A mobile proposal

As broadcasters and technology firms work together to gauge the potential of the 8-VSB digital modulation standard, one question looms large: whether 8-VSB will ever be able to support mobile services, such as receiving stock quotes on a Palm Pilot while traveling in a car.

Although chip makers Zenith, Philips and NxtWave have suggested that 8-VSB may be able to handle mobile reception, no one has been able to demonstrate reception-on-the-move with 8-VSB as European and Japanese broadcasters (and Sinclair Broadcasting at NAB 2000) have done with COFDM.

Most 8-VSB proponents are unfazed, pointing out that the ATSC system was never designed for mobile services. But, with DTV datacasting viewed as a potentially huge revenue stream, mobile reception has definitely become a desire, if not a requirement, of many broadcasters.

"Sinclair's pointing out a lot of problems, and a lot of problems can be fixed," says Clear Channel Director of Engineering Mike DeClue. "But that still doesn't make 8-VSB good for mobile applications."

DeClue thinks he may have a solution: Get local DTV stations to cooperate on carrying each other's programming, so that one station in each market could free up its bandwidth to transmit a COFDM signal aimed at mobile services. For example, Station Y would agree to carry Station Z's core video programming as part of an SDTV multiplex, allowing Station Z to carry mobile services for multiple stations in the market.

DeClue proposes doing this without changing the existing DTV allotment table. The challenge is that the COFDM-modulated station would have to ensure that it wouldn't interfere with existing VSB and NTSC stations in the market, probably by significantly dropping its transmitted power.

"There's a precedent already for drop-in channels" says DeClue. "We would need to show conclusively there was not going to be any interference." To make the idea work, he says, local broadcasters would have to work together, much as they do in coordinating ENG signals, to pick the "least offensive channel" to drop in the COFDM signal. DeClue doesn't expect network O & Os to participate in the plan and will instead look to the "fifth, sixth and seventh broadcasters" in a market.

Other industry sources are much less sanguine about DeClue's idea. They point out that nobody has yet done the engineering work on COFDM interference into VSB and/or NTSC signals, but early indications from Brazil's tests don't bode well.

At particular issue would be COFDM co-channel and adjacent-channel interference with NTSC signals. "You can't just sort of drop this thing in," says one source.