Question about interlacing in 1080i - Displays half the lines or refreshes it?

Discussion in 'Displays' started by Mike B Nguyen, Nov 25, 2005.

  1. Mike B Nguyen

    Mike B Nguyen Auditioning

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    I have a question about how interlaced lines are displayed on screen, especially with 1080i.

    Everysince the Xbox 360 came out, there's been a lot of questions about HD from all the gaming forums (IGN, Gamespot, TeamXbox, etc.). Not surprisingly, one of the most common questions is "Should I set my Xbox 360 to 720p or 1080i?".

    One of the things that's bothered me about these 720p vs 1080i arguments is that a lot of people say that 1080i only displays 540 lines at one time. Meaning you only see half the image in one field.

    I always thought that it "refreshes 540 lines" every field. The words "display" and "refresh" have different meanings. Here's what I mean:

    Display 540 lines:

    This implies that there are only 540 lines at one time. That means in the first field, all the odd lines are shown. That means there are no even lines. In the second field, the even lines are shown, but no odd lines. They're missing. Then in the 3rd field, the odd lines are shown, but the even lines are gone. That is what "displays only 540 lines" implies. Only 540 lines are displayed, the other 540 are gone or are black.

    Refreshes 540 lines:
    This is what I was led to believe. All 1080 lines are shown, but only half are being refreshed. So in the first field, the odd lines are shown. In the second field, the even lines are shown, but the odd lines are still showing the first field. In the 3rd field, the odd lines refresh, but the even lines from field #2 are still there. In other words, whenever an odd field refreshes, the previous even field is still there, and vice-versa.

    So as you can see, changing from "display" to "refresh" changes the meaning a lot.

    So here are my question about 1080i:

    1) Which is it? Are 540 lines only being displayed in one field, switching from odd to even? Or does it just simply refresh from odd to even?

    2) Also, how is NTSC interlacing handled? Are all 525 lines displayed at one time (refreshing only the odds or evens), or just 262 lines at once (meaning blanks in between)?

    3) And finally, is there a difference between the way CRTs HDTVs handle interlacing and the way LCDs and DLPs handle it?
     
  2. Joseph DeMartino

    Joseph DeMartino Lead Actor

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    1 & 2) If I understand this stuff correctly, interlaced systems do indeed only "display" half the lines at one time. By the time they finish illuminating the 540th even line in a 1080i display, the odd lines will all have faded. The reason we don't see this is that same magic "persistance of vision" that makes us see quickly projected series of images as continuous motion when there really is no motion. Same with NTSC. Each frame (of 525 or 1080 lines) consists of two fields (of 262 or 540 lines) displayed at any one time. 60 fields per second, 30 frame per second.

    3) Non-CRT, fixed pixel display systems (DLP, LCD, LCD projection, LCoS - I simply don't know whether plasma is or not) don't do interlacing at all. They are 100% progressive and must ocnvert all incoming signals to the panel's native resolution. So a 720p LCD or DLP set will upconvert a 480p DVD image to 720p and downconvert a 1080i image to 720p as well. 1080p sets upconvert all signals to 1080p.

    Regards,

    Joe
     
  3. ChrisWiggles

    ChrisWiggles Producer

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    joe is correct.

    on a CRT scanning at 1080i, 540 lines are scanned per field, 1080 lines per frame.

    720p scans 720 lines per field.

    This is a very complex debate, but 1080i provides more detail capability per frame by the nature of more lines and more horizontal pixels (sampling) as well. However, interlacing artifacts arise with motion that don't occur with 720p, which has less resolution capability per frame. At a particular given resolution progressive scanning is superior. However, interlaced scanning allows for more per-frame resolution by reducing the data per field. 1080i and 720p are different compromises in a system with limits.

    How this relates to what output a person should choose with the xbox360 is actually less fuzzy, and depends heavily on their display capabilities.
     
  4. Mike B Nguyen

    Mike B Nguyen Auditioning

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    So all CRT TVs do display blank lines inbetween, correct? Odd lines only in the first field, even lines only in the second field.

    And as far as DLPs, LCDs, and Plasmas goes, it doesn't matter since they deinterlace all interlaced signals anyways, correct?

    Okay, thanks.
     
  5. Leo Kerr

    Leo Kerr Screenwriter

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    No, CRTs are strange. The electron beam paints the odd lines, leaving spaces between them. Of course, as soon as the beam passes off of a spot, it begins to fade. However, they do persist somewhat (green usually lasting the longest.)

    After it finishes all the lines in the odd field, it goes back and in the spaces it left before, begins to paint the even lines in.

    Now one of the other things about interlace that CBS and Sony (Broadcast) didn't want you to remember was that in interlaced systems to help protect the system from not quite getting the lines to mesh just perfectly, there is a Kell factor, or vertical pre-filtering, usually reducing the vertical bandwidth by about 20-30%, even if there is no motion what-so-ever.

    Just remember: i is bad; p is good. I've seen situations where a good 860x480x60p looked much better than 1920x1080x30i.

    Leo
     
  6. Allan Jayne

    Allan Jayne Cinematographer

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    I would suggest setting the XBox to the resolution of the TV, 720p for TV's with 720 or 768 scan lines and 1080i for TV's with 1024 or 1080 scan lines.

    How well 1080i (and also 720p to 1080i and 1080i to 720p) is(are) reproduced depends on the quality of the TV.

    While non-CRT 1080 TV's are all non-interlaced, most of them take each incoming 540 line 1080i field and duplicate each line because the plasma or LCD is incapable of being refreshed with just the odd scan lines' worth of material or just the even scan lines' worth of material. Once the material is put on the screen or projection element it stays there until refreshed with the next video frame, the material does not fade as on a CRT face. Depending on how the 1080p frames are constructed, there can be considerable loss of resolution, perhaps your ending up seing just 540 unique lines of material on the screen's 1080 lines. If you already bought your TV you have no choice in this matter.

    Quick example for 1080 non-CRT TV: If the odd lines are (repeated and) laid out 1, 1, 3, 3 and the even lines are laid out 2, 2, 4, 4, the first two positions on the screen will be the same optical blend of scan lines 1 and 2 as the screen is refreshed.

    If you output 1080i to a 720p TV or 720p to a 1080 TV (very few 1080i CRT's accept 720p) some video signal conversion must also take place, and also has the possibility of leaving you with just 540 unique scan lines.

    Vertical pre-filtering is an adulteration of the source material. Where one scan line is bright, the scan line above and/or below is(are) brightened slightly. Vertical filtering is sometimes accomplished de-facto by scanning and averaging slightly-too-wide stripes on the video camera sensing element. It reduces flicker of minute details on interlaced CRT TV's but makes for a softer picture than could otherwise be had if you have a good de-interlacer.

    IMHO the words "refresh" and "display" apply to the TV, not to the video signal. A 1080 non-CRT TV always refreshes and displays 1080 scan lines. A 1080i CRT TV normally refreshes 540 lines at a time and displays 1080 lines. If there is a 1080i/540p setting set to 540p the 1080i TV will refresh and display 540 scan lines regardless of the input, the even lines landing on top of the odd lines instead of going into the spaces in between.

    Video hints:
    http://members.aol.com/ajaynejr/hdtvnot.htm
     
  7. Mike B Nguyen

    Mike B Nguyen Auditioning

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    Thanks for the info, guys! And Allan, you're website is just plain awesome.
     
  8. ChrisWiggles

    ChrisWiggles Producer

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    This is very misleading; ONLY at the same or similar resolution and framerate. For instance a native 480p source is superior to a 480i source. 1080p superior to 1080i. If you are insinuating that 720p is good and 1080i is bad, that would be wrong. Interlaced display is a very important historical artifact to get as good and image possible within very limited broadcast bandwidth.
     
  9. Leo Kerr

    Leo Kerr Screenwriter

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    Leo
     
  10. ChrisWiggles

    ChrisWiggles Producer

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    Have you seen d-theater or other HD at 1080i???

    At both 720p and 1080p it's incredible.

    480p isn't going to touch that. I am assuming good source. Sure, bit-starved HD can look bad, but that's a completely unrelated issue. I've seen HD look like ass compared even with SD 480i sources, but that's not at all related.
     
  11. Leo Kerr

    Leo Kerr Screenwriter

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    I've seen a variety of video displays with variety of source material, including uncompressed camera feeds in shooting galleries. And yes, bit-starvation in any format can make you wish for even VHS tape (well, maybe not that bad. How about S-VHS?)

    I have seen some very nice looking 1080i, too.

    It's just that interlaced source material leaves a fairly nasty foot-print on what it touches that can't be eliminated.

    Now, 1080i distribution of a 24p source ought to do well, given that (at least now,) 1080p displays (even if it says 1080i, a DLP or LCD display is still 1080p at the heart,) are getting more common, and should be able to treat the video as similar to 1080sF and recreate the 1080p source frame.

    I guess it sums up with, HDTV is a major step up from NTSC, and all of its ills. But why, I and some of my friends and co-workers in various branches of government and some in industry, should we settle for a "good" system when we could be striving toward a "better" or "best" system?

    Of course, we've gotten rather far afield from Mike's original questions...

    Leo
     
  12. ChrisWiggles

    ChrisWiggles Producer

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    I hardly care about OTA stuff, my concern is that HD-DVD and bluray will not be as stellar as d-theater titles. From comment's I've heard from the folks working with the studios on this, they're just aiming to get HD encodes better than OTA, which is a pretty low bar, unfortunately [​IMG]
     

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