[opendtv] Scientific American: The Multipath to Clarity

  • From: Mark Aitken <maitken@xxxxxxxxxx>
  • To: OpenDTV <opendtv@xxxxxxxxxxxxx>
  • Date: Mon, 23 May 2005 11:45:45 -0400



Mark A. Aitken Director, Advanced Technology

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?articleID=000DB646-4C3D-1289-837D8 3414B7FFE9F" Scientific American: The
Multipath to Clarity

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  May 23, 2005 The Multipath to Clarity Receiving HDTV over the
airtakes luck and lots of patience By Philip Yam Keep the antenna
level. Rotate it 90 degrees. Move it a few inches to the left. Stand to the
right. Hold it a bit higher ... there--nope. Try again. 

That has been my high-definition television (HDTV) experience. I plunged
intothe alphabet-soup world of digital television (DTV) in 2003, shortly
after I replaced my electron-gun boob tube with a 42-inch plasma flat panel.
I hoped to enjoy beautifully crisp images--only to see what a lousy picture
my Manhattan cable company was piping in. The larger screen amplified flaws
in the analog signals, which not only produced images muted in detail and
color but also added faint lines and speckles, not to mention scratchy
audio.I was too cheap to fork over the $15 monthly fee for digital cable,
which included only a few HDTV channels anyway. So I decided to snag the
signals over the air, just like the old days. 

Local stations around the country are making the change to digital, thanks
toa 1997 Federal Communications Commission mandate (see www.dtv.gov). To
smooth the transition, the FCC allows broadcasters to deliver both analog
anddigital signals over the TV spectrum (channels 2 to 69). If you can get
standard over-the-air television, the mantra goes, then you can get digital.

So I spent some $300 for a set-top box, the Samsung SIR-T351 HDTV receiver,
and then rooted around in one of my storage bins for the right antenna.
Broadcasters in my area beam DTV on the UHF band (channels 14 to 69), so I
grabbed the outline bow-tie antenna. (Rabbit ears work for the VHF channels
2to 13.) I laid the bow tie against my west-facing window, trying to catch
the signals originating from transmitters atop the Empire State Building a
mile to the north. I turned on the receiver and watched my TV screen flash
tolife--with a "No Signal" message. 

Actually the problem was too many signals. Reception in cities is
notoriouslybad, because the broadcast bounces around as it strikes the tall
buildings. As a result, signals arrive at an antenna along many paths and at
different times. The receiver has to sort through this mess and figure out
which signal to lock on to. It's like trying to identify the real lightbulb
in a hall of mirrors. 

In analog TV, such multipath distortion shows up as ghosts. As a kid, I used
to tweak the antenna continually and maybe even pound the top of our TV's
wood cabinet. Reception did not have to be perfect: I could still follow Get
Smart through the multiply warped images. 

No such luck with digital, which is all-or-nothing: if the multipath problem
is severe, the tuner will not produce any image or sound whatsoever. The
onlyrecourse I had was fiddling with the antenna (plasma TVs are too thin to
pound). I managed to pull in digital broadcasts of WPIX (channel 33) and
WABC(channel 45) and only sporadically at that. 

I had stumbled headlong into the problem identified in the late 1990s by the
Sinclair Broadcast Group, based near Baltimore. The company conducted field
tests suggesting that indoor reception may not be possible. The U.S.
transmission format is called 8-VSB (for 8-level vestigial sideband), which
is more susceptible to multipath distortion than the European system, called
coded orthogonal frequency division multiplexing, or COFDM. The 8-VSB format
requires less power to broadcast and packs in more data each second (19.4
megabits compared with 18.66 for COFDM)--useful for "datacasting" services.
But 8-VSB did so poorly in multipath environments that Sinclair urged the
FCCto switch. 

Although my experiences echoed Sinclair's findings, I figured I should give
8-VSB an honest shot with a better antenna. Based on posts on the AVS Forum
(www.avsforum.com), a consumer electronics board, I tried the Gemini Silver
Sensor. Looking like a miniature rooftop Christmas-tree antenna, this indoor
model is supposed to be a ghost buster. 

I also sought help from www.antennaweb.org, which recommends an antenna
basedon street address. The site shows the channels that you can receive and
the direction to point your antenna. It told me to aim at the Empire State
Building, which I knew couldn't work--too many intervening buildings. The
only way I could receive HDTV was to capture a strong reflected signal. 

In the antenna world, bigger is often better, so I also tried something more
radical and not particularly aesthetic: an outdoor UHF antenna, used
indoors.Called a four-bay bow-tie antenna, it spans about two feet by three
feet--among the more compact types of roof antenna. (Even so, you had better
have an exceptionally understanding family.) Sure enough, it enabled me to
pick up WNBC. 

Still, reception was sporadic. I would stand in the corner of the room,
moving the cumbersome antenna slightly this way and that as the receiver
teased me with moments of signal lock. At other times, I located a sweet
spotand left the antenna in place, only to lose the channel when the weather

In January I suddenly picked up WCBS with my Silver Sensor pointed south.
Then came WWOR, followed by WNYW after I put the antenna on top of a couple
of boxes stacked on a speaker. Finally, I could pick up all the channels
being transmitted from the Empire State, albeit with some effort. Something
had changed. I looked down the avenue and saw an apartment tower going up a
few blocks south. New York City's ever changing skyline had redirected HDTV
toward me, at least for the moment. 

I live on the 11th floor, and I cannot imagine that those closer to street
level will be able to pick up local stations reliably, a fact that could be
important once broadcasters cease analog transmissions. At that time they
will give up channels 52 to 69. Four of those will go to public safety; the
others will be auctioned off for wireless services. The target turn-off date
is December 31, 2006, on the condition that 85 percent of the market served
by the broadcasters can get digital (2009 or 2010, realistically).
Unfortunately, the 85 percent figure counts those who have the equipment
evenif they cannot receive DTV because of multipath distortion. In sticking
with 8-VSB, the FCC gambled that improving technology would save the day. 

It is shaping up to be a smart bet, as receivers get better:
first-generationmodels handled ghosts that lagged the main signal by no more
than 10 microseconds and were no stronger than half the main signal. New
circuitry made by LG Zenith can cope with time differences of 90
microsecondsand multipath signals as strong as the main one, the company
claims. Its performance convinced Sinclair to drop its objections to 8-VSB.
As of this past March, however, LG had not offered these units for sale. 

New antennas may also help. Dotcast, based in Kent, Wash., designed an
activeE-field antenna, which is being marketed by Winegard Company and Terk
Technologies. It looks like a mini version of the radar antenna on vintage
aircraft carriers. A special amplifier inside boosts only the electric field
signal picked up by the antenna, Dotcast says, while ignoring other
radio-frequency waves. The narrow, 27-inch-long Dotcast antenna, which
shouldbe available this year for about $120, is supposed to function as well
as a five-foot-long roof antenna. 

HDTV does not transform the viewing experience the way TiVo and DVDs do. And
there are growing pains. On WCBS football games, some on-screen graphics did
not show up. A space shuttle launch momentarily appeared during a high-def
broadcast of ABC's Desperate Housewives (it was not a lame sex joke). But
once you see the clarity and color and hear the digital 5.1 sound, there's
noturning back. I want my HDTV. I just wish it were easier to get. 

[IMG][IMG](C)1996-2005 Scientific American, Inc. All rights reserved.
Reproduction in whole or in part without permission is prohibited. [IMG]

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