Revert to Section 7.2.3

7.3 Digital implementation strategies

Implementation strategies are under consideration around the world. A common feature of these strategies is that for most countries Digital Terrestrial Television Broadcasting (DTTB) will be implemented in the existing frequency bands of VHF or UHF television. A number of schemes under consideration involve simultaneous transmission or simulcast operation or both. To clarify simultaneous transmission involves the transmission of both the existing TV service and the new digital television service with either independent or the same programming. Simulcast operations involve the transmission of both existing and digital television services with identical programming either at the same time or within a specified time period. The implementation strategies presented in this report must be considered in the same development state as digital terrestrial television and should not be construed as the final implementation strategy chosen.

With respect of a suitable implementation strategy for digital terrestrial television two phases have to be considered - the transition period when both analogue and digital TV are present, and the period of only digital TV transmission. For the transition period three options of transmissions seem possible:

Option 1 - Simulcasting: Under this model existing terrestrial broadcasters would be given the option of broadcasting their service in parallel on a digital channel during a transition or "simulcasting" phase. Following this phase the broadcasters would cease broadcasting using the analogue standard so enabling the additional spectrum released to be used either to increase the coverage of the digital service and possibly additional broadcast services or for other services.

Option 2 - New services: Under this model any additional digital capacity would be allocated to new services. Although this would immediately increase the number and range of terrestrial channels available it would also mean that a two-tier level of services would exist so requiring dual standard television receivers and thereby reducing the possibility of an absolute transfer to a digital-only system in the foreseeable future.

Option 3 - Simulcasting and new services combined: This option combines aspects of the previous two options. Under this the simulcast strategy of option 1 would be pursued but any extra capacity identified would be allocated to additional digital services.

Since full coverage of digital TV services may not always be achievable the question arises: what is the appropriate balance between the transmitted net data rate (for the encoded video, audio and data) and the degree of ruggedness with which this data rate is transmitted? These and other system parameters are addressed in other sections of this report.

The available net data rate can then be used for broadcasting one or several television programmes with a picture quality for the individual television programme depending on the allocated data rate for it. One extreme is the transmission of only one television programme in HDTV quality. The other extreme is multi-programme transmission of television where each television programme is of VHS quality or lower. Different service providers are expected to have different priorities concerning the balance between picture quality and number of channels.

It is clear that different service providers will also have different views on the required ruggedness of the transmitted television programmes. Some would prefer a coverage very close to 100% whereas others would accept a lower coverage. A certain transmission ruggedness will lead to different service coverage depending on the receiving conditions. Some programme providers are interested in providing a service that is receivable with portable or even mobile receivers with omnidirectional antennas. Others will assume directional antennas with a certain degree of directivity and a corresponding increase in coverage.

Which of the outlined options to be implemented is a decision yet to be made. However, it should be noted that with respect to the planning and coordination of channels for digital terrestrial television, the shape of the spectrum and the modulation scheme are of importance. Decisions on future digital television implementation strategies will rest on market political and economic considerations as well as spectrum availability and technical criteria.

During the introductory phase it is likely that in most countries DTTB services will need to co-exist with current PAL/SECAM or NTSC services. Further, in countries that currently use 6 or 7 MHz channel plans there may be interest in adopting DTTB services that occupy bandwidths in excess of nominal 6 or 7 MHz channel bandwidths.

In the transition period when PAL/SECAM/NTSC and DTTB services have to co-exist in the same frequency bands it may be possible to accommodate this apparently impossible requirement by allowing a small amount of overlap of the spectrum of the DTTB and PAL/SECAM/NTSC signals..

The primary factor that limits the permissible amount of overlap between adjacent DTTB and PAL/SECAM/NTSC services is the adjacent channel interference rejection performance of domestic PAL/SECAM/NTSC television receivers. Preliminary investigations (Doc TG 11-3/-[AUS-2]) show that by operating the DTTB service at power levels in the region of 12-30 dB below adjacent channel PAL-B services it is feasible to accommodate a small overlap in spectrum of the PAL-B and DTTB signals.

The possible DTTB bandwidths that can be accommodated given either a nominal 6 or 7 MHz vacant channel are shown in Table 16 with the corresponding maximum emission mask values. The ability to successfully accommodate a greater bandwidth for a DTTB service implies an increase in the total data capacity of the digital service.

Table 16

The differences that have arisen throughout the world in the development of digital terrestrial television systems are largely in modulation systems. They are due to a variety of factors, in part the introduction time scales envisaged and the development approaches used, but also a more fundamental factor - the different frequency planning environments which exist in different parts of the world.

There is a wide mosaic of planning environments for the introduction of digital terrestrial television. The different factors include:

• sparse or dense concentrations of population;
• national broadcasting coverage of localised coverage;
• different levels of "taboo" channel availability;
• different levels of maturity in development of analogue television;
• frequency planning criteria of countries and their neighbours;
• requirements for mobile, portable or fixed reception;
• terrain variations.

If a single modulation system were to be used throughout the world, it would have to be based on the most difficult planning environment, because this is likely to need the most sophisticated modulation system. However, the costs and time scales of doing so may outweigh the benefits, for those with less demanding planning environments.

There would certainly be value in as many areas of the world using the same modulation system, as this would lower receiver prices for the consumer. However, there may be over-riding factors, which would have a more significant impact than the economies of scale on the prices of equipment and the services available. This may mean a plurality, but limited plurality, of modulation systems are needed throughout the world. The reasons are just as valid as those for differences in modulation system between terrestrial and satellite systems.

The ITU-R needs to take account of the varying requirements and develop Recommendations for a limited and closely related family of systems. This would allow the nations of the world who have not developed the systems, and who wish to start digital terrestrial services, to select the system most appropriate for them, based on their own circumstances.

Continue to Section 7.3.3

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