And officials say that millions more who thought they were prepared are likely to experience technical problems like poor reception or improperly connected antennas. Their problems arise because the way the digital signal travels is different from analog and can be more affected by topography, weather or even heavy auto traffic.

A list of 49 particularly vulnerable markets includes New York, Los Angeles , Chicago , Philadelphia , Miami , Boston and Dallas-Fort Worth. Officials said Puerto Rico is also among the most susceptible to problems, as it has the highest rate of households that receive their television signals over the air.

In the New York broadcasting market, 92,000 homes are completely unready for the transition and another 348,000 are partly unready, according to Nielsen. That represents almost 6 percent of households in the region.

Early this year, the administration persuaded Congress to postpone completion of the transition to June, from February, and to provide another $650 million, mostly for coupons for converter boxes, on top of the $1.5 billion that had already been spent by the Bush administration.

The Obama administration has enlisted dozens of groups, including AmeriCorps , the national volunteer organization, civil rights groups and even firefighters to help people install the converter boxes and antennas. The program is the most ambitious technology transition effort since the Clinton administration’s enormous Y2K program, which was set up to avert major software problems caused by the inability of computers to process data beginning on Jan. 1, 2000.

Concerned about a possible political reaction, President Obama issued a statement on Thursday urging consumers to take steps so they do not lose television reception. “We have worked hand in hand with state and local officials, broadcasters and community groups to educate and assist millions of Americans with the transition,” Mr. Obama said.

“I want to be clear: there will not be another delay,” the statement added.

Other officials said that the high number of households reflected the inclination of Americans to put things off.

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“There are so many people who are always waiting until the last minute, whether it is college students doing term papers, or people filing taxes , or people like me who wait until Christmas Eve to do their shopping,” said the commerce secretary, Gary F. Locke, in an interview on Friday.

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While applauding the government efforts, he said he was frustrated that the early public service announcements did not provide specific enough information about the problem.

“Earlier on we could have more crisply and clearly indicated who was affected by the switch,” he said. “I’ve been critical of the public service announcements that just say, ‘The switch is coming’ or ‘Are you ready?’ ”

He added: “Too many people don’t know the difference between digital and analog. I didn’t even know myself until a few months ago when my brother-in-law explained it to me.”

Analog technology transmits a video signal through an electronic impulse. Digital technology breaks the signal into 1’s and 0’s, allowing far more data to be transmitted through a far smaller amount of broadcast spectrum.

The transition has already been a huge windfall to television and equipment makers and retailers, as millions of consumers have had to buy digital television sets or converter boxes and special antennas for their old sets.

Shawn G. DuBravac, chief economist at the Consumer Electronics Association, said that sales of digital television sets were up 32 percent this year over the comparable period in 2008, even in the midst of a deep recession . Other officials at the association said the spike in sales was attributable to many factors, including declining prices and availability of more programs in digital, as well as the mandatory transition.

But consumer experts said that many households were buying more expensive television sets and equipment than they needed to continue to receive television signals. Polls by Consumer Reports found that many people were aware of the transition but were confused about how to navigate it, said Joel Kelsey, a policy analyst at Consumers Union .

Bracing for a wave of complaints, the Federal Communications Commission is preparing to fully staff a $40 million call center on Friday and through the weekend. The government will also continue to supply $40 coupons to households toward the purchase of converter boxes.

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Officials advised consumers to rescan the channels of their television sets after the conversion was completed on Friday to make sure they were pulling in all the correct signals.

The conversion is the final step in a long-running plan for more efficient use of the broadcast spectrum. The plan took spectrum licenses from broadcasters, replacing them with other frequencies. It will reallocate some of the broadcasters’ former spectrum to public safety providers. Other frequencies were sold for billions of dollars, primarily to the large wireless telephone companies, whose demand for spectrum has risen with the proliferation of hand-held devices that can surf the Internet and send and receive e-mail.