08.10.2011 09:45 AM
Building Out DTT
Many countries have already made the switch to digital terrestrial or have begun the process. The switch has provided a reason to modernize and improve infrastructure and reduce maintenance costs, as well as provide capacity for many more channels. Those countries that have already made the switch are mainly using DVB-T, but looking at DVB-T2 for newer services. Those still to make the change are choosing DVB-T2 first because of its more efficient use of spectrum.
Using the allocated spectrum more effectively is becoming much more important as each government sells off the “digital dividend” and looks for ways to use existing TV spectrum for new revenue. The need for additional capacity for bit-hungry services like HD and 3D is also helping broadcasters focus on spectrum efficiency.
With these changes in mind let’s explore the improvements and new technologies that have been brought about by the need to change to digital delivery.
Before the start of the DTT revolution, transmitters using transistors and MOSFETs (Metal Oxide Field Effect Transistors) in their final amplifier stages had already started to take over from tube technology. Bipolar transistor amplifiers were fairly nonlinear, which caused problems when multiple carrier digital systems such as DVB-T and ISDB-T were put through them. This focused amplifier development on using the new LDMOS (Laterally Diffused Metal Oxide Silicon) devices. These were becoming available for the 900MHz mobile phone bands and were more linear.
Initially 28/32V devices with gains of around 12dB were used in the final amplifiers. However, higher power 50V LDMOS devices with 20dB of gain have since become available. These higher voltage devices not only make designing wideband amplifiers easier, but the improved gain reduces the number of amplifier gain stages needed; improving the overall transmitter energy efficiency.
Many manufacturers had, and still do adopt, a modular approach to transmitter design—such that a failure of an output module simply reduces the output power slightly, but does not take the transmitter off air. As most of these modules are hot pluggable, a failed unit can be replaced on air without the need to break transmission. Onsite maintenance can be reduced to module exchange with repair at base or by repair at the manufacturer.
Water cooling has also become an option; allowing a significant reduction in the building space occupied by TV transmitters when compared with air-cooled equivalents. The freed-up space has meant that valuable TX site real estate can be used for additional revenue generating services.
Where the digital revolution has made its greatest improvement to TV transmitter design is in the exciter. This said, the switch to COFDM multicarrier transmission systems made it necessary to adopt a mathematical approach to the generation of digital RF TV signals. Fully analogue exciters were no longer possible.
During the design of these new exciters, it was realized that mathematical manipulation could also be used to produce the linearity and phase corrections needed for the output amplifiers. In addition, it was also possible to shape frequency responses and provide group delay corrections for filters and tube amplifiers using the same approach. This resulted in much simpler and more stable transmitter set up.
However, some high-power tube amplifiers using IOTs (Inductive Output tube) are still in use. These tubes have aging characteristics that require adjustment during the life of the tube. To deal with this problem and to reduce the need for skilled setup, self-adjusting transmitters were designed. In these, samples of the amplified output signal are regularly taken and compared with the input signal. If parameters have changed, they are adjusted automatically until correct.
Further exciter technology improvements have made it possible to change from a mainly hardware design approach to one that is more software-based. Using this approach allows design changes to be made without the need to produce numerous hardware versions of the same equipment.
Exciters are now available that can produce signals compliant with many standards. For example, the same exciter can produce DVB-T, T2, ISDB-T, DMB-T or even DAB. All that’s needed “for a change” is a software update. The switch to newer standards and modifications of existing standards can also be accommodated with this type of exciter.
An added benefit of the software approach is that digital exciters can be programmed to produce analogue type TV signals. This capability enables new transmitters to be purchased for analogue service and then switched to digital use when the switchover date arrives.
Even though the first digital terrestrial standards are only now reaching full implementation, the US 8-VSB system, the so-called European DVB-T standard and the Japanese ISDB-T system were proposed and developed many years ago. Since then, the continuous technological review and commercial requirement approach of the DVB Project has brought about many new standards; some of which are already on to the second generation.
This is the case for DVB-S2 for satellite delivery, DVB-C2 for cable and DVB-T2 for terrestrial transmission. These newer standards all provide extra data capacity in the same spectrum space, and are already in use for satellite and terrestrial delivery. The 50-percent additional capacity of DVB-T2, together with improved signal compression technology, is enabling HD and 3D signals to be transmitted on the terrestrial platform in the UK. This is something that was unlikely to occur with DVB-T alone.
The latest profile called T2-Lite for mobile delivery has just been released, giving the DVB-T2 standard an all round capability. However, here in Europe the DVB-H standard for the delivery of TV to handhelds has not taken off. Currently the United States is in mobile hype mode over the ATSC M/H standard for delivery of TV to handhelds.
Looking back over the development of digital TV transmission, one thing is for sure: Technology will continue to evolve and new possibilities will emerge. However, no matter how “cool” a given technology may be, it will die if it doesn’t gain market acceptance.