1、Rep. ITU-R B0.2008-1 REPORT ITU-R B0.2008-1 DIGITAL MULTIPROGRAMME BROADCASTING BY SATELLITE (Question ITU-R 217/11) 1 (1995-1998) 1 Introduction In response to Question ITU-R 217/11, this Report provides background information collected during the preparation of Recommendations ITU-R B0.1211 and IT
2、U-R B0.1294. These Recommendations are for broadcast of television direct from satellite to small antennas, such as used by a direct-to-home (DTH) system, a satellite master antenna television (SMATV) or a direct-to-cable-head system. However, considerations have been given to related Questions, suc
3、h as compatibility with the bit rates, channel coding and modulation techniques used by terrestrial, SMATV and cable distribution systems. The way studies have been organized is summarized in Fig. 1. A list of acronyms used in this report may be found in Q 9. 2 Source coding and multiplexing It is e
4、xpected that satellite digital video broadcasting (DVB) systems will draw upon evolving standards, such as: 1) 2) 3) image coding: International Standard ISOAEC 13818-2; audio coding: Recommendation ITU-R BS.1196; multiplexing: International Standard ISO/IEC 13818-1. Suppliers of digital multiprogra
5、mme television services that are also to be received by SMATV installations should be aware of the bit rate compatibilities required by SMATV systems (Doc. lO-llS/68). 2.1 MPEG-2 video compression architecture The MPEG-2 video compression standard, ISO/IEC 13818-2, defines a syntax or language which
6、 is to be understood by an MPEG-2 compatible decoder. An MPEG-2 data stream describes exactly what actions an MPEG-2 compatible decoder must take to reconstruct the original video sequence. The overall MPEG-2 architecture and syntax were designed such that an MPEG-2 decoder is a much simpler device
7、than an MPEG-2 encoder. This is very important in broadcast applications where there is typically one encoder for many thousands or millions of decoders. This type of architecture greatly reduces the overall system cost. For a given application, certain fundamental MPEG-2 parameters can be selected
8、which set static resolution, acquisition time, use of B-frames and other basic characteristics. The range of parameters used in a given system will determine the fundamental compliance requirements for the systems decoders. Any given video sequence can be encoded in different ways using different co
9、mpression algorithms into MPEG-2 syntax compliant data streams. Each of these MPEG-2 syntax compliant data streams will be different from each other, and yet when presented to an MPEG-2 compliant decoder they can all result in video output sequences similar to the original encoder input sequence. 2
10、Rep. ITU-R B0.2008-1 FIGURE 1 Organization of work Question ITU-R 217/11 “Digital multi-programme television emissions within a satellite transponder” what are : the minimum bit-rate per programme, the performance requirements, the modulation and coding techniques, the multiplexing structure, the li
11、nk overhead capacity, the interference sensitivity . J, Report ITU-R B0.2008 “Digital multiprogramme broadcasting by satellite” Contains information on : - Source coding and multiplexing - Channel coding - Modulation - Satellite channel access - Commonalities with other media - Example of digital mu
12、ltiprogramme satellite broadcasting systems - Feasibility of common functional requirements for satellite multiprogramme TV reception Annex 1 to Report ITU-R BO. 2008 “Feasibility of common functional requirements for satellite multiprogramme TV reception. Architecture and cost estimations” - Generi
13、c reference model for the common functional requirements of a satellite IRD - Universal elements of a satellite IRD - Cost estimations - Summary characteristics of digital multiprogramme TV systems by satellite J, Recommendation ITU-R B0.1211 “Digital multiprogramme emission systems for television,
14、sound and data services for satellites operating in the 11/12 GHz frequency range” Recommends that the DVB-S framing structure, channel coding and modulation methods as specified should be considered in converging to a world-wide standard for the introduction of digital multi-programme television se
15、rvices from satellites operating in the 11/12 GHz down-link frequency range. Special Rapporteur - analyse the commonalities between existing systems - define and describe the functions of a generic system model - identify the processes and the minimum set of parameters of the various sub-systems - d
16、efine the internal interfaces between the modules of the generic system and the external system interfaces J, Recommendation ITU-R B0.1294 “Common functional requirements for the reception of digital multiprogramme television emissions by satellites operating in the 11/12 GHz frequency range” recomm
17、ends : - that one of the transmission systems described be selected when implementing digital multiprogramme television services via satellite; - that the universal elements of the common functional requirements of a satellite integrated receiver-decoder (IRD) serve as a basis for implementation of
18、the services in those areas where more than one system coexists or may coexist in the future; - that further studies be carried out to address the benefits of including other essential satellite IRD functions that are not specified in this Recommendation. Rep. ITU-R B0.2008-1 3 An important point is
19、 that the more efficient encoding algorithms can provide better output video quality using fewer bits per sequence than less efficient algorithms. That is, any given video sequence can be compressed to an MPEG-2 syntax compliant bitstream in a variety of ways, some more efficient than others. MPEG-2
20、 encoders are quite complex devices, and the encoding algorithms used in them are constantly being refined and improved. It is expected that these improvements will continue for some time to come. MPEG-2 decoders currently deployed in many operational digital multiprogramme systems are designed to b
21、e able to properly decode any MPEG-2 syntax compliant data stream. Thus these decoders do not become obsolete as encoders are routinely upgraded with new more efficient encoding algorithms. Thus MPEG-2 video encoders can be improved and upgraded over time without requiring hardware or software upgra
22、des at the decoder. This is a very valuable characteristic of the MPEG-2 video architecture in that it allows for future technical improvements in MPEG-2 video compression efficiency without requiring changes in the MPEG-2 decoders. 2.2 Statistical multiplexing methods Techniques have been developed
23、 in the broadcast equipment industry to provide for variable bit rate operation of MPEG-2 encoders. This technique takes advantage of the statistical nature of video sequences, allocating more channel capacity to difficult sequences and less channel capacity to easier sequences. Statistical multiple
24、x algorithms are operational on many systems, but it is believed that significant technical advances are still possible in this field. 2.3 Number of programmes per transponder The number of programmes that can be carried in a transponder is a function of a number of parameters. These parameters incl
25、ude the available information rate, the type of video source format being used (component or composite), the overall quality of the video source material, the video resolution, the criticality of the video material as it relates to the compression algorithm (i.e., type of programme material), and th
26、e desired video quality. Given the current state of video compression technology, Table 1 provides ranges of information bit rates per programme suitable for providing standard definition TV (SDTV) quality. TABLE 1 Examples of bit rates per programme for SDTV I Movies I 2.5 to 4 Mbitls I Educational
27、 material I 2.0 to 4 Mbit/s I General material I 3 to 7 Mbit/s I Sports I 5 to 11 Mbit/s I Associated audio (two channel stereo)(l) I 64 to 256 kbit/s (1 Significant higher bit rates would be required for multi-channel surround sound. The number of programmes which can be carried in a transponder ca
28、n be estimated by allocating the total transponder information bit rate among the video and audio services of choice. Capacity must also be reserved for programme information and conditional access data channels. It is expected that future advances in compression technology, statistical multiplexing
29、 methods and transmission coding could significantly increase the number of programmes currently carried in a given transponder. 4 Rep. ITU-R B0.2008-1 2.4 Identification of a strategy to distinguish MPEG-2 transport streams (TS) among applications In MPEG-2 systems, the private sections can be defi
30、ned individually according to each application. The framework of the recommendation on service and programme information data for digital broadcasting systems which use the private sections was issued by Working Party 11D in 1994. But different private sections are incorporated into DVB or other sys
31、tems. In Telecommunication Standardization Study Group 9, studies on programme guides and navigation tools were also started by a Special Rapporteur. If no unified consideration is carried out, mutual interference among privately defined sections will occur in the receiver. This problem will be avoi
32、ded if the identification of specific systems is provided in the context of the TS. This point is thought to be essential for realizing the inter-operability of digital broadcasting systems. Therefore, studies should be carried out in Radiocommunication Study Group 11 in cooperation with related Stu
33、dy Groups, Working Parties and Task Groups within the ITU-R and ITU-T and ISO/IEC, for the realization of a unified strategy. 3 Channel coding Channel coding includes energy dispersal, outer coding, interleaving, inner coding and baseband shaping. It is desirable to converge to a common channel codi
34、ng scheme to enable greater compatibility among different systems as explained in Annex 1. 4 Modulation Several types of channel modulation techniques are suitable for use on a satellite. The technique used by most systems currently being planned or implemented is QPSK modulation. QPSK modulation pr
35、ovides a reasonable trade-off of powerhindwidth on the satellite, ability to work with non-linearities associated with satellite transponders, and simplicity in implementation of the integrated receiveddecoder (IRD). Work has also examined compatibility of a satellite modulation technique with modul
36、ation techniques used for cable and terrestrial broadcasting systems. Use of the same modulation technique from satellite could provide a means of achieving maximum compatibility for digital emissions. Orthogonal frequency division multiplexing (OFDM) and single carrier (QAM) or vestigial sideband (
37、VSB) are also currently being investigated for terrestrial broadcasting of digital video. Other investigations Cominetti et al., 19931 and Doc. lO-llS/136 show that OFDM does not provide the best exploitation of the satellite power resources. Also, these modulation techniques require more power from
38、 the transponder and has a high sensitivity to travelling wave tube amplifier (TWTA) non-linear distortions. It should be noted that these preliminary conclusions for OFDM apply to fixed reception only, with direct antennas, for which multipath propagation and selected fadings are not expected. In t
39、he case of portable or mobile reception for television, the advantages of OFDM in terms of selective fading margins might compensate for the losses due to the extra power requirements and non-linear TWTA distortions. However, equalization techniques are also being investigated for QPSK modulation to
40、 compensate for multipath signals. If mobile television from satellite is to be investigated, both OFDM and QPSK using equalization need further investigation. 5 Satellite channel access Two techniques for accommodating multiple television programmes within a single satellite transponder are time di
41、vision multiplexing access (TDMA) and frequency division multiplexing access (FDMA). Rep. ITU-R B0.2008-1 5 In the case of TDMA, the digitized TV programmes are time multiplexed onto a single carrier, while in the case of FDMA the TV programmes are carried by N different independent carriers, sharin
42、g the transponder bandwidth. Doc. lO-llS/135 discusses a comparison of these two techniques. QPSK modulation with convolutional code rate 3/4 is assumed in the study, with total useful bit rates between 34 and 45 Mbit/s in a 36 MHz transponder. Similar bit rates could also be accommodated in transpo
43、nder bandwidths in the range 24 to 33 MHz, modifying the coding rate accordingly. The results show that the C/N penalty of the FDMA approach is relevant. For example, the FDMA approach requires 2.3 dB (two carriers per transponder) and 5.8 dB (four carriers) additional C/N with respect to a single T
44、DMA, for an equal total bit rate of 34 Mbit/s. 6 Commonalties with other media Maximum commonality of digital multiprogramme television broadcasting amongst different delivery media should be pursued; even where transmission environments are quite different, at least the most essential elements can
45、be the same. However, some differences among them are required because of the best optimization of the capabilities of each transmission media. The media considered are: satellite, cable, SMATV, terrestrial and microwave multipoint distribution system. The Recommendation ITU-R B0.1211 recommends tha
46、t the DVB-S framing structure, channel coding and modulation methods should be considered in converging to a worldwide standard for satellite multiprogramme broadcasting services. A draft new Recommendation on digital multiprogramme system for television, sound and data services for cable distributi
47、on, which recommends the use of the DVB-C system for distribution on cable, has been submitted to ITU-T. Similarly, a draft new Recommendation on digital multiprogramme, system for television, sound and data services for SMATV distribution, which recommends the use of the DVB-CS system for distribut
48、ion on satellite master antenna systems has been submitted to ITU-T. It is expected that whatever the media is, the characteristics of video source coding, audio source coding and transport multiplex may be common ensuring the maximum flexibility of the transport multiplex to be interconnected betwe
49、en different media. The basis could be the MPEG-2 multiplex (the Transport Stream) structure, which is a fixed length packet structure which has 188 bytes of data. Also the MPEG-2 TS has some service information (SI) elements and makes adequate provision for more developed SI systems to be added. The framing, synchronization and randomization can be common. When outer coding is needed (depending on the media requirements) the same scheme and ratio should be used. Also, when interleaving is defined (depending on the media) the same depth should be used. When inner coding is required, it shou
