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And of course , these channels feel the need to fill the whole screen , I guess partially because it 's there and partially to prevent screen burn - in . Let 's compare , shall we ? Above are two screen shots of the same scene , taken from " It 's the Great Pumpkin , Charlie Brown . " On the left , we have the standard definition ( SD ) aspect ratio , 4 units across and 3 units down ( 4:3 ) . On the right , the same scene stretched out to fill an HD screen 's aspect ratio , 16:9 .
What might be the reason that they are talking about screens ?