Weak signals could wreak havoc as US goes digital

In preparing for America's big switch to digital-only broadcast next year, the US government has neglected to say much about trees. And hills. And buildings. These can disrupt digital signals and result in blank screens. It's a significant oversight that may cost millions of viewers money and aggravation.

In some respects, the country is proceeding nicely toward the switch-over. After February 17, in accord with a congressional mandate, full-power TV stations must stop transmitting analogue signals and change to digital ones that offer a sharper picture and more viewing choices.

Most broadcasters are already sending out digital signals along with analogue ones. They'll turn off the analogue signals come February.

And most viewers won't be affected by the transition because they subscribe to cable or satellite services, which will make the conversion on their end. But about 17 million households watch only free-of-charge, over-the-air analogue signals via "rabbit ears" and rooftop antennas. It's here where the government is stumbling.

These viewers must buy set-top converter boxes at $40 to $80 each (the government is supplying two $40 coupons per household, available at www.dtv2009.gov or by calling 888-388-2009). Or they can buy digital TVs or subscribe to cable or satellite.

But the coupons expire after 90 days, and only about half of the nearly 23 million coupons distributed have been redeemed. Meanwhile, some people with new converter boxes find the boxes don't work fully or at all due to weak digital signals. Even new digital TVs may have reception problems.

The reason is the "cliff effect". When analogue signals are disturbed, the picture gradually degrades. But digital signals are much more sensitive to disturbance, and screens can suddenly break into pixels or freeze, or just be blank. There's no way to fix this except to re-aim an antenna, upgrade to a more expensive and stronger one - probably on the roof - or subscribe to cable or satellite (www.antennaweb.org can help viewers select an antenna).