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Old January 28th, 2009, 01:54 PM   #1
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What is the difference in Structure & Quality between "I" Frame & Regular MPEG 2 ?

HI friends. Could someone more learned than I give me an explanation of what are the structural differences, as well as quality (If any ?) differences between regular and I frame MPEG 2 in day to day useage ?
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Old January 28th, 2009, 02:44 PM   #2
 
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MPEG2 frame makeup is composed of several different types of compression, I-frames, B-frames and P-frames. I-frames are the most basic type. Typically, I-rames represent "uncompressed" images. All the other types derive some compression as a difference from the basic I-frame.

In reality, some intra frame compression is going on in the I-frame. While the other types need an I frame to be decompressed, the I frame is a stand alone that can be decompressed and stand on its own merits.

For a deeper explanation, look it up on Wikipedia.
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Old January 28th, 2009, 05:57 PM   #3
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Quote:
Originally Posted by Bill Ravens View Post
MPEG2 frame makeup is composed of several different types of compression, I-frames, B-frames and P-frames. I-frames are the most basic type. Typically, I-rames represent "uncompressed" images. All the other types derive some compression as a difference from the basic I-frame.

In reality, some intra frame compression is going on in the I-frame. While the other types need an I frame to be decompressed, the I frame is a stand alone that can be decompressed and stand on its own merits.

For a deeper explanation, look it up on Wikipedia.
....Hi Bill. Thank you for responding. So if I have an MPEG 2 movie composed of IPB frames and another comprised of only I frames, would the IPB frame MPEG look sharper ?
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Old January 31st, 2009, 06:59 PM   #4
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At the same bitrate? Your standard MPEG 2, with its full frames and its partial frames, will be better, it's much more efficient since in many videos, there is not much change between one frame and the next, which might be only a 24th, 25th, 30th of a second later. So since nearly the whole frame looks the same, compressing the whole thing again is pretty wasteful.

So you can have, for a given bitrate, sharper I-frames and more detailed changes than if you spent that encoding for a series of virtually identical images.

However...if you have bitrate to burn, a pure intraframe codec (like DV) is great for action scenes, where the relationship between frames is less close- say an explosion, a fast moving car...smoke is a bugger to compress...then they really shine. And you can place an edit anywhere and not have to do any re-encoding to generate new I-frames. Related to that was the saving grace that a tape dropout (ho ho, tapes, eh?) on an intraframe codec affects just what was dropped, but on an interframe codec, it will mess things up until the video stream has a new I-frame.

It's very swings and roundabouts.
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