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See Inside June 2005

The Multipath to Clarity

Receiving HDTV over the air takes luck and lots of patience
DIRECT TV



JOHN FRASER
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 into the 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 to a 1997 Federal Communications Commission mandate (see www.dtv.gov). To smooth the transition, the FCC allows broadcasters to deliver both analog and digital 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 2 to 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 to life--with a "No Signal" message.

Actually the problem was too many signals. Reception in cities is notoriously bad, 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 only recourse 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 FCC to 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 based on 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.

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