WBOC in Salisbury, Md.--which has analog channel 16 and DTV channel 21--asked the FCC to deny an application for a new analog channel 21 station in Virginia Beach, Va. Previously, WBOC had complained about interference to its analog signal from WHRO-DT channel 16 in Hampton Roads, Va. and feared the new analog channel 21 analog would cause similar problems for its DTV channel 21. Copeland Channel 21 LLC responded that there have been no complaints about interference from WBOC analog channel 16 to WHRO-DT channel 16. As readers of my RF Technology columns know, DTV signal can handle analog interference much better than analog signals can handle DTV interference. Copeland used this argument in its response.
In its decision denying WBOC's Petition to Deny, the FCC was not persuaded by the station's speculative engineering showings that "ducting" interference may occur in this case to the extent that Copeland's analog operations will cause harmful interference to the WBOC DTV operations.
The commission described ducting as a "weather-related phenomenon and may be highly variable in both direction and intensity. We recognize that the highly variable phenomenon of ducting may occur near or over water. The evidence presented by WBOC to support its contention that ducting is likely to occur here, however, is not persuasive or probative. The commission has not made a determination concerning the alleged ducting interference between WHRO-DT and WBOC-TV. Furthermore, WBOC-DT's digital signal is much more robust than its analog signal, and should be more resistant to impermissible interference from an analog station."
The commission found Copeland's analog operation to fully comply with the commission's technical rules including the interference protection requirements.
"Furthermore, should impermissible interference actually occur upon the initiation of service by Copeland's NTSC station, the commission has at its disposal a number of different options to which it may avail itself in conjunction with the DTV channel election process. These include proposing to change the channels for either WBOC's or Copeland's stations or both. It would be premature to condition Copeland's construction permit in the absence of a proven interference problem. Any such condition would be pure speculation without technical support. The above facts considered, we find Copeland fully qualified and that grant of its application will serve the public interest, convenience, and necessity," according to the FCC.
Details on the WBOC case are available the FCC Letter to WBOC
Last week the FCC issued a Memorandum Opinion and Order
denying petitions for reconsideration filed by ABC and United Communications of the Report and Order amending the digital Table of Allotments changing the DTV allotment for Clear Channel station WXXA-DT in Albany NY from channel 4 to DTV channel 7. ABC, licensee of WABC-TV on analog channel 7 in New York City and United, licensee of WWNY-TV analog channel 7 in Carthage, N.Y. claimed WXXA-DT's use of channel 7 would interfere with reception of its current analog service. While they agreed that Clear Channel's use of channel 7 DTV for WXXA-DT would affect less than 2 percent of the population, the FCC MO&O said, "They argued that Clear Channel did not offer any justification for its proposal sufficient to overcome that loss of service. Moreover, even though Clear Channel's proposal would affect less than 2 percent of the population served by each station, they argued that the actual number of viewers to be impacted could not be justified under the commission's public interest mandate."
The FCC did not buy this argument. They responded:
"We will deny the reconsideration petitions. Despite the fact that the rulemaking proposal meets all coverage and interference requirements, the petitioners continue to argue that Clear Channel has otherwise failed to justify the channel reallotment in light of predicted interference to WABC-TV and WWNY-TV."
Those stations are not entitled to the level of protection they seek, according to the FCC.
"Some disruption or loss of service to analog services is anticipated during the DTV transition, and the Commission's 2 percent interference standard was established to provide appropriate flexibility for construction of digital facilities while maintaining existing NTSC service. Moreover, the commission has encouraged the collocation of digital and analog facilities to provide the flexibility to accommodate operation of nearby stations. However, because the channel-change rulemaking proposal meets our technical requirements, its grant was not premised on Clear Channel's interference or cost claims as ABC and United contend."
The FCC added the petitioners remaining arguments opposing this proposal have been thoroughly considered, and reconsideration will not be granted merely to again address matters that have already been resolved.
Both of these cases raise an issue I discussed in my Aug. 4, 2004 RF Technology article Choosing a Final DTV Channel
. As I noted in that article, if an interfering analog signal is more than 2 dB below the desired co--channel DTV signal, OET-69 says there will be no interference. However, if two digital signals share the same channel, the interfering signal has to be attenuated by at least 15 dB for no interference to result under OET-69 criteria. DTV interference into an analog TV signal will appear as noise, and if the noise is not attenuated at least 40 dB viewers may notice it. If you are not familiar with dB, when referring to power a reduction of 40 dB requires the interfering signal be reduced to 1/10,000th the power of the desired signal, or, if voltage is compared, one-hundredth the voltage.
In the last case, WABC-TV and WWNY-TV may also have been concerned that WXXA-DT's use of channel 7 for DTV would prevent WABC-TV and WWNY-TV from using channel 7 for their DTV operations at a power level sufficient to replicate their analog Grade B contour. In the channel 21 case, it appears WBOC incorrectly assumed that if co-channel interference from a DTV station was a problem for its analog station, co-channel interference from an analog station to its co-channel DTV station would have the same effect. Although the WBOC Petition to Deny was rejected, Copeland Channel 21, LLC is likely to face more serious problems when analog broadcasting ends, which now appears will happen Dec. 31, 2008 To protect the WBOC-DT service area on channel 21, Copeland will either have to reduce its coverage area if it stays on channel 21 or find another channel it can move to and modify its equipment to work on that channel.