Making Digital Tv Go The Distance

Unlike current analog television broadcasts, digital TV pictures would be free from annoying "snow." And because more information can be packed into the signal, digital technology would allow efficient transmission of high-definition TV. But here's the problem: the dreaded "edge effect."

With today's TV sets, reception gradually gets worse farther away from the broadcast tower. TVs tuned for digital signals would get a crystal clear image--or no picture at all. Where the signal cuts out depends partly on the weather, so TVs in outlying areas might suddenly go blank.

Martin F. Vetterli, co-director of Columbia University's Advanced Image & Television Laboratory has a solution. A mathematical technique known as wavelets selects key parts of the picture in order to create a multi-tiered signal. One tier provides a full HDTV image, while another omits extra detail but travels farther. A third contains only basic information to give a lower-quality, but still viewable, picture. Vetterli has tested the technique in the laboratory with promising results.

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