Nearly six million Americans with digital receivers may still lose TV signals when digital-only broadcasts begin next February, a new study says.

The study by Centris, a market research firm in Los Angeles, found gaps in broadcast signals that may leave an estimated 5.9 million TV sets unable to receive as many channels as they did before the changeover. It may affect even those who bought the government-approved converter boxes or a new digital TV. To keep broadcast reception, many viewers may have to buy new outdoor antennas, the study found.

The Centris study predicts greater disruption of service than government agencies like the Federal Communications Commission have acknowledged.

The federal government estimates that 21 million American households have primary TV sets that receive only over-the-air signals. But it says most will continue to get a digital signal by means of a digital-to-analog converter box, which costs about $50 to $70. It is helping to underwrite the cost of a converter box by issuing $40 coupons.

Centris said it looked at a more detailed method for predicting the coverage pattern of TV signals than the government had used.

However, the problems with reception could be far worse, according to engineers who have taken signal measurements. One study of the first HDTV station by Oded Bendov, the consultant hired to replace the broadcast antennas on the Empire State Building, found that digital signals did not travel as far as either model had predicted.

“For the people with rabbit-ear antennas, I would say at least 50 percent won’t get the channels they were getting,” Dr. Bendov said. “I would say a lot of people are going to be very unhappy.”

Digital reception is more affected by hills, trees, buildings and other interference than analog has been. An analog TV picture degrades gradually, getting more snow or ghosting as a signal becomes weaker.

But digital TV is subject to the “cliff effect” — the picture is excellent until the signal gets weak and the picture suddenly drops out.

The number of sets that the Centris study projects will fail varies from city to city, based largely on the landscape. In Las Vegas, which lies in a flat basin, the study estimates that 2.5 percent of over-the-air TVs would lose at lease one of five major networks. In Philadelphia, which has more hills, 5 percent of over-the-air TVs would lose reception, while in St. Louis, 10 percent would lose reception.

Centris says, based on the F.C.C.’s data, a digital signal would travel 60 to 75 miles in those three cities. However, Centris says its own model showed that the signals would degrade at 35 miles.

Whether a TV gets a strong digital signal may depend on seemingly minor impediments, said David Klein, executive vice president of Centris. “Are there big trees in your area? Is there a big retaining wall next your house?” he said. “It’s not a matter of, ‘is reception good in your neighborhood’; it’s a matter of, ‘can I get the signal in the bedroom?’ ”

Centris also estimated that of the 117 million TVs not connected to cable or satellite, up to 80 percent have set-top rabbit-ear antennas that may not be able to pull in an adequate digital signal. Many of those sets will require a better antenna or a cable or satellite connection to do so.

Electronics manufacturers say the quality of the TV’s receiver and converter will play a role.

Story source: nytimes.com.