The workplan of the VALIDATE 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 SPECIFICATION

To obtain an early verification of the Specification, VALIDATE participants compared simulations of a DVB-T modulator developed independently by different laboratories. Once the simulations had been verified, real DVB-T modulators could be compared with the simulations, giving designers confidence that no errors had crept in during hardware design. Hardware interworking, verifying the Specification completely, could then be expected with some confidence.

2.1 Comparison of simulations

Five partners (BBC, Bosch, CCETT, Tele Danmark, Teracom) had developed software models of a general DVB-T modulator, including all specified DVB-T modes. Teracom proposed a 'Specification of test signals of DVB-T modulator' and conducted a series of comparisons of software-generated DVB-T signals from their own and other Partners' models. All the software models generated identical outputs, showing that all participants had the same understanding of the Specification. This encouraging result was reported to DVB in September 1996.

2.2 Interworking tests

Two DVB-T compliant modems have so far been completed by VALIDATE participants. These are the BBC modem and the modem belonging to the RACE dTTb project.The BBC modem implements all non-hierarchical 2k modes of the Specification; the dTTb modem, which was developed by Thomson Multimedia, ITIS, and CCETT, implements eight combinations of code rate, modulation level, and guard interval in both 2k and 8k.

In December 1996 interworking was demonstrated between the BBC modem and the dTTb modem. Signals were transmitted by the modulator of one modem and received and decoded by the demodulator of the other 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 also 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.

The BBC has since achieved interworking at the COFDM level with a modem built by the UK company DMV.

These interworking tests have shown that the DVB-T specification is sound. But they have 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.

Further DVB-T compliant equipment is currently nearing completion in the laboratories of other VALIDATE participants and will be tested for interworking when it becomes available.

3. TEST RESULTS

3.1 Laboratory tests

Detailed laboratory tests have been conducted with the dTTb modem at the RAI laboratories in Turin and with the BBC modem. Detailed results are given in a companion paper [4].

Some of the most important results to date are given in Table 2. They show that the performance of the DVB-T system is very much as expected from simulation results, thereby verifying the assumptions made when the Specification was written. Two points are particularly worth noting:

• the effect of a 'quasi-domestic' tuner* is very small, showing that phase noise effects are limited and do not penalise the 8k mode;
• reception is possible with a 0dB echo, in line with simulations, justifying the claims that have been made for COFDM.

Table 2: Key VALIDATE laboratory test results.

Test Condition	Result	Comments
Gaussian channel	Implementation margin 3 dB	Of this, 1.6 dB is the effect of channel correction
Effect of quasi-domestic tuner	<0.3 dB	8k, 64-QAM, r=2/3 8k mode not penalised by phase noise
Impulsive noise	C/I better than about 12 dB for pulse frequency >300 Hz	Comparable to results with previous OFDM systems
Echoes	C/N 6dB for 0 dB echo inside the guard interval' sharp failure outside the guard interval for (=1/4)	Expected by computer simulations. Receiver enhancements could improve performance outisde guard interval.
Protection ratios	CCI, DVB-T wanted: +1 to +8 dB, with PAL/SECAM interferer	64-QAM, r=2/3
	CCI, PAL-I wanted: 34 to 37 dB, with DVB-T interferer	Grade 3 (tropospheric interference)
	ACI, PAL-I wanted: -8 to -9 dB, with DVB-T interferer	Grade 3 (tropospheric interference)
Doppler channels	Max. Frequency difference between two paths: 270 Hz max.	0 dB echo, 55 s delay, Doppler shift applied to one path only. 2K, QPSK, r=1/2. UHF channel 59
Receiver synchronisation	<1s	Very rapid

3.2 Field trials

The BBC has been broadcasting trial DVB-T signals from transmitters in London (Crystal Palace) and Newcastle (Pontop Pike) since June 1996 [5]. These broadcasts have allowed fairly extensive field trials to be performed. Early results were reported to ITU Task Group 11/3 [6]; full results will be given later this year. Figure 1 gives some representative results.

Fig. 1 - Representative results from BBC field trials:
Distribution of minimum C/N values for Pontop Pike.

The field trial results are generally as good as or better than predictions, showing that the values measured in the laboratory are achievable under practical conditions. These results again verify the assumptions made when the Specification was written.

4. PREPARATION FOR LAUNCH OF SERVICES

Once the principles of the DVB-T specification have been verified, broadcasters need to prepare for the start of services as soon as possible. This will verify that the Specification meets their requirements.

As mentioned above, the BBC has been broadcasting trial DVB-T signals since June 1996; other VALIDATE participants are also preparing trials. The experience gained from preparing for these trials has been made available to broadcasters not involved in the Project in the form of Implementation Guidelines [8]. 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. The following sections give an indication of the scope of this aspect of the Project's work.

4.1 Basic aspects of DVB-T networks

Two approaches are possible to the planning of DVB-T networks: multi-frequency networks (MFNs) and single frequency networks (SFNs).

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.

SFN techniques can be used on a smaller scale to fill gaps in coverage, or even within a house where a domestic gap-filler could improve portable reception. VALIDATE partners have been studying such domestic gap-fillers. They have 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 has given encouraging conclusions and a prototype is being built for tests in a planned SFN in Madrid (see Section 5).

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.

4.2 Network planning

Digital television service coverage is characterised by a very rapid transition from near perfect reception to no reception at all and it thus becomes much more critical to be able to define which areas are going to be covered and which are not. In a section based on a contribution from the EBU, the Implementation Guidelines give definitions of service planning terms as used for digital television, give details of the field strengths needed in different bands for different reception conditions, and consider the protection ratios that must be used to allow for the effects of interference when digital services share the UHF band with analogue services.

4.3 Setting up DVB-T transmitters

The digital television transmitters will, in general, re-use the same sites as existing analogue television transmitters, so that a large part of the existing analogue infrastructure may be re-used. In some cases a new antenna will be needed; if the existing antenna is to be used, then the digital signals have to be combined at high power with existing analogue signals or the ensemble must be passed through a multichannel amplifier. Different problems of filtering and non-linearity arise in each of these cases.

When a digital channel is to use a channel adjacent to an existing analogue TV signal, it may be necessary (depending on the analogue standard in use -- System I PAL and System L SECAM have somewhat wider bandwidths than System G PAL) to apply an offset to the digital signal or to restrict the lower (vestigial) sideband of the analogue signal to reduce interference. For instance, in the UK it has been proposed to restrict the nominal 1.25MHz bandwidth vestigial sideband of System I to the 0.75MHz bandwidth of System G; this is not expected to have any significant effect on the quality of the received analogue picture.

Recommendations on these and other aspects of transmitters are given in the Implementation Guidelines and in a specification for transmitters that VALIDATE is currently preparing.

4.4 Primary distribution

A digital primary distribution network will be needed to distribute MPEG-2 transport streams from TV studio centres to remultiplexing sites (if the network has regional variations) and to transmitters. Possible choices are optical fibre, PDH or SDH networks, ATM, and satellite distribution; of course a real network may use a combination of these techniques. The timing of the primary distribution must be controlled to ensure that it does not induce jitter in MPEG-2 decoders and to ensure stable synchronisation of the MPEG-2 multiplexers and the COFDM modulators.

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 cascaded with 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.

4.5 SFN synchronisation

All transmitters in an SFN must be synchronised so that their broadcasts are frequency identical and bit identical. VALIDATE partners have devised a method of synchronising all the transmitters in an SFN by identifying a megaframe in the MPEG-2 transport stream using a megaframe identification packet (MIP). This specification has now been accepted by DVB and proposed to ETSI [9]. The megaframe length has been chosen to contain an integral number of OFDM frames, of Reed-Solomon packets, and of the energy dispersal sequences, thus ensuring that it is possible to produce identical waveforms at each transmitter. The MIP contains a timestamp indicating the time at which the megaframe should be broadcast, related to a universal time and frequency reference such as that available from the GPS satellite system. By comparing the timestamp with the universal time reference at the transmitter, all transmissions can be time synchronised.

4.6 Availability of equipment

Perhaps the most critical requirement for the adoption of a new standard is the availability of equipment to support it. DVB-T equipment for broadcasters' networks (such as modulators and transmitters) has been announced by a number of manufacturers, including several of the VALIDATE participants.

Domestic receiver manufacturers will require integrated circuits for demodulation and decoding of DVB-T signals. Such ICs have been announced by at least four large manufacturers, all of whom are expecting to deliver samples in the coming year.

5. TIMESCALES FOR SERVICES

Two countries have announced their intention to start services: the UK and Sweden. Planning is well advanced in several other countries although political and commercial decisions are still awaited before a service can be announced. For instance, Spain has already set up an experimental digital terrestrial TV network in the Madrid area [10], and in Germany the trials in Cologne and Berlin are evidence of an increasing interest.

In Sweden, pre-operational DVB-T services are expected before the end of 1997 and fully commercial services with two multiplexes are expected in March 1998, in synchronisation with expected availability of terrestrial consumer set-top-boxes. The service planning is partly based on SFNs. An Act of Parliament enabling the start of digital terrestrial television was passed in April 1997.

In the UK, Parliamentary approval for the start of digital terrestrial television was given in 1996. Service planning is based on MFNs [11], with six multiplexes available at most transmitting sites. All the existing terrestrial broadcasters have expressed their intentions to take up the multiplex space that has been offered to them, and two new consortia are bidding for the remaining multiplex space. Services are expected to start in mid-1998.

6. CONCLUSIONS

The DVB-T standard has been technically verified by software comparison of simulations from different laboratories and by interworking tests with conformant modems. Laboratory tests have verified that the performance of the system matches the assumptions made when the Specification was prepared, and field trials have given excellent results.

Members of VALIDATE have worked on all aspects of distribution networks and transmitters, and have published their results in the form of Implementation Guidelines to help other broadcasters to implement the system quickly.

Transmission equipment and ICs for receivers have been announced by a number of leading manufacturers and services will be launched in the UK and in Sweden in the coming year.

Thus the DVB-T specification has been thoroughly tested and is well supported. It offers an excellent choice for the future of terrestrial broadcasting.

7. ACKNOWLEDGEMENTS

The author wishes to thank the many colleagues in the BBC and other organisations participating in the VALIDATE project who have contributed to the work described in this paper.

VALIDATE is partly funded by the European Commission through the fourth Framework programme.

8. REFERENCES

1. ETS 300 744 (1997): Digital broadcasting systems for television, sound and data services; framing structure, channel coding and modulation for digital terrestrial television.
2. MØLLER, L.G., 1997. COFDM and the choice of parameters for DVB-T. Proceedings of 20th International Television Symposium, Montreux.
3. STOTT, J.H. 1997. Explaining some of the magic of COFDM. Proceedings of 20th International Television Symposium, Montreux.
4. MORELLO, A., BLANCHIETTI, G., BENZI, C., SACCO, B., and TABONE, M., 1997. Performance assessment of a DVB-T television system. Proceedings of 20th International Television Symposium, Montreux.
5. OLIPHANT, A., MARSDEN, R.P., POOLE, R.H., and TANTON, N.E., 1996. The design of a network for digital terrestrial TV trials. Proceedings of the 1996 International Broadcasting Convention, IEE Conference Publication No.428, pp. 242-247.
6. ITU R, 1996. Digital terrestrial television broadcasting service coverage studies and field trials. TG 11/3 draft Report.
7. WECK, C., 1997. Receiving DVB T: Results of field trials and coverage considerations. Proceedings of 20th International Television Symposium, Montreux.
8. VALIDATE project, 1997. Draft implementation guidelines to DVB. Obtainable from: http://www.bbc.co.uk/validate/
9. Draft EN 301 191 (1997): Specification of a Megaframe for SFN synchronisation.
10. CAÑIZARES, P., TORRES, J.L., MARTÍNEZ, J.A., 1997. VIDITER: Spanish experience on DTT. Proceedings of the International TV Symposium, Montreux.
11. MADDOCKS, M.C.D., TAIT, B., LAFLIN, N.J., and DOEL, G., 1996. The plan for digital terrestrial television in the UK. Proceedings of the 1996 International Broadcasting Convention, IEE Conference Publication No. 428, pp. 172-177.

*	That is, a laboratory prototype that is similar to a domestic tuner -- rather than a professional one -- with performance representative of what manufacturers believe could be achieved with a domestic tuner in mass production. Return to Text