Going digital means a whole lot more than changing an exciter in the transmitter. DTT deployments also involve detailed coverage modeling, careful equipment selection, and skilled installation and maintenance teams.
As digital broadcasting steps up a notch in the Asia Pacific region, broadcasters poised on the verge of deployment can benefit from the experiences of those who have pioneered DTT technology. For six years, Broadcast Australia has been building and operating one of the world's most extensive digital television network deployments. It includes more than 300 individual DVB-T services, with 85 services in single-frequency networks (SFN). This national network provides an example of what is achievable with careful planning, system design and build-out, and expertise in digital technologies.
One of the most important criteria is to recognize that digital technology is vastly different from analog. This necessitates a new approach to almost every facet of network deployment and operation — including the development of new skills and the ability to adjust to continuously changing technology. It is, therefore, important to consider the technical challenges associated with deploying a digital network, as well as keeping it on-air long-term.
The digital migration
Detailed RF coverage planning using purpose-built computer modeling tools is an essential first step in network deployment. It is usually necessary to work within the framework provided by the government authority responsible for channel allocation and required effective radiated power (ERP) levels. Both have significant implications for planning the network and controlling interference. For example, the use of adjacent RF channels promotes co-siting of services and requires advanced system design. And electromagnetic emissions need to fall within local regulations.
The decision to use SFNs (as opposed to multifrequency networks) introduces a host of additional challenges. Highly spectrum-efficient, SFNs require state-of-the-art engineering to achieve the precise site launch timing that enables multiple adjacent broadcast sites to operate on a single frequency. It is most effective to limit the degrees of freedom associated with the network, such as location and number of sites, ERP, site launch timing, and control over the modulation scheme and guard interval. Even with the most advanced modeling tools, SFN planning is still a best-estimate scenario and requires post-installation fieldwork to optimize the network.
Another key element of network planning and design is the assessment of existing infrastructure on a site-by-site basis. This includes working out how the new services can be accommodated. Aspects under review include equipment room floor space, electrical and transmitter power, antenna system capability and analog equipment performance — all of which affect capital and operational expenditure.
System design and integration is also a crucial issue in a DTV network. The range of program feed types and new technologies make effective equipment evaluation essential to achieve interoperability. It is, therefore, important to complete as much equipment evaluation and trial integration as possible before network deployment.
The types of system issues that may ultimately be experienced and addressed vary widely. For example:
- Burstiness of asynchronous serial interface (ASI) sources, if excessive, can prevent interface with a following device.
- Receiving program feeds via other off-air DVB-T signals can be problematic, as receivers can be sensitive to changes in path conditions, and generally need extra filtering due to the wideband nature of their first RF stages and the possibility of adjacent channel services.
- For SFNs, the transport stream must not be modified after it has left the headend SFN adapter, because injudicious use of bit rate adaptation will cause the SFN to fail.
- Power conditioning or redundancy can prevent equipment resets and lengthy start-up times in the more susceptible microprocessor-based digital equipment.
- Digital systems require different RF redundancy configurations than equivalent analog services.
- For SFNs, each site's identical frequency must include any offset used for combiner optimization purposes.
In light of these considerations and other system issues, it is important that the installation team is appropriately skilled. Appropriately trained operators will also help maintain tight control over equipment supply, testing, installation and commissioning. The aim should be to minimize mobilization costs and maximize installation quality by deploying specially trained project teams on a region-by-region basis. System integration is considerably more complex in the digital domain compared with analog.
On-air and ongoing
Deployment of the digital network is only the first stage. Once on-air, a myriad of ongoing challenges needs to be met to ensure the quality of the service, because the change in technology is significant. Staff training is an important part of this — whether in engineering, installation, monitoring or maintenance.
DTV monitoring and management is more complicated than for an analog network, yielding an increased number of indications and faults that need to be assessed and dealt with. The incorporation of multiplexed services fed into a single transmitter may also need to be monitored at the individual service level, representing a change in philosophy.
Monitoring and management in general are improved by the move toward IP networks, which have stimulated a marked leap in the ability for remote identification of system status, interrogation and rectification. But such advanced IT-based functionality requires an equivalent improvement in IT skills on the part of technical broadcast staff. Whatever is installed will almost certainly demand some aspect of IT network engineering.
Clearly, the migration to digital broadcasting involves multiple deployment stages, including detailed coverage planning, equipment selection, system design and installation, and development of the required skills to effectively operate and maintain the network for the long-term. With ongoing technology developments continuing to provide significant network benefits, broadcasting has entered a new level of dynamism, requiring a high level of technical expertise.
Stephen Farrugia is engineering director for Broadcast Australia.
