The current state of digital transmitter technology
Sep 1, 2010 12:00 PM, By Richard Redmond
The Swisscom transmitter site at Gotschnagrat, located at an elevation of 2200m near the Austrian border, houses three Harris low-power DVB-T transmitters.
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.
Flexible standards
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.
Figure 1. Shown here is a typical DVB transmission system.
Select figure to enlarge.
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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