Tackling the digital black spots
After DTTB network deployment, there are inevitably some places the signal won’t yet reach. Stephen Farrugia, Broadcast Australia Technology Director, explores the options in tackling coverage of these digital ‘black spots’
The first Australian digital terrestrial television broadcast (DTTB) services were launched on 1 January 2001 in the main metropolitan markets of Sydney, Melbourne, Adelaide, Brisbane and Perth. Since that date, services have been progressively rolled out through regional centres, translator sites, major tourist areas and remote rural regions to cover more than 97 percent of the nation’s widely dispersed population.
Now, all that remains before analogue is switched-off are the final pieces in the patchwork-the extension of coverage into those regional and remote areas that are currently digital ‘black spots’.
As their name suggests, black spots are coverage gaps in the digital signal, and may be present during the digital/analogue simulcast period for a number of reasons. The main one is spectrum scarcity during the simulcast period: there is often simply not enough spectrum to permit conversion of every single existing analogue service and maintain a simulcast, resulting in some services for smaller communities missing out in the short term.
Beyond the cliff
There are other, more indirect, reasons as well. Digital signals fail differently to analogue signals; this is exemplified by the well-documented ‘cliff effect’ exhibited by digital video broadcasting (DVB) signals, where the picture and audio quality are excellent, then abruptly deteriorate to become unwatchable, as opposed to the more graceful degradation of analogue.
If sections of a community reside ‘beyond the cliff’, so-to-speak, then, despite their ability to receive watchable (if not perfect) analogue television, they will unfortunately reside in a digital black spot. A similar scenario can arise where populations have expanded, if this hasn’t been taken into account during the digital network planning stage.
Broadcast Australia has recently analysed digital coverage in the Australian states of Victoria and Queensland to explore where the black spots are and what might be required to fill them. In Victoria alone, there are more than 30 existing so-called analogue ‘self help’ sites-where a metropolitan, regional or remote community broadcasts services under license. Such facilities arise most commonly where geographical obstructions to the signals from main transmission sites exist, or where the communities are very small. Assuming these self-help sites are converted to digital transmission, the study suggests that some 17 additional sites would still be required to fill gaps in digital coverage using terrestrial broadcasting.
Spectrum availability is undoubtedly one of the central considerations. It will always be a limiting factor, especially during simulcast periods; moreover, there is global widespread interest in what has become known as the ‘digital dividend’ once analogue services are switched off.
In particular, the upper portion of the UHF band (>700MHz) is attracting interest from government regulators all around the world as a candidate for next-generation services-such as wireless mobile broadband, enhanced digital terrestrial services (standard or high-definition), mobile TV and some non-commercial applications. Additionally, digital radio services-such as DAB+ being deployed in Australia-could be assigned to liberated VHF Band III spectrum.
Whether the target is 20MHz or 120MHz for the digital dividend, or merely the introduction of new services, it is helpful to start planning with the end in mind. By establishing the policy framework up front, networks can be deployed to greatest effect and lowest total lifecycle cost to the country as a whole.
For example, if a specific dividend is targeted, the spectrum plan can take this into account and ensure that a minimum number of digital services are assigned to frequency channels in the nominated spectrum. Moreover, if the targeted dividend is significant, a large number of single-frequency networks (SFNs) may be necessary to make most efficient use of the available spectrum.
When it comes to the digital black spots, the spectrum scenario becomes more challenging. In order to address spectrum congestion, many countries will likely be faced with the prospect of managing a hard cutover from analogue to digital services in these smaller communities.
The UK experience is demonstrating that such hard cutovers are achievable, given an appropriate level of public education and retail support to ensure the right equipment is available. The UK rolled out digital television a decade ago with a mere 80 sites; at the end of the current digital switchover project, these will ultimately have grown to more than 1150 sites, over 1000 of which will have experienced a hard cutover with no simulcast period.
A tempting alternative to the hard cutover of multiple small terrestrial services might be to provide blanket satellite coverage for those regions not serviced by the existing digital network. Here, decisions need to be made as to the balance between the overall cost and benefit to the population.
Terrestrial services present the best outcome for the consumer for several reasons. One of the most compelling arguments is the provision of localised content-such as news and advertising-which would be limited with a blanket satellite service. Yet there are cost and quality benefits as well. The purchase and installation of satellite receive systems is a greatly more expensive proposition than the use of terrestrial receive systems. Moreover, terrestrial services are more resilient to environmental factors such as heavily treed areas and rain fade.
It will rarely be possible or practical, however, to service 100 percent of a population terrestrially, and an element of satellite coverage will always be warranted. The end result is a hybrid coverage model, where satellite coverage is used as a safety net. For example, direct-to-home satellite might be more viable for communities of only around 30 homes; whereas terrestrial services might be justifiable for remote communities of around 500 homes.
Ultimately, the approach taken to addressing digital black spots will vary from country to country, depending on government policy, spectrum availability and other market factors. The important thing to remember is that deploying terrestrial services is a viable option for most of the population, and, given intelligent spectrum planning up front, will certainly permit a significant digital dividend if desired.
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