Digital TV in the U.S.

Ghost story: antennas for DTV

With digital TV (DTV) broadcasts, no ghosts, snow, or other distortion will appear on screen—and maybe no picture, either. Viewers will need an indoor or outdoor antenna to find out.

With analog T\/, in contrast, picture quality deteriorates over distance and under adverse reception conditions: Causes of distortion include distance from the transmitter and signal obstruction by atmospherics, terrain, or moving objects. But so long as the receiver is within range of a station, something ought to be visible on screen, at least theoretically. Whether it is viewable or not is a subjective judgment.

DTV, though, is an either-or proposition. When the signal received is adequate—not even full strength—the picture should be great. But if too few bits make it to the set, there is either a frozen image or no picture at all. In any event, until DTV comes routinely by cable or satellite, viewers will need an antenna to pull the best possible DTV signal from the air.

The consensus is that a properly functioning outdoor antenna will handle DTV just fine, although local conditions will determine if omnidirectional types are preferable to directional arrays. Households that get good VHF/UHF reception now with an indoor antenna probably will receive a ”valid bit-rate” for DTV, too. Signal processing in the set will assemble a picture even if some bits are dropped in transmission, unless the dropouts are too great.

The venerable Sarnoff Corp. laboratory in Princeton, N.J., equidistant from New York City and Philadelphia, is a case in point. On 9 February, Spectrum visited the now-independent facility, which has midwived many TV innovations since its 1942 founding as an RCA think-tank.

But for all the facility's accomplishments, it never could receive viewable National Television Systems Committee (NTSC) signals, the lab being located in RF limbo just beyond the FCC-standard Grade-B reception contours of each city. Notwithstanding, the DTV reception viewed last month from each city was immaculate, with nothing special riding Sarnoff's mast besides a bidirectional rooftop antenna available at most consumer electronics stores.

As the Sarnoff situation suggests, early analysis indicates that signal strength will be less a problem than multipath. Consequently, DTV sets are designed to analyze multiple signals to pick the most robust—a process called adaptive equalization.

The advent of DTV has prompted antenna manufacturers to develop better indoor mousetraps than the ubiquitous rabbit-ears dipole—specifically, antennas with no need of the meticulous element-tuning that few people understand anyway, nor of the semi-permanent attachment of a household member to increase gain. In fact, some of the newer designs purport to minimize signal deflection from the movement of individuals around the antenna.

Meanwhile, whatever the design, an indoor antenna that picks up a signal in some parts of the residence might come up empty in others. Just as with analog T\/, it depends on the nearby construction materials, some of which block or deflect signals. These include aluminum siding, foil-backed insulation, the metal mesh used for stucco walls, and even the metal particles in tinted glass windows.

Regardless of antenna location or type, the multipath issue has already raised alarm in some quarters. At the recent Consumer Electronics Show in Las Vegas, Nev., one

broadcast executive warned that multipath might force some DTV viewers to repoint their antennas every time they change stations—a time-consuming annoyance even with a motorized outdoor aerial.

Nat Ostroff, vice president of new technology for Sinclair Broadcasting, Baltimore, Md., contended that if this causes DTV to flop, the United States might have to consider switching from its current use of the 8-bit vestigial sideband (8-VSB) modulation to the European standard, called coded orthogonal frequency-division multiplexing (COFOM). The latter spreads the signal over thousands of narrow channels, each at a low data rate, making it possible for the receiver to identify and reject unwanted reflected signals [see ”Digital TV comes down to earth,” IEEE Spectrum, October 1998, pp. 22-29J. — S.A.B.

Advertisement
Advertisement