Broadcasters are maintaining an interference "watch list" after signal conflicts began cropping up as digital stations came on line.
To build a record showing that DTV stations ramping up to full power may be creating interference to analog stations on the same channel, the Association for Maximum Service Television (MSTV) plans to chronicle reports of unanticipated interference.
Three such complaints have been reported to the FCC; another two are being dealt with under stations' agreements.
"We are collecting information to help determine whether these are unique instances of interference or indications of a more systemic problem with DTV," said MSTV President David Donovan.
Some industry officials worry that models used to assign channel allotments do not accurately predict digital propagation characteristics. Rather than face the monumental task of extensive field tests of DTV, the government based allotment tables on small amounts of real-world data combined with predictive mathematical models. Now the fear is that, as the real-world data grow, the predictive models won't hold up.
So far, all the reported interference has occurred in markets close to large bodies of water—an indication that the culprit may be duct-skipping, a phenomenon that allows TV signals to travel farther over bodies of water. But there are indications that DTV signals travel much farther than expected over land as well.
Last week, Viacom Executive Vice President Martin Franks said he can pick up digital signals from Baltimore using a rooftop antenna on his home just outside Washington.
The number of interference problems may be limited for now because roughly half of digital stations rely on inexpensive lower-power transmitters that reach only their community of license, not their entire coverage area. Interference reports so far involve stations transmitting at power levels sufficient to cover their entire markets.
With many stations happy to save on construction and energy costs by foregoing full-power digital broadcasts, FCC staffer Shaun Maher says the agency has a little breathing room to determine whether the industry faces a widespread problem.
The FCC's hope that stations will cooperate to resolve signal conflicts has mixed results.
LIN Television's WOOD-TV Grand Rapids, Mich., was forced to use a satellite station to boost its signal to Muskegon viewers, whose picture quality was being impaired by WMVS-DT Milwaukee on the other side of Lake Michigan. Both channels use ch. 8. LIN was on the other side of the problem when it launched digital service for WAPA-DT San Juan, P.R., on ch. 27. After virtually wiping out the signal of a St. Croix analog station on the same channel, LIN reduced power.
In other instances, stations have not been able to reach accommodation, and complaints are pending at the FCC. WBOC-TV Salisbury, Md., is locked in a dispute with WHRO-DT Hampton Roads, Va., over interference on ch. 16. WHRO-DT initially reduced power but powered back up after deciding that the primary cause of WBOC-TV reception problems was many viewers' decision to point antennas toward Baltimore stations. The Salisbury station rejects that explanation.