djtoltz
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Aug 19, 2005, 9:39 PM
Post #2 of 2
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Re: [dansen] Progressive vs. Interlaced
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With interlaced video, a single frame is made up of two distinct images, captured at two distinct periods in time and interlaced into a single frame. Therefore, a frame is not like a snap-shot of a moment in time, rather it is two fields that are 1/60 of a second apart (in NTSC, 1/50th of a sec in PAL). Really, interlaced isn't approximately 30 frames per second, it is 60 fields per second. The result is that when objects are moving, their edges will appear jagged, with what is called a "comb effect". When viewed on a field-based monitor, like a standard television, the fields are displayed separately, so there is no image degradation. However, when viewed on a frame device (like a computer) or progressive dispay, where the two fields are displayed together as alternating lines in the same frame, some temporal distortion is often evident.
As you may have guessed by now, progressive captures the entire image at one time, so a frame is like a still photo, with no interlacing artifacts. Motion may not appear as smooth on an NTSC monitor with 30 frames per second as the 60 fields per second (interlaced), but the frames will look better on a progressive monitor or computer, and they will look more like film.
I hope this explanation helps. I'm not the best explainer. I do have an example of interlacing on my web-site:
http://www.oakstreetsoftware.com/~doug/interlaced/
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Douglas Toltzman
Hubert, NC
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