a virtual laboratory to accelerate the launch of digital terrestrial television
Andrew Oliphant BBC Research & Development Department,
ABSTRACT
1. INTRODUCTION 1.1 The DVB-T Specification The European DVB project has prepared standards for digital television transmission by cable (DVB-C), by satellite (DVB-S), and most recently for terrestrial transmission (DVB-T). Of these three, the DVB-T specification [1] presented by far the greatest challenge. Terrestrial digital TV transmission has to share the broadcasting spectrum with analogue TV transmissions. It must therefore use the same VHF or UHF channels as used for analogue transmission, and is subject to the familiar impairments of terrestrial TV reception -- noise and echoes -- and also to interference from more powerful analogue TV signals. The existing usage of the TV broadcasting spectrum varies significantly from country to country, and there are different applications foreseen for digital television in different countries. These considerations require coding and modulation that is robust against echoes and interference, with the flexibility to serve a wide variety of applications and implementation scenarios. Some compatibility with the cable and satellite specifications is also desirable. The choice was to use COFDM (coded orthogonal frequency division multiplex) modulation, with the outer interleaving and Reed-Solomon coding used in the DVB cable and satellite specifications, inner coding used in the satellite specification, and an inner interleaving appropriate to the COFDM parameters. The DVB-T specification and the choice of parameters have been described by L.G. Møller [2], and the advantages of COFDM in dealing with echoes and interference have been explained by J.H. Stott [3]. Flexibility is obtained by specifying a wide range of options for different modulation constellations, guard intervals, and code rates. Thus different compromises are possible between payload and robustness for the different applications foreseen in different countries. Inevitably, this robustness and flexibility led to a complex specification which needed careful verification, a task needing the combined expertise of broadcasters, network operators, and manufacturers. This verification was the first aim of the VALIDATE project. 1.2 The VALIDATE Project The VALIDATE project started work in November 1995, when the DVB-T specification was close to being finalised. Its aims are to verify in detail the DVB-T specification and carry out technical work that will speed up the start of services. VALIDATE is part of the ACTS programme sponsored by the European Commission as part of the fourth Framework programme. It is led by the BBC and includes several broadcasters and the EBU representing other public service broadcasters in Europe. Other participants (see Table 1) include broadcasting research centres, telecom operators (which in some countries are responsible for distribution and emission of broadcast signals) and both professional and domestic equipment manufacturers.
VALIDATE partners have in effect set up a 'virtual laboratory', standardising test procedures and exchanging and comparing test results from laboratories all over Europe. The workplan of the project includes upgrading existing COFDM modems to conform with the DVB-T specification and conducting laboratory tests and field trials both to verify the Specification and to supply the parameter values needed for service planning. VALIDATE is also studying all aspects of transmission and distribution of the signals including primary distribution networks, transmitters, sharing with existing analogue services and re-broadcast transmitters (gap-fillers) -- when COFDM is used, domestic gap-fillers become a real possibility that could provide 'portable' reception throughout a house or flat even in areas of low signal strength. 2. VERIFYING THE DVB-T SPECIFICATION To verify the DVB-T specification, VALIDATE had to show:
2.1 Verifying the Specification
2.1.1 Comparison of Software Models
2.1.2 Hardware Interworking
In December 1996 interworking was demonstrated between the BBC modem and the dTTb modem. The test was successful with both senses of interoperability, and for all modes that were tested. (All 2k non-hierarchical modes that the dTTb demonstrator is capable of working with were tested, except one, which was omitted in one direction by an oversight!) MPEG-2 coded video and audio were transmitted successfully in both senses. The test was successful on the first interconnection of the equipment. This was the first demonstration of interoperability between a modulator fully compliant with the DVB-T specification and a compatible demodulator. This success was an important step in the work of VALIDATE. These interworking tests showed that the DVB-T specification is sound. But they also uncovered some areas where, although the Specification is correct and unambiguous, some clarifications would be helpful to equipment manufacturers in the future. VALIDATE has prepared an Informative Annex to the DVB-T specification drawing these points to the attention of all users. Because of the very powerful error correction coding in the DVB-T specification it is possible for a modulator and demodulator to interwork even though one or both of them may not be totally conformant with the Specification. To demonstrate full conformance it is necessary to demonstrate not just interworking but error free interworking before error correction is applied and interworking with the expected level of performance in a difficult channel. Since the important tests described above, more equipment has become available to VALIDATE: a modulator from Bosch, a modem from CCETT (the Sterne IV modem), a modem from ITIS based on the Sterne IV modem, a modem from Rohde & Schwarz, and a modem from Teracom. Some of these modems are still being optimised, but all have been demonstrated to interwork with at least one other modem. 2.2 Laboratory Tests Detailed laboratory tests have been conducted with the dTTb modem at the RAI laboratories in Turin and with the BBC modem. The first BBC demodulator was optimised for ultimate carrier-to-noise ratio performance in a Gaussian channel, and achieved performance very close to the theoretical figures given in the DVB-T specification - the second aspect of verifying the Specification. However, when the channel equalisation is modified to give improved performance in time-varying channels, the results in a Gaussian channel are degraded by a small amount, accurately predicted by simulations. Detailed results were given by Morello et al [4] and by Nokes et al [5]; a summary of the most important results is given in Table 2.
2.3 Field Trials Field trials in Germany have been reported by C. Weck and R. Schramm [6], and field trials in the UK by Nokes et al [5]. The trials in Germany compared several different modes of the Specification at a number of sites near Munich, whereas the UK trials used one mode continuously (2k, 16-QAM rate 3/4 -- later changed to 64-QAM rate 2/3), but at a large number of sites in London and in north east England The results showed that coverage is at least as good as early planning studies expected, and confirmed the values of planning parameters being used by CEPT. Since the tests reported in [5] and [6] VALIDATE partners have conducted further trials in France, Germany, Spain, Italy, Denmark, Sweden, Ireland, and The Netherlands. These trials have addressed different aspects of DVB-T transmission including: field strength variation, building loss, comparison of actual and predicted service area, portable reception, mobile reception, reception in a single-frequency network, and the influence of co-sited or non-co-sited transmitters. A synthesis of the results is given by Weck [7]. 3. PREPARING FOR THE LAUNCH OF SERVICES 3.1 Implementation Guidelines VALIDATE participants have amassed a great deal of experience in all aspects of the implementation of DVB-T services. To make this experience available to broadcasters not involved in the Project they have prepared Implementation Guidelines These Guidelines draw attention to the technical questions that need to be answered in setting up a DVB-T network and offer some guidance in finding answers to them. They give an explanation of the DVB-T specification and the basic characteristics of transmission networks; they then cover transmitters and issues of sharing with existing services, distribution networks, SFN operation, and network planning. The Guidelines were submitted to DVB and to the EBU and were first published on the VALIDATE website (http://www.bbc.co.uk/validate/). They were welcomed by DVB as "a major contribution to the list of deliverables of the [DVB] project [which] will become part of our marketing campaign to the rest of the world". The Guidelines have now been published as an ETSI Technical Report [8]. 3.2 Service Planning Parameters Results of VALIDATE laboratory tests and field trials were reported as they became available to the EBU, ITUR, and CEPT. In June 1997 a massive document (120 pages) bringing together all the results relevant to service planning was sent to CEPT PT FM24 which was preparing for a conference on coordination procedures for digital terrestrial television held in Chester (UK) in July 1997. Delegations from 37 countries attended this conference; many of them had no direct experience of DTT, but the comprehensive results available, confirmed by a demonstration of reception in the BBC measuring vehicle, reassured them that the Conference proposals were realistic. All approved the output document [9] which gives details of procedures for international coordination of frequency allocations and transmitter powers and tables of parameters to be used. 3.3 Transmission and Reception
3.3.1 Network Configuration
MFNs are planned in the same way as analogue networks, using an individual set of radio frequencies for each transmitter. This approach might be considered when an Administration wishes to re-use some or all of the spectrum used for analogue broadcasting. SFN planning relies on the relative insensitivity of COFDM to delayed signals arriving within the guard interval. It is possible, if a suitable frequency is available and a sufficiently long guard interval is chosen, for all transmitters in a region, or in a country, to use the same frequency. The Implementation Guidelines give advice on the relative advantages of the two approaches and on the modes of the Specification that might be appropriate for different kinds of networks and modes of reception.
3.3.2 Primary Distribution
Standards for transporting MPEG-2 signals in PDH, SDH, and ATM networks have been prepared by DVB and early equipment is being tested by VALIDATE participants in their trial networks. In one VALIDATE trial conducted by the BBC an SDH network was cascadedwith the JAMES international ATM network to feed a satellite uplink station. The signal received from the satellite was then remultiplexed with local programmes, distributed via an optical fibre link, to simulate a regional opt-out. The resulting transport stream was COFDM modulated and perfect reception was demonstrated. This showed that all the challenges of network synchronisation in a mixed primary distribution network can be met.
3.3.3 SFN Synchronisation
To test this synchronisation technique, VALIDATE partners RTÉ and ITIS set up, with the assistance of TDF, a DVB-T Single Frequency Network (SFN) using two transmitters in the Dublin area on 8 November 1997. The transmission mode used for this experiment was 8K, 64-QAM, R=2/3, guard interval=1/4. An MPEG-2 Transport Stream generator, an SFN adapter, a DVB-T modulator, and a 1KW TV transmitter operating at 50 W were set up at the site of Three Rock. A second DVB-T modulator and a 25 W transmitter were set up at the site of Donnybrook. Both transmitters used UHF channel 30. A 34 Mb/s PDH link was established from Three Rock to Donnybrook which fed the second DVB-T modulator with the MPEG-2-TS output from the SFN adapter. This complete SFN arrangement was synchronised by using GPS receivers. At a site near Donnybrook where the signals from the two transmitters were at similar levels, the signals was received successfully with a small omni-directional antenna and a professional DVB-T receiver. This field arrangement represents the world's first SFN operation based on a real primary distribution network according to the ETSI specification [10].
3.3.4 Transmitter Parameters
Transmitter performance is largely based on the idea of Equivalent Noise Degradation (END) as the main (perhaps the only) performance criterion. The degradations in performance produced by different impairments can be expressed in terms of the loss of noise margin that they produce in a Gaussian channel, and these noise equivalents can then be added as noise powers to derive an END figure for the transmitter. The work of VALIDATE has shown that this procedure is valid provided the individual impairments are small (each significantly less than 3 dB loss of noise margin). An alternative method of specifying the overall performance of a transmitter is the Equivalent Noise Floor (ENF). To measure ENF the transmitter is connected to a demodulator and noise is added to achieve quasi-error-free reception (QEF -- bit error ratio of 2x10-4 before Reed-Solomon correction, corresponding to about one error an hour after correction). The transmitter under test is then replaced by an undistorted laboratory test modulator and noise is added from a second noise generator in parallel with the first to obtain QEF reception again. The level of noise from the second generator then represents the ENF of the transmitter. It will often be helpful to broadcast DVB-T from the same sites as analogue TV signals, re-using transmission infrastructure and receiving antennas and maximising the number of households able to receive the signals at the start of services. A prototype 8-cavity high power combiner for combining a DVB-T signal with adjacent channel analogue TV signals has been designed and built by Thomcast and will be installed at a TDF experimental site at St Pern near Rennes. Much useful information about the issues raised in sharing sites with analogue transmissions is included in the Implementation Guidelines.
3.3.5 Gap-Filler Transmitters
A professional gap-filler was demonstrated by Mier and DT Berkom in Berlin to provide coverage to the Potsdam area that is shadowed by hills from the main transmitter at Alexanderplatz in the centre of Berlin. With both receiving and transmitting antennas mounted on the same concrete tower an isolation of 105 dB was obtained. The ripple on the output DVB-T signal spectrum was less than 3 dB with an output ERP of 100 W. Field trials in Potsdam showed that portable reception was possible at all locations with a reasonable field strength. One important conclusion is that some trials have shown that no signal degradation has been observed for a ripple amplitude up to 10 dB peak-peak.
For the domestic gap-filler, Televés developed a channel
model based on field tests in houses and a device model to study
the configuration, the antennas that might be used, and the gain
that might be achievable. As a domestic device, the safety of
such a gap-filler and its cost have been important considerations.
A feasibility study gave encouraging conclusions and a prototype
was built. In a first test conducted by the BBC in the London
area, the domestic gap-filler gave sufficient field strength to
provide portable reception in all rooms of a house with an output
power less than 200
3.3.6 Mobile Reception
More details about mobile reception of DVB-T are given in another paper by Burow et al [11]. 3.4 Trials and Demonstrations The first broadcast conforming to the DVB-T specification was made on 9 April 1996 by the BBC from the Crystal Palace transmitter in London. It was received at the BBC's west London centre at White City, and at BBC Research and Development at Kingswood Warren, south of London. In June 1996 the BBC started a trial service, broadcasting a multiplex of four TV programmes with sound and data from Crystal Palace and from the Pontop Pike transmitter serving Newcastle-upon-Tyne in north-east England; the broadcasts from Pontop Pike carried BBC North East regional variations, demonstrating one of the important advantages of terrestrial transmission. This trial service was demonstrated to a wide range of broadcasters and the broadcasting industry. The trial broadcasts from Pontop Pike have now ceased; the broadcasts from Crystal Palace will probably continue until replaced by operational services. The first public demonstration was given at the International Broadcasting Convention in September 1996 when the MPEG-2 transport stream providing the BBC trial service in London was carried to Amsterdam over an international ATM link kindly provided by ACTS project JAMES and broadcast by the local broadcaster NOZEMA with the helpful collaboration of several other VALIDATE participants. Because this demonstration showed live transmission of network TV programmes with some high quality widescreen material and a realistic EPG, it was perceived as more than an engineering demonstration: for many delegates it was a first exciting experience of what digital TV broadcasting can offer.
Another major demonstration was led by TDF and CCETT at the
Montreux International TV Symposium in June 1997. A multiplex
containing four TV programmes was assembled on the TDF stand and
COFDM modulated. The COFDM signal was transmitted to Thollon
on the other side of Lake Geneva from where it was broadcast in
UHF channel 49 (698 MHz). The signal broadcast from Thollon was
picked up at Clarens on the Montreux side of the lake and re-broadcast
on the same frequency. Reception was demonstrated on the VALIDATE
stand in the exhibition with a rotatable antenna. The signal from
Clarens arrived about 1.5 IBC and Montreux are exhibitions for the broadcasting industry. DVB-T was shown to the general public at the Internationale Funkaustellung (IFA) in Berlin in September 1997 where both Deutsche Telekom and IRT gave demonstrations. Eight TV programmes and a data service were broadcast using three UHF channels, two of which were adjacent to PAL services broadcast from the same mast. Fixed and portable reception were demonstrated at the exhibition site with good reception even indoors. Mobile reception was demonstrated in a car and buses. The availability of the dTTb modem has allowed several Partners to arrange short field trials, often including demonstrations to influential national or international groups. Such trials and demonstrations have been held in Turin, Munich, Madrid, Copenhagen, and Dublin and have contributed greatly to the increasing interest in DVB-T. 4. CONCLUSIONS The work of the VALIDATE project has verified the very complex DVB-T specification, technically proving the excellent behaviour of DVB-T in critical broadcasting situations including co- and adjacent-channel interference and portable reception. This work has ensured its unanimous acceptance by ETSI members less than one year after the completion of the Specification. This success has been achieved thanks to the excellent teamwork of the Partners in exchanging and comparing test results from laboratories all over Europe. VALIDATE partners have developed a range of prototype equipment and have contributed to open standards. The commercialisation of VALIDATE prototypes will ensure that a wide choice of equipment is available to support the launch of digital terrestrial TV. VALIDATE has studied all technical aspects of the implementation of DVB-T networks and services. It has reported its work to the DVB project and has made its experience available to other broadcasters in the form of Implementation Guidelines. In particular it has pioneered the mobile reception of DVB-T signals and has developed the concept of the 'gap-filler' transmitter for DVB-T. A collaborative project of this kind involving broadcasters, network operators, and equipment manufacturers is an excellent vehicle for verifying standards and to ensure a common basis for the early start of services. 5. ACKNOWLEDGEMENTS The Author would like to acknowledge the active participation of all of the Partners in AC106 VALIDATE in the work described in this paper. VALIDATE is supported by the Commission of the European Union through the fourth Framework programme. VALIDATE has relied heavily on the work done in earlier collaborative projects including RACE dTTb, the Nordic HD-DIVINE project, and the German HDTVT project. REFERENCES
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