Why 720x480 for DVD?

Discussion in 'Archived Threads 2001-2004' started by John_Berger, Oct 30, 2002.

  1. John_Berger

    John_Berger Cinematographer

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    This is only marginally related to my previous thread about 480p, so I'm going to just create a new thread.

    I was wondering why 720x480 was selected as the resolution for DVD. It seems to be a very awkward resolution.

    Assuming 480 lines as the base numeric (NTSC obviously), a standard TV (4:3) would be a resolution of 640x480. Obviously 640x480 is a very common computer resolution, so a resolution of 640x480 would have made sense.

    Now enters the widescreen TV at 16:9 (1.78:1). One would think that the 720x480 resolution would be meant for widescreen TVs; however, a 16:9 AR with a 480 line base would actually yield an expected resolution of 853x480.

    Upon some further mathematical calculations, 720x480 turns out to be an aspect ratio of 1.5:1, which is not even a half-way point between 1.33:1 and 1.78:1.

    So, the resolution of 720x480 seems to be a very, very awkward resolution to use as the NTSC DVD standard. Does anyone have some insight as to why this resolution was selected?
     
  2. Jeff Kleist

    Jeff Kleist Executive Producer

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    It's not selected, it's pre-existing NTSC standard

    The additional 80 lines are the overscan area, 40 on each side
     
  3. Bill Slack

    Bill Slack Supporting Actor

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    Like Jeff said, it's the maximum resolution NTSC can allow. And it's also the resolution used for widescreen enchanced discs (but the pixels are no longer square.)
     
  4. Vince Maskeeper

    Vince Maskeeper Producer

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    Jeff, you've potsted this twice now, and sorry but it's not correct.
    -V
     
  5. Allan Jayne

    Allan Jayne Cinematographer

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    I believe that 720 x 480 was chosen for DVD as the best compromise between a good sharp picture and the amount of playing time available on the disk. The next best NTSC source material is LD (laser disks) where the smallest detail reoproduceable is about 1/560'th the screen width.
    For live NTSC video or video taken off of films, it doesn't matter whether it is 640 x 480 or 720 x 480, The TV stretches it any which way you want subject to its width, height, zoom, etc. controls. Except that 720 x 480 is slightly sharper assuming good production techniques. Either 640 x 480 or 720 x 480 can represent either a 4:3 or 16:9 picture.
    The video is analog as it enters the TV set (not counting DVI) so the TV doesn't care whether it was 720 x 480 or 640 x 480.
    ATSC (U.S.) digital TV includes both 720 (sometimes 704) x 480 and 640 x 480 sub-formats. So long as the 720 x 480 has pixels 1/720'th the scan line forward horizontal span (active area) and 640 x 480 has pixels 1/640'th the scan line forward horizontal span, you don't have to adjust the TV overscan when changing back and forth. It is the responsibility of the DVD player or HDTV tuner box to space out the pixels properly when generating the analog component video output.
    If you are creating cartoons using a computer for burning onto a DVD, for a 640 x 480 video using 640 x 480 computer resolution leave the computer monitor as-is. For a 720 x 480 video using 640 x 480 computer resolution, shrink the width of the picture so it occupies 5/6 the screen width. Then draw what you want in in the proper looking proportions.
    Video hints:
    http://members.aol.com/ajaynejr/video.htm
     

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