720 p vs 480 p

Discussion in 'Displays' started by RussellTodd, Mar 25, 2005.

  1. RussellTodd

    RussellTodd Stunt Coordinator

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    I have question about 720 vs 480 progressive signals. I'm trying to calibrate my new Panasonic TH-42PWD7UY with a resolution 852 x 480 with the MOXI cable box. The cable box does not have an "override" function that recognizes the difference between HD and non HD sources. I like the aspect ratio the 480p setting gives me because it allows the Panny to do the stretching for me on non HD signals. However the HD images seem more crisp on 720p but then on the non HD channels the image appears poorly strecthed since the cable box is doing the stretching instead of the Panny.

    My question is if the Panny only has a resolution of 852 x 480(EDTV) why does the HD image look better on 720p. Shouldn't 480p and 720p look the same since the TV can only display 480?

    Thanks
     
  2. Allan Jayne

    Allan Jayne Cinematographer

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    1. Your TV may be doing a better job of scaling and stretching the video signal to fit the number of rows of pixels (or scan lines) on the screen.

    2. If the cable box letterboxes the HDTV signal when outputting as 480p, the quality will be degraded considerably.

    Video hints:
    http://members.aol.com/ajaynejr/video.htm
     
  3. RussellTodd

    RussellTodd Stunt Coordinator

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    But not taking into consideration stretch modes shouldn't 480p and 720p look the same since the EDTV can only display 480? I swear to me the 720p looks a little crisper even when using the same stretch modes for both 480 and 720. Is it just my imagination?
     
  4. Pat Frank

    Pat Frank Stunt Coordinator

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    Sure, and it's by the same reasoning that films are now transferred at thousands of lines of resolution when no display handles more than 1080. You tend to get a better image when you downsample from a higher resolution than when you sample at the intended resolution.

    There are various reasons behind it, but in a nutshell it has to do with what kind of choices your algorithm is making in each situation. When you're building an image at intended resolution, you have the whole picture to worry about. But when you're downsampling you get to cherry-pick the best available bits.

    Something to remember is that any digital representation of analog data is always inherently flawed. Anything you can do to remove or reduce those flaws is always a good thing. Two passes through the decision-making process is therefore (generally) going to be better than one. (Quite a lot different from analog-world thinking, isn't it?)
     
  5. John S

    John S Producer

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    yes, the source is key here. More information to be displayed. Color depth, details, ect..ect...

    It should look better. Keep in mind, DVD is really only 480i at the source level.
     
  6. RussellTodd

    RussellTodd Stunt Coordinator

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    That does make sense. So it isn't just my imagination. Do any of you know whether or not 480p is typicaly broadcast in the 4:3 aspect ratio? I know the 720p output on my cable box is 16:9 I think the 480p is 16:9 as well but I can't really tell since the TV makes sure all images fill the whole screen. But is doesn't appear to be any strectching going on when I watch an HD broadcast in 480p just doesn't seem quite as sharp as the 720p. My real problem is how to make the cable box recognize the difference bewteen HD and non-HD broadcast. Meaning it would automatically switch to 720 when on a HD channel and then switch back to 480 when on a SD channel. My old Motorola DCT 6200 would do this but I have not found a way to make the MOXI box automatically does this. You have to go in and manually change the video output depending on what you are watching.
     
  7. John S

    John S Producer

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    Might be worth a fully programmable remote, and macro's.....

    I have no expeirence with that MOXI box though.

    See... Just when you get the Motorola's down, they go and start using different boxes. [​IMG]
     
  8. RussellTodd

    RussellTodd Stunt Coordinator

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    A new programmable remote might be worth looking into. However, the programmable remote I have now does not work with the MOXI signals sadly. Also even a macro funtion would be somewhat annoying as you have to go thru about six menu functions to switch from 480p to 720p. I wish the Moxi box had the 4:3 override like thier other HD boxes. For now I may just be stuck with function over form and live in 480p world. I guess thats the price you pay for having an HD DVR box. Its not a huge difference between the 480p and 720p but a slightly noticable one that is annoying none the less.
     
  9. Michael TLV

    Michael TLV THX Video Instructor/Calibrator

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    Greetings

    Don't forget the pixel counts too. DVD starts at 350K worth of pixels ... and the panel has 410K worth of pixels.

    720P has 922K worth of pixels.

    The DVD is scaled upward to fit the panel and the HD is scaled downward to fit the panel.

    Scaling is better done when you can drop information rather than make it up out of thin air.

    Don't forget that the HD color palette is also much better than that of DVD and conventional NTSC.

    regards
     
  10. Pat Frank

    Pat Frank Stunt Coordinator

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    That's interesting, I didn't know that. Makes sense, of course, since DVD is based on NTSC.
     

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