Interlace VS Progressive Scan

HD video images can be transmitted and displayed through two common formats, progressive and interlaced. Both formats display images left to right and top to bottom, but progressive and interlaced formats present the series of images that make up video very differently.

An interlaced signal splits the picture into two parts and sends them separately one after the other. The first image displayed contains all the even horizontal scan-lines while the second image displays the odd lines. Each of these images stays on the screen for one sixtieth of a second before the next image is displayed. When the odd and even parts of the signal are combined, it works out to an entire frame being displayed every thirtieth of a second. One drawback to this method of display is that some jaggedness is noticed in videos with motion, like sports or action movies. This is because only half of the lines are keeping up with the moving object in the image, while the other half is waiting to be displayed.

With a progressive signal, all of the lines are displayed when the screen is refreshed instead of alternating odd/even lines. Progressive video displays each frame for 1/60th of a second, this allows for a smoother picture because there are no split, alternating images as there is with interlaced video. One drawback to 720p video is that it can only display 720 lines of resolution compared to 1080i.

Both formats use the same amount of bandwidth, so broadcasters will only use one format or the other. Fox and ABC stations use 720p while CBS and NBC stations use 1080i. Viewers also had to choose between TVs that natively display 720p or 1080i video. Even though 1080i video could be de-interlaced and rescaled for display on 720p displays and visa-versa, the result is degradation in video quality one way or another.

A new signal format has become the new standard for HDTV, and that is 1080p. 1080p is progressive, meaning that it displays an entire image every 1/60 of a second, and each image is 1080 lines of resolution in height. 1080p delivers a smooth image that also has the maximum detail.


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