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Thread: Can someone explain the difference please...

  1. #1
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    Default Can someone explain the difference please...

    I've never managed to find a simple description to the difference between Progressive and Interlaced frame-rates.

    Anyone have a short answer or know a link?

    Thanks a lot, Ben

  2. #2
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    Nutshell layman's answer:

    CRTs made up pictures by guns firing a beam of electron thingys at the screen thingy (note the technical terms may not be quite correct

    The picture was made up of a series of horizontal lines. So the guns would fire line 1, then line 2 etc right down to line 480(NTSC) or 576(PAL).

    There is a problem in that by the time the beam has reached the bottom of the picture the lines at the top are beginning to fade.

    To overcome this effect to some extent, "interlacing" was developed. Using this technique the first pass of the beam would display lines 1,3,5,9 etc and the second pass lines 2,4,6,8 etc. So a complete scan would take half as long and whilst lines would begin to fade just as fast, it was only every OTHER line that faded, which gives an overall consistent picture.

    A side effect of this was that fast motion can display better. Without interlacing, by the time the bottom line had been reached and it started on line 1 again the image will have changed noticably - especially in fast moving images. Effectively you would constantly see the top part of the image and the bottom part "out of sysc" with each other. The other problem was that by the time the beam was firing at the bottom of the screen

    With interlacing, as it only takes half the time for the scan to reach the bottom of the picture, the movement in the image would be only half as far.

    In progressive shooting each "frame" is a complete picture. With interlacing each "frame" is only half a picture, made up of either the odd or the even scan lines.

    See also What is deinterlacing? The best method to deinterlace movies and [ame=http://en.wikipedia.org/wiki/Interlacing]Interlace - Wikipedia, the free encyclopedia[/ame]
    Tim

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    Thanks so much for that!

    So would progressive suggest the lines form one by one down the screen, progressively?
    Also how does this technology apply to modern liquid crystal displays?

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    Quote Originally Posted by benesotor View Post
    Thanks so much for that!
    You're most welcome
    Quote Originally Posted by benesotor View Post
    T
    So would progressive suggest the lines form one by one down the screen, progressively?
    Yes
    Quote Originally Posted by benesotor View Post
    Also how does this technology apply to modern liquid crystal displays?
    LCD TVs handle interlaced footage automatically.
    On a PC some software does, and some doesn't. When you see jagged horizontal lines especially along vertical edges which are moving, this is interlaced footage being played using software that doesn't correctly de-interlace.
    Tim

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    Quote Originally Posted by TimStannard View Post
    Yes

    LCD TVs handle interlaced footage automatically.
    On a PC some software does, and some doesn't. When you see jagged horizontal lines especially along vertical edges which are moving, this is interlaced footage being played using software that doesn't correctly de-interlace.

    Ah okay makes sense now. I have actually had footage off a camera that records interlaced PAL, and (especially at low quality) I get very visible horizontal lines. However I cut the video in Sony Vegas Pro, I would have thought the software would automatically handle it like you said?

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    By default, Vegas will render footage as interlaced as most movies are put onto DVD and watched on a TV. Unless you specially tell Vegas to render the clip in progressive (ie: for the web) then interlaced footage is normal. However only the TV has the circuitry to handle it.

    If you want web video and need to rid yourself of interlacing lines then when you render in Vegas click the "Custom" button and under field order select "None - Progressive" ..You also have to remember that web video cannot interpret widescreen horizontal pixels so if you are doing a clip for the web you need to render it with dimensions that are widescreen ratio (like 640x360) and in the custom render window also set the aspect to square pixels.
    If you shoot widescreen for the web make sure your video dimensions have an aspect ratio of 1.7777 !!!

    Chris

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