Distributed transmission systems help fill the gap
Dec 15, 2008 8:18 AM, By Michael Grotticelli
Axcera’s Distributed Transmission Adapter adds single frequency network (SFN) differential timing and synchronization information in the transport stream to multiple SFN transmitters operating DTS or ATSC M/H.
It’s widely acknowledged that signal propagation in urban and mountainous areas will be problematic with digital television transmissions, as it has been in the analog world. Various experiments have been conducted over the past few years — notably one in State College, PA — that use several low-power transmitters ringing a topographically challenged coverage area, in order to fill in the gaps.
To date, these experiments required a Special Temporary Authority (STA) license from the FCC in order to conduct them, but now the commission has issued a final Report and Order (08-256) that outlines rules for formal authorization for the use of Distributed Transmission Systems (DTS). This new ruling will facilitate newer DTS projects in the New York area and elsewhere. The idea is to ensure proper coverage after Feb. 19, 2009, and until stations can relocate to a new 1776ft Freedom Tower, which is being built on the site of the former World Trade Center (where NY area stations previously transmitted from).
The deployment of a DTS infrastructure involves the use of multiple transmitters operating on the same frequency in what’s called a single frequency network (SFN). It helps fill the gaps within a broadcaster’s coverage area that can’t be reached with a single, high-power transmitter. Some have tried increasing power levels, but this has often proved to provide only slight improvements in coverage and inefficient use of power, adding cost. Also, the use of translators can be prohibitively expensive, and it requires additional spectrum. That’s why SFNs are so attractive.
DTS deployment has been pioneered by S. Merrill Weiss, president of Merrill Weiss Group of Metuchen, NJ, who owns several patents necessary for proper operation. The idea is gaining momentum with broadcasters across the country. Some are now on the air with some type of SFN while others are soon to be, thanks to the new FCC Report & Order. One is Penn State Public Broadcasting, which went on-air in 2003 with synchronized transmission sites in Clearfield, State College, Altoona and Johnstown, PA, and is fully operational. There’s another in Virginia, operated by Shenandoah Valley Public Television; an eight-transmitter network in Philadelphia that will soon be on the air; two systems in New York City; and another one developed by Tribune in Indianapolis, which was used for experimental purposes.
Transmitter vendors have cooperated with these early tests and expect to continue to develop error-correction technologies that cut down on signal delay and ensure a reliable signal to every home in a coverage area. Companies like Acrodyne, Axcera, Harris, Larcan, Rohde & Schwarz, and Thales (Thomson) have begun to offer standard ATSC versions with DTS slave exciters and mobile TV-compatible exciters, all in the same hardware platform. These new generation of DTS exciters can be retrofitted to existing transmitters, making any transmitter DTS and ATSC M/H compatible.
Jack Wilson, director of marketing and business development for Axcera said his company has worked with several of these new tests and applauds the FCC’s action.
“The FCC authorizes a broadcasters to cover a certain service area but due to terrain shielding issues, it will be necessary for many broadcasters to enhance their digital TV transmissions in order to effectively serve their coverage area.” Wilson said, adding that “the big advantage of DTS is that broadcasters can implement the technology to improve digital TV coverage using the same frequency [channel] they have been allotted for DTV service.”
DTS transmitters are synchronized to a single frequency, so that when the signal gets to a specific set-top receiver in a home, no matter which transmitter it is coming from, it will be able to lock on that requested channel and display it.
As broadcasters move to deploy the ATSC’s Mobile Handheld (M/H) TV services (now in the process of being finalized), DTS and SFNs will be critical to signal reliability. In order to deliver a robust signal to devices on the move, many acknowledge it will require more than one transmission site.
“Terrestrial broadcast networks are designed to reach viewers who are at fixed locations and normally have outside, directional, high-gain antennas,” Wilson said. “Mobile broadcasting serves small mobile devices that are in someone’s hand, they have low-gain antennas which are not directional, and they’re often moving or inside a building. We have found for quality mobile services, especially in-building coverage, it is advantageous to broadcast from multiple transmission sites which are synchronized in a single frequency network. So, DTS is a highly effective technology to improve mobile TV coverage using the ATSC M/H standard.”
Axcera participated in an early project for Crown Castle’s now defunct mobile TV service in and around the Pittsburgh and New York City markets, which used dozens of sites (including building rooftops) operating under the European DVB-H modulation scheme. U.S. broadcasters are mandated to use the ATSC-approved 8-VSB modulation. Technically the test was highly successful, although it was limited by a low licensed power level (about 600W) and operation with the L-band spectrum.
Mobile TV systems will ultimately have to use a hybrid of a traditional broadcast transmission infrastructure and a cellular network. In a cellular network there are normally many more sites because the service has be able to pick up a return channel from a low-power handset. There are also capacity limitations, because multiple users are demanding bandwidth simultaneously. For broadcasters, a hybrid approach might be to deploy one high-powered transmitter and two or three lower power, synchronized sites. Power levels of the new Axcera models range from 5W to 80 kW, depending on what a station is licensed for.
“The FCC’s authorization of DTS allows broadcasters who face signal propagation challenges to effectively serve their coverage area,” Wilson said. “DTS is a technology that allows stations to improve service without requiring the use of additional spectrum. This very spectrum efficient and cost-effective technology is a win-win for everyone the government, broadcasters and the American public.”
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