What is the difference between an interlaced and progressive image?

What is the difference between an interlaced and progressive image?

Now that we know there are 525 horizontal scanning lines stacked one atop the other, we should also be aware that only 480 of these lines form the visible image. The remaining 45 lines are in the background and form the vertical blanking interval (VBI).

In an interlaced system, the very first type of television broadcast, the lines were painted in fields. One field painted all the odd lines… 1,3,5,7 etc… These lines were painted in 1/60 second. The second field, comprised of scanning lines 2,4,6,8 etc, is painted in the second 1/60-second. The two fields create a frame. Television images are shown 30 frames per second. Persistence of vision ensures we see a complete image as the inherent latency of the human eye makes it impossible to perceive visual changes that occur in less than 1/30-second.

Computer monitors utilize a different method of display. Because in the early days of computers there was no need for motion in the images, the data could be painted (the refresh rate) at a higher speed because of the lower amount of data being carried. A computer monitor displays all 480 lines sequentially in 1/30-second. This is progressive scanning - the horizontal lines are "painted" sequentially.

With the advent of digital video in the later part of the twentieth century, we adapted the progressive scan model to video transmission in order to ease the "marriage" of data and video sources and displays. A byproduct of this advance is that progressive scan video has become a standard for digital television broadcasts and storage media. Progressive scan images aren't any "sharper" than interlaced images, but they can provide greater color saturation and better image "density", resulting in a more film-like experience for the viewer.

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