DTV interference issues loom - Broadcasting & Cable

DTV interference issues loom

As stations move to full power, problems among stations are expected to increase
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Industry experts warn that, when broadcasters get on-air with full-power DTV signals, interference issues like that being experienced by WBOC-TV Salisbury, Md., may be the norm rather than the exception.

"I think it's inevitable that there are going to be a lot of these problems, and a lot of it was predicted," says William Meintel, president of Chantilly, Va.-based TechWare, which was hired by the FCC to create the software used to make the DTV propagation tables. "We just haven't seen it because there aren't a lot of stations on the air and an even smaller number on-air at their full power. And I think that, once these stations start firing up with their full-power signals, we'll see a lot of interference to NTSC, and it may be worse than what was originally predicted."

Two weeks ago, WBOC-TV filed an emergency request asking the FCC for relief from interference that its signal is getting from the 950-kW DTV signal of WHRO-DT Hampton Roads, Va., located nearly 120 miles away. In April, when WHRO-DT made the move to full power, WBOC-TV began receiving consumer complaints (even from cable subscribers) not only from the area of its viewing audience closest to Hampton Roads but also from Dover, Del., which is about 150 miles away from Hampton Roads.

The cause of the interference in the case of WBOC-TV is "duct skipping," a phenomenon in which the signal skips across a body of water, causing interference where it usually wouldn't occur. It is not just a DTV phenomenon. Analog stations, typically in Florida or Louisiana, have been subject to duct-skipping interference for years.

WBOC-TV General Manager Rick Jordan says the situation is unique because ducting was not taken into account in the FCC's DTV interference modeling. WHRO-DT's tests showed interference of only 0.9%, well within acceptable limits, but they do not take into account the ducting interference.

"I wish that 0.9% was all that we were receiving, but I have cable systems as far north as Dover, Del., receiving an unbelievable amount of interference," he says, adding, "It's also unbelievable that propagation can have that much impact" from so far away.

The concern is obvious, Jordan says: The analog signal is the station's livelihood, and there are only a handful of DTV viewers.

"I'm hoping that this will be resolved with a gentlemen's agreement," he adds. "And I hope everyone sits down at the table and says this is a real-world issue and not that the computer model says this doesn't exist so we're done."

WHRO-DT plans to file a reply to the FCC concerning WBOC-TV's complaint, according to spokeswoman Donna Hudgins. "Everything that we've built and constructed has complied with the FCC's regulations," she adds. "And our engineers don't see that the facts presented really demonstrate a reception problem that needs extraordinary action by the FCC."

The situation is similar to one experienced in 2000 on Lake Michigan. WMVS-DT Milwaukee's signal was interfering with that of WOOD-TV Grand Rapids, Mich. The solution then was for WMVS-DT to voluntarily cut its transmission power by 75%.

That was a temporary solution, because the FCC told WOOD-TV that the interference from WMVS-DT was acceptable within the Grade B contours.

Today, WMVS-DT is back on at full power, and WOOD-TV has spent about $200,000 putting in an LPTV translator transmitting its signal on ch. 46 to serve cable headends and viewers within that Grade B contour.

"The rules provide up to 10% interference within a Grade B signal," explains Mike Laemers, director of engineering for WOOD-TV, WOTV(TV) Battle Creek, Mich., and WXSB(TV) Grand Rapids. "That doesn't sound like a lot of people, but, if you have a cable headend that is considered one of those 10% being affected, that equals a lot of people. That headend is serving 40,000 households."

The potential number of complaints about DTV interference seems destined to rise to flood levels when it comes to stations on or near large bodies of water.

"Anytime there's a large body of water with an unobstructed path," says Laemers, "interference is going to happen a lot, not only when temperature inversions occur."

Complaints may spread to land-locked stations as more stations increase DTV power levels, although just how large of a problem it will become remains to be seen.

"The root problem is that some people think that the DTV spectrum was spectrum that wasn't being used," says one industry source. "For all practical purposes, there was no available spectrum on the East and West Coasts."

MSTV Vice President, Technology, Tom Gurley points out that the spectrum of a digital station is noise-like: Any interference raises the noise level and creates snow and sparkles in the signal. Analog-station signal spectrum is very concentrated around the visual and aural carrier, causing interference that is much more distracting to viewers.

Meintel concurs. "Digital is different than analog, and the interference characteristics are different. The FCC knew there was going to be interference, but it was the only way to assign the digital channels."

So what is the solution? Meintel says lower power levels may be the only way to solve interference troubles because changing channel assignments is not easy and may just move the problem elsewhere. "You can play with the statistics all you want, but it's not going to solve the problem."

The problem with lowering the power is that digital broadcasters then lose service in some areas.

Meintel says that the extent of interference problems from DTV signals remains to be seen. But everyone seems to agree that it could become a fairly large number. The question is, what can the FCC do?

"If the interference is a temporary thing, then the commission is not likely to do anything," says Meintel. "If the station causing the interference is complying with all of the commission's regulations, I don't know if there is anything the commission can do about it."

Gurley says that, if the digital station is operating at a power it was licensed for and the allotment table says it can operate at, what the FCC can actually do remains to be seen. "If the FCC does something for a station that is getting interference from that signal, then they almost have to start looking at the whole table again."

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