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W O R K S H O P

Shrinking the Video: How Codecs Work

February 18, 2002
By Darrin Woods

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Getting Video to the Viewer


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Video is displayed in one of two ways: interlaced or noninterlaced. Noninterlaced is also known as progressive scan. Interlacing is used for television broadcasts and laces two pictures together to create one. For each picture to be drawn on the screen, the TV starts at the top with the first row of pixels. It then skips the second row and draws the third, and so on.

Interlacing

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This displays the odd field on the screen. When the last odd row is drawn, the TV starts back at the top and draws the even field starting with the second row. Each field is drawn within 1/60th of a second, so a total of almost 30 pictures or frames are being represented on the screen every second. Actually, 29.97 frames are drawn every second on television screens in the United States.

The noninterlaced or progressive scan technique is used in everyday computer CRTs; it is also used in some HDTV (High-Definition Television) signals and newer DVD players. A noninterlaced image is drawn starting on the first row, then the second, the third, the fourth and so forth. Because the rows of pixels are drawn in a sequential manner from top to bottom, flicker is eliminated.