The unknowns surrounding the quality of off-air reception
of new digital broadcast-TV signals add up to a potential problem that could bite the
Ongoing tests of in-home antenna reception of digital TV
signals in Washington, D.C., as well as tests of signal performance in high-rise urban
environments, are generating results that suggest that no one yet knows how easy it will
be for consumers to get high-definition television and other digital services off-air.
The fear within cable circles is that if off-air-reception
performance turns out to be much worse than leading broadcast engineers say it will be,
cable reception could become the last resort in a government-mandated push to convert the
nation to digital broadcast.
It is an extremely sensitive subject that cable executives
are reluctant to talk about in public, given their efforts to come to amicable business
terms with broadcasters on myriad issues associated with digital TV, which is to be
broadcast in major markets starting this fall.
"They keep telling us that everything is great, but
our information says otherwise," said one engineering executive in cable, asking not
to be named.
The potential source of contention is multipath reception
of broadcast signals modulated in the VSB (vestigial-sideband) format mandated by the
Federal Communications Commission-approved digital TV standard.
Some sources insisted that the problems in high-rise
environments -- and for in-home antenna reception in non-high-rise situations, as well --
are much more acute than broadcasters are willing to admit, but, so far, the results of
testing in these situations have been kept under wraps, making it hard to decipher what
the real dimensions are.
"Indoor-antenna reception wasn't originally a
design criterion for HDTV, but we on the UHF side of the industry have made it clear that
the ability to receive signals over indoor antennas is absolutely necessary" said Nat
Ostroff, vice president of Sinclair Broadcasting Group, which has asked Congress to look
into the issue.
"We think that there's a problem, but if there
isn't, then the people doing the testing should be willing to make the results
public," Ostroff added.
More information is definitely needed, said Richard Green,
president of Cable Television Laboratories Inc.
"In view of the short time schedule to deploying the
equipment, we need to have access to data as soon as possible and as openly as
possible," Green said.
"A corollary is that if there are data available that
show the propagation characteristics in big cities, where the biggest problem with
multipath appears to be, they should be disclosed," Green added. "Or, if such
data haven't been obtained, studies should be undertaken to determine how big the
problem could be."
Industry sources said that at least one broadcast network
has performed such tests showing that there are significant problems. But the network
hasn't shared the results.
Multipath interference -- which can cause ghosting or, in
the case of digital signals, can make the screen go blank -- occurs when signals operating
in the same frequency arrive from different directions at the antenna at approximately
equal levels of strength in slightly different time sequences.
Within the closed space of a room, the small separation
between the two ends of a "rabbit-ear" antenna can be sufficient to create
multipath interference from signals reflecting off different surfaces.
The problems for digital TV are no worse than they are for
NTSC (National Television System Committee), said Victor Tawil, vice president for
technology at Maximum Service Television (MSTV), the broadcast-industry technical group
that has been conducting indoor-reception tests over the facilities of WHD-TV, the
experimental TV unit of NBC affiliate WRC-TV in Washington, D.C.
"People are making a lot of ill-informed claims about
these tests before we've even finished compiling the results and without recognizing
that we're performing these tests to determine signal performance under worst-case
conditions," Tawil said.
Those conditions include broadcast from a transmitter only
400 feet or so off the ground, which conforms with Washington's zoning requirements,
but which is not the norm elsewhere.
Typical broadcast-tower heights are in the 1,000- to
2,000-foot range, said Ron Gibbs, president of Lodestar Towers Inc., a leading supplier in
"None of this is news," said Robert Graves,
chairman of the Advanced Television System Committee, who noted that the reception
performance in Washington was being tested using two-year-old prototype gear.
"We've been aware for a long time that multipath
discrimination would be a factor in the performance of receiver manufacturers'
products and that they would respond by improving their antenna designs, which they
are," Graves said.
He added: "People on the fringes of the broadcast
footprint will have to do extra things to enhance their reception, just like they do now
But the fringe parameters for digital may differ, others
noted. "There are sets working off-air today that show that HDTV reception is good
with the use of rabbit ears within the Grade A [broadcast] contour," said Art
Allison, senior vice president of engineering for science and technology at the National
Association of Broadcasters, in a discussion on digital TV at the recent Society of Cable
Telecommunications Engineers Emerging Technologies Conference.
"But outside of Grade A, where you need more antenna
support, there is a potential for confusion among consumers."
Most broadcasters today transmit at powers sufficient for
indoor reception well into the Grade B segment of coverage, which typically begins 25
miles or more from the transmitter and extends for another 15 miles or so.
Bob Weirather, director of TV-product lines at
transmitter-supplier Harris Corp., stressed the need for more information, which
won't be available until people are broadcasting digital TV on a much wider scale
than is possible today.
"When you're testing things, you're looking
for problems, which are bound to show up," he said. "But you need to know what
the real-world percentages are."
While Sinclair officials have suggested that the multipath
problem might require a change-out to the COFDM (code orthogonal frequency-division
multiplexing) modulation technique used in Japan and Europe for digital TV -- which was
examined and rejected by MSTV two years ago -- there seemed to be little, if any, support
for this option elsewhere.
Not only doesn't COFDM perform any better than VSB,
but a sudden shift at this point would be a "show-stopper," contrary to
Sinclair's assertion that it could quickly be done, said Bruce Babcock, vice
president for new products at Thomson Consumer Electronics.
But U.S. broadcaster assertions that COFDM isn't more
robust than VSB are wrong, and they don't contribute to an honest discussion of the
issues, asserted a cable-engineering executive who has discussed testing results with both
European and U.S. broadcast interests.
"The technology was too early in the development
curve, and it posed cost and timing problems for the United States when it was tested, but
now that it's being used in Japan and Europe, the situation is different," the
engineering executive said.
Nonetheless, this engineer added that it probably makes no
sense to start over again with a new modulation technique. "There are corrective
measures that you can take for VSB in the receivers that are probably as effective as
using COFDM," he said.
The real problem is that until the actual real-world
performance parameters are understood, manufacturers won't know exactly what these
corrective measures should be.
Not getting it out on the table and fixing it now is where
the cable industry's vulnerability comes in, the engineer added.
"The bottom line is that the remedy should be in
giving people equipment that will ensure adequate reception off-air, if they want to get
DTV that way," he said. "The last thing that we want is for the government to
decide that the fail-safe remedy is cable."