Mobile DTV Gets Its Road Test
By Glen Dickson -- Broadcasting & Cable, 11/6/2006
If a test on the highways of Buffalo, N.Y., is any indication, broadcasters may be able to use their digital television (DTV) spectrum to reach mobile and portable devices. That's the word from Sinclair Broadcast Group, Samsung Electronics and transmitter manufacturer Rohde & Schwarz, which teamed up to put mobile reception to a tough test.
The trio tried out a new transmission technology, called Advanced-Vestigial Side-Band (A-VSB), that they'd like to see added to the Advanced Television Systems Committee (ATSC) digital-TV standard. The informal field tests demonstrated the successful reception of an A-VSB broadcast at highway speed, as Samsung engineers drove a minivan outfitted with special receive-and-display equipment.
Broadcasters like Sinclair are eager to master mobile TV, as it represents a way to reach viewers that cable cannot.
A-VSB, shown in a simulated laboratory demonstration at the NAB show last April, uses a forward-error-correction (FEC) technique called “turbo-coding” that can dramatically improve reception in difficult signal environments. Turbo-coding trades bits for signal robustness; a turbo-coded stream of 1.5 megabits per second (Mbps) might deliver usable video of only 375 kilobits per second.
The idea behind A-VSB is that it is backwards-compatible with the existing 8-VSB transmission system and thus would allow turbo-coded streams of lower-resolution video, suitable for viewing on PDAs or cellphones, to be delivered alongside an HD broadcast within a single TV channel.
Sinclair, which raised concerns in the late '90s over the ATSC standard's inability to support mobile or portable reception, is bullish on the technology.
The broadcast group was also in a unique position to help A-VSB's cause. Its Buffalo digital station, WUTV-DT, had been operating on a special temporary FCC license as it finalized frequency coordination with Canadian broadcasters. That gave Samsung and Rohde & Schwarz engineers a chance to go to Buffalo in early September and test A-VSB in a real-world environment.
“What we did was prove what had been done in the laboratory in terms of simulating an RF channel in a mobile environment,” says Mark Aitken, director of advanced technology. “That testing indicated the system should hold together in speeds of up to 150 mph.”
While Samsung's engineers didn't go that fast, they were able to receive turbo-coded pictures at up to 80 mph. Rohde & Schwarz had modified WUTV's digital transmitter to emit two A-VSB turbo-coded streams, each 1.5 Mbps, alongside an 8-VSB high-def signal. Both signals were received in a van outfitted with DTV antennas on the roof and a prototype Samsung A-VSB receiver, about the size of a set-top box, in the backseat.
A Samsung YEPP portable digital media player decoded and displayed the MPEG-4 video in the turbo-coded streams, and an ATSC receiver and LCD display demonstrated the limited mobile-reception capabilities of standard 8-VSB broadcasts, compared with A-VSB. The high-def stream had to be played off a server and fed into the transmitter, because existing multiplexers can't combine a normal 8-VSB off-air stream with an A-VSB one.
The tests were, more or less, a fair approximation of real-world conditions. “The results were pretty encouraging,” says Mike Simon, manager of advanced technology for Rohde & Schwarz.
Formal ATSC testing of A-VSB, being conducted at the Communications Research Centre in Ottawa, began late last month. Field tests will follow, and if all goes well, a draft standard of A-VSB could be circulating within the ATSC by mid 2007. “Whether it's this technology or another, we're very confident we're going to have the capability in the ATSC standard to do mobile and handheld,” says ATSC President Mark Richer. “It will just take a bit of time.”
One thing that hasn't been tested is A-VSB reception on a small portable device itself. C.B. Patel, a consultant on the project, says shrinking the front-end receive technology to that size is feasible for cellphone manufacturers like Samsung but such expensive chip development will probably wait until A-VSB becomes standard. In the meantime, he hopes to organize a broadcast demonstration of the system for the CES or NAB show next year, which will obviously require local broadcasters' cooperation.
While it was done as a test for mobile TV this time, receiving A-VSB in a car-based device may have its own merits. Patel notes that General Motors joined ATSC in September: “They didn't become a member just for the heck of it.”
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