for the generation of MHP interactive applications
The elements called "MHP Editor and Publisher" and "MHP Server" are usually placed at the premises of the company who created the interactive applications. To create them there is no need to deeply know the Java language, there are software platforms (called "Authoring Tools") similar to the ones used to create web sites for the internet, fundamentally differing for the graphical formats and display options. Once the data stream has been created, the MHP Server routes it (by means of an IP connection) to the TV studio, where it is received by a device called MHP Carousel Generator, placed immediately before the multiplexer. The Carousel Generator transforms the IP stream into an ASI stream that can be managed by the multiplexer and is used to generate inside it some data streams structured according to the DSM-CC (Digital Storage Media – Command and Control) protocol to interact with the operating system of the set-top boxes in the management of interactive services. The Carousel Generator is often used also to send to the end users the software for the periodic update of set-top boxes.
58
Among the more interesting interactive applications, on which current experiments are focused, there are the ones related to public utility services. For example, several local entities in Public Administration are currently experimenting services to the citizens such as archives consultation, reservation of clinical analysis, tax payment, all just by using the remote control of the digital decoder. ETSI TR 101 202 ISO/IEC 13818-6
In the last two-three years, Digital TV also opened its doors to the so called "Mobile TV", that is TV received on cellular phones anywhere and anytime, even when walking or moving with other means of transport. Mobile reception is undoubtedly a technologically complex goal, considering the remarkable existing difficulties in receiving a signal while moving, indoor, in crowded urban areas, with a battery-powered device and in presence of several interference sources. The DVB Project has developed an extension of the DVB-T standard, called DVB-H (Digital Video Broadcasting – Handheld) which solves the technical issues related to mobile reception in order to offer to the end user a satisfactory visual experience in a car or a train, but also at home or at a restaurant. DVB-H standard has been designed so to be compatible with DVB-T. This means that an existing DVB-T network can be upgraded at any time (adding proper devices) to broadcast programs to DVB-H compatible cellular phones, using the same transmitters and frequencies. An operator who wants to build a network for DVB-H only is still able to do it without broadcasting DVB-T programs on the same frequency as well. This undoubtedly achieves better performances in terms of coverage and picture quality for Mobile TV programs, since it is possible to devote a given band entirely to DVB-H services, not sharing it with existing DVB-T ones. The DVB-H technique is based on the generation of an IP stream broadcasting video contents (at a low resolution as they will be displayed on small screens) encoded with the H.264 standard in multicast mode, and on the subsequent encapsulation of this IP stream into an MPEG-2 standard Transport Stream.
High-added-value applications
Interactive services are potentially the real strength of Digital Terrestrial TV in comparison to traditional analogue TV. TV broadcasters have the task of developing this huge potential in order to offer increasingly more attracting services, thus maintaining the fidelity of their audience, in turn creating a return in terms of business opportunities.
PAGE
59
High-added-value applications
Besides, the DVB-H broadcasting technique does not consider a continuous broadcasting of the usable signal over time, it being made at regular intervals called "time slices". DVB-H compatible phones demodulate the signal only during active time slices (thus allowing a longer duration of the battery) and extract the IP stream encapsulated into the TS. A playout software similar to the ones used to watch TV over the internet is used on the phone to display the selected DVB-H program, while an operating system in the phone manages interactive services in the program, if any. Figure 14 shows a schematic example of a network broadcasting only three DVB-H programs. The signal path shown by the arrows implements an SFN DVB-H network, which is the most used network for Mobile TV as it prevents the users from losing the signal when moving from one cell to a neighboring one (cell handover). Figure 14 – Network for broadcasting three DVB-H services MPEG-4 Encoder
MPEG-4 Encoder Ethernet Switch*
IP Encapsulator
SFN Adapter
DVB-H Transmitter
MPEG-4 Encoder
ESG Server CAS Server GPS Receiver Receiver (*) or generic IP multicast network
Figure 14 contains several network elements, a brief description of which is given below.
PAGE
- MPEG-4 Encoder: encodes the video in MPEG-4 (H.264) and the audio in AAC and generates the IP output signal. One encoder is needed for each DVB-H service to be broadcast; - ESG Server: adds the Electronic Service Guide (ESG) to the broadcast stream. The ESG allows DVB-H phones to lock to the existing services and decode them; - CAS Server: adds the Conditional Access information (encryption key) to the broadcast stream. CAS allows reception only by authorized users (smart card management); - Ethernet Switch: combines the IP streams generated by the encoders, the ESG server and the CAS server; - IP Encapsulator: transforms the IP stream coming from the Ethernet Switch into an MPEG-ASI Transport Stream and manages the time slicing and the DVB-H MPE-FEC
60
characteristics. It also adds the SI tables as per ETSI standard; - SFN Adapter + GPS Receiver: used for SFN synchronization (if SFN mode is used); - DVB-T/H Transmitter: used for the COFDM modulation and the RF broadcasting; - DVB-H Receiver: mobile phone with integrated DVB-H reception chipset and decryption capabilities. The decrypting operation needs a SIM with a pre-loaded software application which only allows the reception of services broadcast by the operator which distributes the SIM itself. http://bit.ly/8S3KXm www.dvb-h.org
High-added-value applications PAGE
61
8 Atoquickthe futureglance
PAGE
A quick glance to the future
While the world of television is under the current revolution of the transition to DVB-T, experts of the DVB Project are already thinking about the future of Digital TV in Europe after the definitive switch-off of analogue TV. As it already happened with the definition of the second generation DVB-S2 standard for satellite TV, also for digital terrestrial TV the DVB Project has defined the necessity for a second generation standard. DVB-T2 is indeed a new standard for digital terrestrial broadcasting. After an accurate technical and commercial study, the DVB Project has concluded that a new standard is needed in order to provide to the terrestrial channel a higher transport capacity and a better robustness, mainly (though not exclusively) for high definition broadcasting to stationary and mobile receivers. The positive results obtained with the DVB-S2 standard in providing a higher transmission capacity to the satellite channel (about 30% payload more than the DVB-S standard, with the same radiofrequency bandwidth) have been the inspiration for starting to work on the DVB-T2. The key elements leading to the definition of the new standard can be summarized into the following considerations: - DVB-T2 must be able to reuse the antenna systems already used by DVB-T; - DVB-T2 must serve both stationary and mobile receivers; - DVB-T2 must be able to provide a payload increase of at least 30% compared to DVB-T, under the same conditions; - DVB-T2 must offer superior performances in SFN compared to DVB-T; - DVB-T2 must provide some mechanism to differentiate the protection level offered to different services within the same bouquet, for instance protecting more a channel devoted to mobile reception than others used for stationary reception; - DVB-T2 must be flexible in frequency and bandwidth; - DVB-T2 must possibly use a system for decreasing the peak factor of the signal so to require less-linear amplifiers, thus decreasing the costs for broadcasting. In June 2008 the DVB project issued an official document describing the new
64
standard and during late 2009 it has been officially approved and published by ETSI in EN 302 755. DVB BlueBook A112 en.wikipedia.org/wiki/DVB-T2 www.dvb.org/technology/dvbt2/index.xml
From a strictly technical point of view, the new characteristics of the DVB-T2 standard can be summarized as follows:
Obviously the reception of the future standard will require a new decoder, thus the end user will have to face a new expense to upgrade. However we are still at the beginning of this new standard and the technical and commercial conditions of the transition to the second generation DVB-T standard will be defined in the next years. At present we can only wait for the new developments while we all prepare to switch off the analogue TV for the definitive transition to DVB-T. The transition to Digital TV is for sure the second revolution which has invested the world of television, after the introduction of color. DVB-T2 probably will not be the "third revolution", but it will have a remarkable technical-commercial impact, as promised by the experts of the DVB Project.
A quick glance to the future
- higher-performance FEC (Forward Error Correction) schemes; - 256QAM constellation; - "rotated" constellations; - IFFT 1k, 16k, and 32k modes; - 1/128 guard interval; - 1.7MHz and 10MHz bandwidths; - multiple antenna configuration; - flexible multiplexing with multi-encapsulation support to manage multiple services within the same bouquet; - MPEG-4 encoding for audio and video with HD support.
PAGE
65
9 Frequently asked questions
What should I do to transform my old analogue transmitter into a DVB-T transmitter?
Frequently asked questions
Transforming an analogue TV transmitter into a DVB-T transmitter requires the careful execution of some steps. The Customers Service of Elettronika is always available to help during this process. First of all it is necessary to take care of the frequency to be used to broadcast the DVB-T channel, requesting the proper license to the Ministry of Telecommunications. Usually the final transition to digital is preceded by an experimentation stage in which both analogue and digital signals can be broadcast. It is possible to decide to broadcast both signals simultaneously using two different channels, or to broadcast them individually on the same channel at different times of the day, for instance the analogue one at daytime and the digital one at night. To implement this scheme, Elettronika proposes a timed changeover which can be programmed to switch the signal at given times. Transforming the analogue transmitter into a DVB-T one requires replacing the analogue exciter with a DVB-T exciter. Who owns a latest-generation television exciter (http://bit.ly/cyJM1f) may even replace only the IF modulator and keep using the frequency converter of the analogue exciter. The existing amplifier can be used for the digital signal after applying and adequate back-off to the output power. Thus the analogue band-pass filter will have to be changed with a more selective cavity filter for DVB-T. The number of channels which can be broadcast in the bouquet is not a fixed value, it is a compromise between video quality and channel capacity. Anyway it is recommended to encode a TV program at least at 3.5-4Mbit/s in MPEG-2 to ensure a satisfactory video quality. A general rule is that a program with high sport contents needs a higher bit-rate than a program with few details and motion, such as news or documentaries. What should I do to transform my old analogue radio link into a digital one?
PAGE
Also existing microwave radio links can be reused for Digital TV. As the transmitters, the links may carry more programs on a single RF channel. An existing link can be reused to build a new link by replacing the analogue FM modulator with a QPSK modulator with IF output and ASI input. The typical back-off to be applied to the output power used in analogue broadcasting is
68
about 3dB and the limiter circuits on the conversion stage have to be replaced by automatic gain control (ACG) stages. The IF output of the receiver of the link has to be converted in L-band with a simple and inexpensive module, and sent to a QPSK demodulator with built-in decoder, which will provide the ASI output signal and the audio/video signals of one TV program. Is it better to use a critical or a non-critical cavity filter at the transmitter output?
How can I choose the best value for the Guard Interval? The highest value of the Guard Interval (1/4) ensures the maximum immunity to multi-path conditions. The lowest value (1/32) provides a minimum immunity to multi-path. Thus if the multi-path distortion strongly affects reception in the covered area, the Guard Interval should be increased accordingly. It must be taken into consideration that, leaving the other network parameters unchanged, an increase of the Guard Interval will always decrease the net bit-rate of the DVB-T mode. For example, a DVB-T mode with 8MHz bandwidth, 64QAM constellation, 2/3 FEC and 1/32 Guard Interval has a maximum bitrate of 24.13Mbit/s. Increasing only the Guard Interval to 1/4 will decrease the maximum bit-rate to 19.91Mbit/s, thus a better immunity to multi-path effects is paid with a lesser channel capacity. So even in this case the best solution is a compromise.
Frequently asked questions
Usually a non-critical filter is made up by six cavities, and in most cases it provides enough suppression of out-of-band signals. In some cases a further attenuation is needed, especially when the DVB-T signal interferes with very low analogue or digital TV signals present in the same area on neighboring channels. In this case a filter with critical mask, which may have up to eight cavities, is used. It is to be taken into consideration that the higher out-of-band rejection of the critical filter comes along with a higher insertion loss, due to the higher number of cavities.
When is the 2k IFFT mode better than the 8k? 2k mode is to be preferred for mobile DVB-T reception, because it better tolerates the Doppler Effect generated by moving receivers. But it is also the PAGE
69
less adequate for SFN, because it allows a lower maximum distance between the transmitters of the network. Thus if mobile reception is important, 2k mode should be used, while when building an SFN it would be better to use 8k mode, because it allows to use a lower number of transmitters in the network. Most DVB-T networks today are for home stationary reception thus, as mobile reception is not important, they use 8k mode, which guard interval is four times the one of 2k mode, granting a better immunity to multi-path effects. What do I need to encrypt DVB-T programs?
PAGE
Frequently asked questions
Encryption is implemented by means of a Conditional Access System (CAS) which is installed at the production studio, between the multiplexer and the TS distribution network. The Conditional Access System is composed by a Scrambler, a CAS Server and a billing system. The Scrambler and the CAS Server work together to insert encryption keys into the selected programs and the billing system is used to manage the fees paid by the subscribers. Only authorized subscribers can access the encrypted programs and the authorization is checked by means of a smart card purchased by the user. Several subscription modes can be used: monthly subscription, pay-per-view using a prepaid smart card, "scratch cards" or others. What is the CI Plus standard? CI Plus (CI+) is a technical specification which adds additional features and a superior safety level to the DVB Common Interface (CI) standard used for Pay-TV management. The extended features of the CI Plus standard will allow to CI+ compatible “consumer” devices (such as integrated digital television sets and set-top boxes) to access a wide range of Pay-TV services by means of plug-in CI+ modules where CI Plus technology will be supported by the local Pay-TV provider. The CI Plus standard is born mainly to offer high definition Pay-TV services and for the protection of copyrights even in case of recording on hard drives in the last-generation digital television sets. A television set or set-top box compatible with the CI standard will not be upgradable to the CI+ standard, but a CI module will work without any problem into a CI+ standard television set.
70
Can the DVB-T platform support High Definition programs? Yes, this support is feasible. High Definition (HD) programs are better encoded using MPEG-4 encoders, since an MPEG-2 encoded HD program would need about 20Mbit/s, while only 6-7Mbit/s are needed using an MPEG-4 encoder. Thus it is necessary to add an MPEG-4 HD encoder for each HD program to be added to the bouquet. The outputs of these encoders will be totally compatible with the ASI inputs of the multiplexer, thus the programs would simply have to be added to the multiplexer itself. The end user who wants to receive these HD programs must have an HD TV and an HD MPEG-4 compatible DVB-T set-top box allowing to receive both high-definition and standard-definition programs.
The main performance parameters of a DVB-T transmitter are basically the MER and the "shoulders", which indicate respectively the quality of the radiated signal and the entity of out-of-band emissions going onto the two neighboring channels. When it is installed, a DVB-T transmitter should typically have a MER at least equal to 35-36dB and shoulders at least equal to 40dB, if using a non-critical mask band-pass filter. Usually in case a degradation of the MER is detected, the shoulders should be seen degrading as well and this effect might be related to a malfunctioning inside the amplifier. In case the MER degrades remarkably but the shoulders are unaffected, most likely there is a problem in the conversion oscillator of the DVB-T modulator. If, instead, the shoulders degrade but the MER is unchanged, most likely the cavity output filter needs recalibration. A quick visual inspection may be useful, for a "skilled" observer, even by checking the spectrum and the signal constellation. In both cases the assessment is purely qualitative, but the visual information provided by these two diagrams may be very useful in addition to the merely numeric information provided by MER and shoulders.
Frequently asked questions
Which parameters should I check to continuously monitor the quality of the signal broadcast by my DVB-T transmitter?
Can I use a DVB-S digital radio link to transport a TS for feeding DVB-T transmitters in an SFN? Yes. The fundamental issue in transporting a TS within an SFN network is that it is not possible to make any kind of modification on the stream to adapt PAGE
71
Frequently asked questions
the bit-rate at the various interfaces within the transport network. In particular, for connections with radio links, there would be the need to change the output bit-rate of the SFN Adapter to adapt it to the modulation bit-rate correspondent to the DVB-S parameters selected on the modulator. For SFN networks this cannot be done because not even a single bit can be added to nor subtracted from the stream, otherwise the synchronization between the transmitters of the network would be lost. Digital radio links by Elettronika, thanks to a new, sophisticated real-time processing algorithm, allow to implement for a competitive price radio link paths to be used for the TS distribution inside DVB-T SFN networks. ItÂ’s enough requesting to Elettronika a digital radio link equipped with the "SFN Transport" option, which ensures perfect operation even under very strict conditions such as the transport in SFN networks. Is it possible to provide an internet connection service on the DVB-T platform? Yes, it is. Each DVB platform (satellite, terrestrial and cable) can be fed by a generic data stream, by using a proper protocol conversion depending on the specific application. In case of the supply of an IP stream for internet connection through the DVB-T platform, the production studio must be equipped with a "DVB Gateway", which cares about the protocol conversion and about the encapsulation of the IP stream coming from an Internet Service Provider (ISP) into a TS to be sent to the multiplexer. End users can receive the internet stream using a PC or laptop equipped with a DVB-T receiver module which demodulates the DVB-T signal and extracts the transmitted TS. A software application extracts the IP stream from the received TS and a common internet browser (Internet Explorer, Mozilla Firefox and others) can browse the web pages provided by the ISP. This system can be very useful for providing internet service to rural areas not reached by cabled infrastructures, for which the cost of a wireless service is less compared to building brand new structures with cables or optic fibers. What is an EPG?
PAGE
EPG is the acronym of Electronic Program Guide. It is a series of information used by the decoder to provide the users with real-time information concerning the programs broadcast on the DVB-T channel that they are receiving at the
72
moment. The information of the EPG is contained into the EIT (Event Information Table) tables, which are generated by the production studio and updated in real time so that the information provided to the users is constantly updated for the current on-air programs of the day and even for some following days. What is the difference between Pay-per-View and Video-on-Demand?
Frequently asked questions
Both are Pay-TV modes using a Conditional Access System to allow only authorized users to watch the programs. There is anyway a substantial difference between the two modes. Pay-per-View (often referred to with the acronym PPV) allows users to pay for watching one or more programs selected from an archive provided by the broadcasting network, with a chance to buy them at a time immediately before their availability. In simpler words, the network broadcasts a given number of programs with a defined schedule, properly advertised days in advance, and the user who wants to watch a movie which will be broadcast at 21:30 on Friday can use a prepaid card to buy it some time before it starts. The Video-on-Demand (often referred to as VOD) has a different concept, not based on a prearranged schedule but on a large archive of programs made available to the users, who can watch them at any time. In this case, the user could select the movie from a list provided by the television provider, pay the relevant price, download it to his decoder and watch it on his TV. Technically speaking, the fundamental difference is that Pay-per-View programs are broadcast according to a given schedule even if no user has requested them, while Video-on-Demand programs are "sent" only when a user requests them. So, while a classic broadcasting platform such as DVB-T well fits Pay-perView services, for Video-on-Demand a point-to-point connection network between the provider and the end users is more fitting to the kind of service offered. For this reason Pay-per-View offers are very common on DVB-T and DVB-S platforms, while Video-on-Demand is more frequent especially with TV operators broadcasting over the internet or over private telecommunication networks. There are some instances of Video-on-Demand platforms over DVB-T, implemented using DVB-T set-top boxes equipped with hard disk storage, thus able to support the download of a chosen event and its viewing at a later time. For the DVB-T platform, however, the operating cost of the Video-on-Demand solution is higher that the one required for Pay-per-View,
PAGE
73
which remains for this reason the largely more used Pay-TV solution for Digital Terrestrial TV. I have a question which is not covered by the ones listed so far. How can I obtain an answer?
PAGE
Frequently asked questions
Just write an e-mail to latvdigitaleintasca@elettronika.it and you will be answered satisfactorily in a short time. Even this useful service is available thanks to digital technology!
74
Notes
PAGE
75
We broadcast technology to the world ELETTRONIKA has been operating in the field of broadcasting for more than thirty years. Born by the intuition of Eng. Raffaele Fasano, it has its premises in Italy, in Palo del Colle (Bari), where three modern buildings contain the Research & Development Department, the production area and the Management and commercial department. It also has four separate branches: in Miami, USA (Elettronika America); in Tirana, Albania (Elettronika Albania); in Brasov, Romania (Elettronika Research), and in Ghangzhou, China (Guangdong Jinyi Broadcasting Equipment). One of the leading companies in the radio-TV broadcasting field, ELETTRONIKA is among the few companies which manufacture both radio and TV broadcasting equipment. It is present in more than 50 countries and in all continents, either directly or through distributors, partnerships and Joint Ventures. The large number of installation of its products in Africa, Asia, Europe, America and Australia are the result of a production able to satisfy any need, from low-power to high-power and even "turn-key" installations, all followed directly from the feasibility study to the after-sale assistance.
PAGE
The pull to research continuously new technologies and applications, the dynamism and the care for all of the potential evolutions of the market allow Elettronika to be always on the market
76
with technologically advanced, cost-competitive products with increasingly smaller dimensions . Thanks to a very efficient assistance department, Elettronika is also able to help and support its customers before and after the sales, by providing all of the information needed, optimizing the configurations of the equipment depending on the targets, providing complete designs and training the technical staff of the customers. Its current production includes: - TV transmitters and transposers - low- and medium-power TV amplifiers - high-power TV amplifiers - Auxiliary TV equipment - FM transmitters - medium-power FM amplifiers - high-power FM amplifiers - Auxiliary FM equipment - Changeovers - Telemetry equipment - TV microwave links - FM STL
The whole line of the new digital system in the DVB, ATSC and DTMB standards for TV broadcasting and in the DAB and IBOC for radio broadcasting. A wide range of accessories, including, but not limited to, signal distributors, stereo and RDS encoders, antenna systems, coupling systems, etc.
For thirty years one step ahead in the research of innovative solutions and advanced technologies to assure always better performances, lasting and realiability. A company policy to be ready for the needs of the market beforehand, as testified by the brand new ATSC and DVBT/H digital systems PAGE
77
Palo del Colle (Bari) ITALY S.S. 96 km 113 Zona Industriale Tel. +39 080 626755 Fax +39 080 629262 elettronika@elettronika.it www.elettronika.it
Marco Fiore, Project Manager at Elettronika, has been working for ten years in the design of equipment and networks for Radio and Television Digital Broadcasting. His education includes specialized training courses at Politecnico di Bari and at the Rohde&Schwarz GmbH Training Center. He has managed the implementation of DVB-T pilot projects in Europe, Asia and Africa and coordinated the teamwork with several European Research Centers during research projects within different application fields. Currently he is also Technical Director of Elettronika Research, the research center of Elettronika located in Brasov, Romania, oriented towards the new technologies of digital convergence. In February 2010 he has been officially appointed as Expert by the European Economic and Social Committee for a technical evaluation of the digital dividend benefits in the EU.
Info and questions:
latvdigitaleintasca@elettronika.it
The word digital is frequently used in the everyday vocabulary, a clear evidence of the huge impact that the new numerical tech nologies are having on the lifestyle of everyone. For television, in particular, the word digital assumes an almost revolutionary sense, in the sense that the transformation of traditional analogue transmission facilities into the new digital equipment is imposing to TV networks a radical technological change. In addition to the purely economic issues related to the investment needed for the renovation of transmission equipment, the advent of digital technology in the television world is also accompanied by a cultural problem. TV operators, in fact, find themselves suddenly thrown into a world of new stuff without having time to "digest" the technical concepts underlying the new digital broadcasting platform devised by the DVB Project. Digital TV in Your Pocket does not claim to be an exhaustive manual about digital transmission techniques, but aims to be a practical and useful support to the daily work of broadcasting operators that must take important decisions on the implementation and management of their networks. One Manual, Many Solutions.