1080i-720p confusion

Discussion in 'Displays' started by Mark:F, Nov 21, 2004.

  1. Mark:F

    Mark:F Stunt Coordinator

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    Hope this is the right forum for this issue. TV is Sony 34XBR910 which supposedly displays HDTV signals natively up to 1080i. Time Warner HDTV box can be set at 480i, 480p, 720p or 1080i. I had been using the 1080i setting for all signals, but began to notice that the horizontal bars on the ABC and ESPN HDTV signals were of uneven width...went to the 720p setting and bars were straight, I guess because ABC and ESPN transmit at 720.
    Questions....do I lose any PQ on 1080i HDTV signals if I set the box at 720p?...I don't see any difference.
    Are analog 480 signals really "upscaled" to 720p when the box is at 720? What is upscaling really? I didn't think a signal with a fixed number of 480 detail lines could somehow be made "better". What's going on here?
     
  2. ScottLR

    ScottLR Stunt Coordinator

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  3. Arthur S

    Arthur S Cinematographer

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    If the other networks change over to 720P the TV manufacturers will put 720P back in the majority of their sets. By that time I will probably be able to afford a 65 inch DLP or Plasma. In the mean time I am not suffering with 1080I. The image quality is great. We are not talking hughe differences in picture quality between 720P and 1080I.

    Never the less, 720P should have been the standard. Oh well, can't fight the system.

    Artie
     
  4. Allan Jayne

    Allan Jayne Cinematographer

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    The middle diagram in the link above is an apples to oranges comparision. Also I disagree with the naming convention.

    I do not think the terms 540i and 240i mentioned should be introduced here. This makes for confusion with the video formatting process.

    Format wise, 1080i and 540P are essentially the same. (Content wise, 1080i has 1080 unique scan lines every 1/30'th second while 540p has 540 unique scan lines every 1/30'th second.)

    If you were to display 1080i and 480p as closely the same as you can (I don't know of any TV set that can), you would get one of two scenarios:

    1. 480p as 480 scan line positions on the screen looking like 480p, and 1080i as 540 scan line positions on the screen, fields juxtaposed with slightly more (540/480 times) the vertical resolution. Neither will show the motion artifacts suggested in the above link.

    2. 480p as 960 scan line positions on the screen, alternate frames staggered, still with 480 lines of resolution vertically, subject wise. 1080i will have 1080 scan line positions on the screen. Both will show the motion artifacts suggested in the above link.

    Actually a TV with a 540p(see 1 preceding)/1080i(see 2 preceding) menu selection comes close to doing the above, although the 480p and 480i incoming video will have already been converted to 540p so the screen always has either 540 or 1080i scan line positions in use.

    If the cable TV box is outputting 720p, incoming 480i and 480p shows must be upscaled to 720p otherwise the picture won't fill the screen. This upscaling can be done as simply as duplicating a scan line here and there, and the result can be be horizontal stripes of uniform thickness being non-uniform. Just because there are more scan lines, the picture will look smoother. Better? That's your opinion and depends on the quality of the video processing too.

    Video hints (not including the above link's diagrams):
    http://members.aol.com/ajaynejr/video.htm

    You do get more picture detail horizontally with the 1080i or 540p whatever you want to call it from HDTV shows, compared with 480p digital shows.

    Note that some set top boxes may convert Fox' and others' 720p into 1080i for TV sets requiring 1080i, using 540p as an intermediate stage. So the final picture has just 540 unique scan lines, not 720 or 1080. Also you might lose quality by outputting 1080i programs as 720p because that conversion might use 540p as an intermediate stage too.
     
  5. Mark:F

    Mark:F Stunt Coordinator

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    Thanks for the replies guys. I read the pro-720 story and Allan's response. I'm not ashamed to be confused! I make a lot of money explaining finance and investments, but this stuff is brutal!
    I think my TV accepts both 720 and 1080 signals natively....is that right? So it would seem best to set the HDTV tuner at 720 for ABC and ESPN, and 1080 for all others ( no Fox HDTV available)....is that correct?
    I could set the tuner to "pass" too...that gives me all signals native - 480 720 etc- but when using "pass" the 480 comes through on a very small 4:3 image requiring serious zooming, whereas when the tuner's on either 720 or 1080 the 480 4:3 image almost fills the screen without zooming.... probably because 720 or 1080 automatically sets the TV to "full". Any further comments or suggestions?
     
  6. Chris Gerhard

    Chris Gerhard Screenwriter

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    I am using an HD TiVo with my XBR910 and I can't tell any difference having the TiVo set to 720p or 1080i when viewing the 720p channels. I just leave the TiVo at 1080i all the time without problems.

    Chris
     
  7. Mark:F

    Mark:F Stunt Coordinator

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    Chris- I was using 1080 also...but with ABC and ESPN HD the side bars were uneven, but straight when I'd switch to 720...have you noticed uneven bars on ABC and ESPN?
     

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