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Mobile DTV Transmission Surprises

(Feb. 20, 2009) PALM SPRINGS, CALIF.: There are a few bugs to be worked out of mobile DTV transmissions, according to Merrill Weiss, a veteran TV signal expert based in Middlesex County, N.J. Weiss presented a summary of transmission test results at the Hollywood Post Alliance Technology retreat on the high desert this week.

“There are so many reflections, there’s no dominant path,” he said.

Engineers conducting the tests found higher signal echoes than originally expected. Weiss focused on a trial in San Francisco, but results were reproduced elsewhere. Based on temporal deduction, the test pattern revealed signals were bouncing off support structures on the Bay Bridge.

“Longer and stronger echoes really do exist,” he said, complicating the prospect of urban transmissions of mobile DTV.

The tests were conducted by members of the Open Mobile Video Coalition, a group formed to hammer out a transmission standard and coordinate the roll-out of mobile DTV. Broadcasters mobilized on mobile in the last 18 months or so, hoping to shore up local over-the-air loyalty and create a new revenue stream. The standard was rapidly established, and 63 stations are on deck to launch mobile DTV service in 22 markets throughout this year.

Reaction within the broadcast community is enthusiasm versus raised eyebrow. CBS is happy to let established carriers like Verizon and Sprint chop the wood and haul the water. CBS’s Bob Seidel notes that no one is making money on mobile TV, and the carriers have two-way service, where broadcasters could just offer one-way.

Jim Starzynski of NBC said, well oh, contraire. Broadcasters could take some of the video load off of carriers in return for two-way capability.

However the business model plays out, Weiss’s presentation indicated that more technical work may be in order. The receivers ready to come to market can handle signal spread of up to 50 microseconds, but the echoes were coming in at levels that could overpower some receivers, both stationary and mobile. Weiss, who helped pioneer multi-transmitter, single frequency networks for television, said the SFN architecture is one way to solve the echo problem. -- Deborah D. McAdams