The current state of digital transmitter technology
The ever-advancing technology of digital transmitters has created a new era for suppliers in digital multimedia broadcasting. Such rapid change raises many questions and can be viewed simultaneously as exciting, daunting and challenging. As such, it is crucial to become familiar with this increasingly complex technology to ensure that the right choices are made when developing plans for network transmission infrastructure.
There are several key considerations to evaluate when developing implementation plans for new terrestrial digital multimedia networks. These include the various digital and mobile TV standards, analog-to-digital transition, new technologies, implementation and the impact on space planning, power levels and costs, and their resulting effect on infrastructure requirements.
While the path to a digital future may seem complex and confusing, understanding a few major points about the various network transmission solutions will help determine the right transition path.
Rapid standards development has become the norm for the delivery of content to consumers, either in their homes or on the move. Long gone are the days of determining which version of PAL to use for color television. Now, networks must deliver content with the most modern approach, while leaving open the possibility for easy and affordable future upgrades. For example, many broadcasters need to upgrade aging analog networks to ensure complete nationwide coverage, but they are not ready to make a “hard cut” to digital. Transmission infrastructure must have an upgrade path from today's analog operation to deliver crystal clear content to existing users yet still provide a smooth transition so broadcasters can make the switch with minimal upgrade costs and downtime.
In addition to the analog-to-digital transition, several existing digital standards are now seeing either extensions or next generations to allow for more payload capability. The DVB family of standards is one example — the extension of DVB-T to DVB-H for handheld use, either by offering a separate DVB-H mix for mobile or via hierarchical modulation techniques to use some of the bandwidth in the same transmitter as the DVB-T, or the newly announced DVB-T2, which boasts greater payload capacity targeted at offering HD content. (See Figure 1.)
ATSC has a similar migration path with the evolution of ATSC mobile in the ATSC-M/H standard. Standards in Asia continue to evolve, such as China's CTTB for terrestrial and both CMMB and TMMB for mobile content.
These two examples highlight the need to employ a flexible, adaptable solution in preparation for the future standard changes that will undoubtedly take place over the long life of a transmission system.
A cornerstone of ensuring future compatibility is a true multimedia exciter, making it an important part of the evaluation of technology suppliers. In addition to supporting multiple standards, the exciter should offer adaptive, digital precorrection to maximize the transmitter operating performance and ensure technical compliance. While some older systems offer fixed precorrection, adaptive correction constantly monitors the performance of the transmitter and adjusts for nonlinearities in the amplifier and filter system, which can be affected by changes in weather and the antenna system.
System power levels
The early promise of digital transition boasted a dramatic reduction in transmitter power levels, because the digital signal offered high quality up to the point that the receive level was simply too low to decode. In actual practice, however, many network operators have discovered that while digital power levels are indeed lower than the equivalent analog, they are higher than originally calculated.
Careful planning is needed to ensure the network provides adequate coverage for the type of reception desired. For example, many planning models assume that antennae are located 10m above the ground with a certain amount of antenna gain. In reality, many homes use indoor antennae attached to set-top boxes to receive digital signals. Signal reception in these conditions is much more challenging. Similar difficulties are also apparent in many mobile digital networks for TV and radio that employ far less optimized receive antenna systems. Deploying a 10m pole with a directional antenna connected to a handheld receiver is simply impractical. In fact, elevated digital radio power levels were allowed in Europe under the Geneva 2006 agreement to combat poor indoor reception from early networks. In addition to the impact on reception, many operators are now planning a greater number of higher power sites in a network to reduce ongoing operating expenses such as site rental, antenna system costs and maintenance.
The implication of this movement to higher power levels in networks requires transmission systems that can support elevated power levels from the beginning or as an upgrade path. For example, many networks have been designed for high-power transmitters ranging from 3kW-4kW. This has been the typical maximum size of a single-cabinet UHF system offered by many suppliers. Using a traditional transmission system, one could simply stack enough cabinets to reach higher powers. However, the system is penalized with a larger footprint, reduced efficiencies and increased complexities. New developments in RF device technology, driven by new 50V DC devices used in the mobile phone network industry, have enabled unprecedented power levels in compact systems. Systems are available today that offer power levels equivalent to double the previous benchmark for analog or digital power in a single cabinet.
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When evaluating the costs associated with deploying a new digital network, one's immediate thought might be the acquisition costs of the infrastructure equipment needed. However, over the life of a typical network, the upfront costs are paid many times over by the ongoing operating expenses. When planning the deployment of a multimedia transmission network, it's important to carefully evaluate the long-term costs, such as power consumption, delivery and installation, building floor space, site rental charges, lifetime maintenance and repair, and eventual disposal costs. Although the topic of power consumptionseems straightforward, it is often overlooked in the planning and procurement process in favor of purchase price, which can lead to elevated operating costs and reduced profits over the life cycle of the product.
A recent evaluation of contemporary products for digital mobile transmission found significant power consumption differences that, in a few months, covered any price difference and, within several years, paid for the transmitter on the power savings alone. Multiply the savings per transmitter across a typical network, and the savings are quite large.
The size of a transmitter used to be a consideration in a network only if there were sites to which transporting equipment was difficult or where doors had limited openings. However, in today's world of shared transmission sites and multiple overlaid terrestrial media networks, the space available at each of the transmission sites is limited. Increasingly, costly real estate places greater emphasis on the size of the transmitter and, as previously discussed, this is exacerbated by the move to higher power levels.
When planning the network deployment, there are three main ongoing costs affected by the transmitter footprint: cost of floor space, availability of floor space and cost of delivery. However, not so obvious is the impact on long-term costs that parts and modules shipped from either the supplier of a centralized parts depot or an operator would have on the network. Additional costs are incurred for each item shipped; therefore, any reduction in the weight and volume of the units and components can have a sustained effect in reducing operating costs.
The ongoing maintenance of the network factors into cost figures as well. The ability to easily repair units in the field without having to ship massive assemblies back to the factory is a key driver in reducing the ongoing costs of any network. To facilitate field repairability, systems must employ careful, simple design techniques that support simpler RF system and module design to eliminate complex alignment procedures. This architecture replaces the previously complex and widely used approach of maximizing gain stages within an RF module. The earlier approach supported low-level signal distribution with a return to the traditional and more reliable higher-level signal distribution followed by lower-gain modules. The improved method employs lower-gain RF modules that allow for easy troubleshooting and repair in the field using simple test fixtures and rapid replacement pallets. (See Figure 2.)
Another attribute worth investigating when evaluating network options is the commonality of assemblies used in transmission systems. The more common the assemblies across the network, the lower the holding costs of the parts. An ideal situation has the same simple RF pallets used in both low-power, air-cooled systems and high-power, liquid-cooled systems. In this case, the same parts used throughout the system reduce not only the stocking of spares, but also the training of operations staff on field service. This reduced complexity helps dramatically at a time when broadcast RF engineers are not readily available in many markets and a wider range of skills may be deployed to maintain the transmitter network.
Increasingly, stringent rules to protect the environment also have affected the operation of multimedia networks. The movement to Reduction of Hazardous Substances (RoHS) impacts infrastructure products due to the lead used in electronics components, assemblies and paints used on other treatments. RoHS will soon be mandatory in every country. Even if this does not influence an ongoing network operation, it can affect the complexity and future cost of environmentally safe disposal of noncompliant components.
In any network, by the very nature of the design, there will be different sites with different power levels working together to create a seamless coverage for the user. To provide the needed signal levels in some areas, small gap fillers or single-frequency network transmission sites are used to provide consistent coverage. Most broadcast operators are familiar with the installation of transmitters in a building or shelter located at the base of a tall mast. However, mobile phone networks have been successfully using weatherproof outdoor enclosures to hold transmission and technical equipment to reduce operating costs and increase the number of locations in which these lower-power fill-in systems can be deployed. Now, most notably in mobile TV deployments, broadcast networks are starting to use this approach for lower-power systems. As planning begins for a multimedia network, it is important to give careful consideration to this approach, because there are solid benefits, such as lower site deployment and rental costs. The units are typically weatherproof enclosures that hold rackmount transmitters, support equipment, and contain heating and cooling systems.
Selecting technology partners
In addition to evaluating products, prices, features and operation costs, selecting the right technology partner can make the deployment and long-term operation of the multimedia network problem-free and cost-effective. Understand the financial stability of the potential partner. Will they be able to provide support if problems arise? Do they have global service capabilities? Can they help commission and deploy the system? Another critical consideration is whether they have established training facilities so staff will be trained and knowledgeable on the system deployed.
Keys to successful deployment
Planning a digital multimedia network and overseeing the implementation is not a simple undertaking. Diligent evaluation, careful network planning, good financial modeling and critical partner evaluations are all part of making the right choices to ensure a successful deployment from the beginning and over the long term.
To recap, consider these key points as must-haves in the selection process:
- Flexible, software-defined multimedia exciter technology with adaptive correction;
- Operating cost efficiency — best-in-class power consumption;
- Compact footprint;
- Plan for elevated power levels — high power density;
- Leading technology;
- Fully compliant with regulations — technical and environmental (RoHS); and
- Solid technology partner with support for the infrastructure rollout and the long term.
While examining many of the considerations of building a digital multimedia network, it is imperative to understand the local market consumer trends and business model approach. Ensuring that the network covers the consumers and delivers the content they desire when they want to consume it will ultimately determine the network's success.
Richard Redmond is director of strategic marketing, transmission, at Harris Broadcast Communications.
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