[转帖] DVB QA

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This Questions and Answers document deals with some of the queries that the DVB Project Office regularly receives. If you have any further questions, please contact the DVB Project Office.
Q Why did DVB choose to use MPEG-2 ?
A The MPEG-2 coding and compression system was chosen after an analysis and comparison of potential alternatives. Unlike other compression tools which claim to provide greater degrees of compression for given quality, but which are as yet unproven for a wide range of programme material or over different broadcasting systems, MPEG-2 is tried and tested. It has been repeatedly shown to be capable of providing excellent quality pictures at bit rates which are practical for the services that will be required.
From a commercial point of view, the adoption of MPEG-2, an existing, proven standard, was advantageous because it allowed DVB to concentrate its efforts on finding ways of carrying the already well specified MPEG-2 data packets through a range of different transmission media, including satellite, cable, SMATV, MDS, and terrestrial.
DVB can effectively be regarded as the bridge between broadcasters and the home, over which MPEG-2 data packets can be carried.
Another important reason for choosing MPEG-2 was that it includes techniques for the inclusion of Programme Specific Information (PSI) for the configuration of decoders. DVB extended these techniques to provide a complete SI (Service Information) capability, enabling receivers to automatically tune to particular services, and to decode a mixture of services and service components, including television, sound, and data. SI also allows services to be grouped into categories with relevant schedule information, making it possible to provide user-friendly programme guides.
Another important consideration was that the design of the complete MPEG-2 system makes it possible to freeze the design of a decoder whilst still retaining the flexibility to make quality improvements at the encoding end of the chain. Any new performance improvements due to encoding developments will automatically be reflected in the picture quality provided by the domestic decoder. Chip-sets for MPEG-2 coding and decoding are commercially available now, with those for decoders being on the market at a cost of only a few francs. It made sense to adopt a proven system, and time has borne out our faith in MPEG-2.
Q Why did DVB choose OFDM for the DVB digital terrestrial broadcasting system? Is it not true that tests in the US have shown single carrier systems to be superior?
A The European and North American broadcasting environments are very different. One stark example of this is that the US has 1600 TV transmitters in total whereas Europe needs many times more, with 1000 transmitters operating on every available frequency. Tests of the Orthogonal Frequency Division Multiplexing (OFDM) system have shown that it operates extremely well in the conditions likely to be found in heavily built-up areas such as towns and cities. OFDM makes it possible to make constructive use of the unavoidable 'multipath echo' signals which are caused by reflected signals from buildings arriving at the receiving antenna. Whereas conventional television systems can suffer degradation from these multiple signals, OMM receivers can actually add together the various out of phase signals arriving at the antenna, in order to re-assemble perfect pictures and sound from the incoming digital signals.
The 8 Vestigial Side-band (8-VSB) modulation system to be used by the US ATSC DTV digital terrestrial system can provide excellent results in suitable conditions, and is less complex and might therefore be initially cheaper to implement than OFDM. The US system is markedly less rugged than OFDM in conditions where multipath interference is likely to occur. In order to approach comparable coverage from the two systems in built-up areas, the planned US system would require receivers to incorporate a complex channel equaliser, which is likely to make receivers comparable in complexity and price with those using OFDM.
Q Is the DVB terrestrial system DVB-T only suited to the European broadcasting si tuation?
A No, the rugged DVB-T terrestrial system is suitable for use anywhere in the world. It is already being implemented in the United Kingdom, and plans for implementation are already advanced in Spain, Sweden, Denmark, and several other countries. Versions of the DVB-T specification have been developed to cater for different frequency planning arrangements and different channel spacings, i.e. 6,7 and 8 MHz, and the system will work in all parts of the world, and, very importantly, will co-habit with all existing analogue systems. Countries in many parts of the world have expressed great interest, and there is a growing feeling that it makes sense for countries to adopt the DVB standards rather than to develop proprietary or separate systems.
Q Having decided that DVB for terrestrial transmissions will use a multiple carrier OFDM system, why have you not settled on one single standard for the number of carriers to be used. Doesn't the fact that you have adopted both 2k and 8k systems suggest that the DVB system has been 'designed by committee' and will be less efficient because of that? Surely it would make sense to make a firm decision on one or the other, making chip-sets simpler and, ultimately cheaper?
A In the longer term it would be advantageous for all DVB-T users to use the same number of carriers, but there are practical problems if the earliest implementation date needed is to be achieved. Chip manufacturers are confident that they can have bulk supplies of chips suitable for processing 2k signals available by the end of 1997, when the first services are expected to begin, whereas it is generally believed that it will take longer before 8k chips can be manufactured in quantity. Those wishing to begin services in 1997 will probably have little choice but to utilise a 2k system, if they are to ensure that receivers are available for the start of service.
In any case, tests have shown that the 8k system is not strictly necessary for some applications. The 8k system is needed in countries where geographically large 'Single Frequency Networks' (SFNs) can be implemented. In countries like the United Kingdom, with a regional television system, the possibilities for using wide area high power SFNs are limited. There are also other constraints on the use of large SFNs, mainly concerning the likelihood of interference from such transmissions to other users in adjacent areas and countries. The UK would therefore gain little benefit internally from waiting for 8k chips to become available, particularly when market forces are demanding the earliest possible start to digital terrestrial services. Future receivers designed to decode 8k signals will in principle also be able to cope with transmissions using the 2k system, without modification.
Q Surely your tests showed the relative advantages and. disadvantages of using 2k and 8k systems?
A In a practical situation there are several parameters other than the number of carriers which can be adjusted to help provide the performance match needed for any particular application. The application of forward error correction, the type of modulation applied, and the guard interval can all be modified. The DVB systems all allow for a range of parameter sets, and this is part of their strength and usefulness.
Q Why should I choose DVB? Surely an 'open' system like DVB, where different pieces of equipment from different manufacturers are connected together is likely to give more problems than a proprietary system where all the component parts come from one single manufacturer? What do we do if we assemble a system with equipment from different manufacturers and it doesn't work, in spite of being DVB-compliant? To whom do I complain? Who will sort out my problems? Surely I would be better staying with the single manufacturer of proprietary equipment who offers me a 'one-stop shop' and guarantees that all his equipment will work together, though not to the DVB standard.
A DVB has over 200 member companies from 29 different countries, each committed to interoperability of both systems and equipment. All DVB members have access to the combined knowledge and expertise of the other members. If you adopt DVB as your digital broadcasting standard you can be sure of finding someone to help with your technical queries. The DVB Project Office in Geneva will be happy to point you in the right direction. In many ways this is more satisfactory than being tied to one proprietary system, where you may have only one possible source of help, the original manufacturer, and where equipment is not made in a competitive environment.

greycat

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seed

好东西 顶上去

haleliu

顶一下。

slp_yj

长见识了，让我这个菜鸟知道了DVB的一些知识