DVD Aspect Ratio Question

Discussion in 'DVD Video' started by George Graves, Dec 8, 2005.

  1. I have a technical question about DVD aspect ratios. I'm not sure
    exactly how to phrase it so that it's clear, but I've come here because
    I can find nothing on the web that covers this.

    As you probably know, many HDTVs and HD-ready sets automatically assume
    that any time they see a component video input that has a 31.5KHz raster
    or higher, it's indicative of a high-definition program. This causes the
    TV to automatically apply the anamorphic stretch to the standard NTSC
    4:3 frame to make it 16:9. In some sets, this anamorphic stretch is user
    defeatable, but in most, it is not.

    Most modern DVDs made from old TV shows, older, non-widscreen movies, or
    even DVDs that have the widescreen picture letterboxed rather than
    anamorphically squeezed, display the progressive picture output fine on
    sets where the anamorphic stretch can not be defeated, but other DVDs
    will display the progressive output as 16 X 9 when they should display
    it as 4:3 - with the inevitable distortion to the picture, of course.

    My question is simply this. Why is it that some 4:3 DVDs get stretched
    to 16:9 when played on a progressive player (irrespective of what one
    does with the "aspect ratio" controls in the set-up menu) while other
    4:3 DVDs don't get stretched? Is there a 'bit' that can be set in the
    DVD headers at mastering which tells the player that the picture is 4:3,
    and when that bit isn't set, the player outputs the picture as 16:9? Or
    is the content of the discs that display correctly mastered
    anamorphically squeezed complete with side-bars? If the latter, how come
    they still look correct when played back on a normal NTSC monitor using
    composite or S-VHS inputs?

    I need to know this because I review a lot of DVD players for an on-line
    video magazine and I've not seen ONE player that will allow some of
    these 4:3 DVDs to display properly when output progressively or
    up-sampled and I'm at a loss to explain exactly why this happens.

    Thank you in advance for any insight into this phenomenon that you can
    give me.
    George Graves, Dec 8, 2005
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  2. George Graves

    Joshua Zyber Guest

    It would help if you could specify which DVD player is being used and
    what DVDs.

    All DVD video contains 720x480 pixels in a native 4:3 format (DVD pixels
    are not square). Anamorphic widescreen discs are mastered in such a way
    that the image is stretched vertically, and if viewed on a 4:3 TV
    without proper downconversion will look distorted. However, when viewed
    on a widescreen TV this anamorphic image will be stretched horizontally
    to restore the proper picture geometry, but the resolution is still
    720x480 pixels.

    The problem you describe occurs when you upscale DVD video to a higher
    resolution such as 720p or 1080i (and on many older sets is problematic
    with 480p as well). The TV sees the higher number of pixels coming in
    and thinks, "Ah ha! This is an HDTV signal. All HDTV is natively 16:9.
    Therefore, I must automatically lock into 16:9 mode." Of course, not all
    DVD content is natively 16:9, so when the TV enforces this stretch
    you're left with a distorted 4:3 image.

    The answer to your question is yes, DVDs contain an aspect ratio flag
    that indicates whether the disc is 4:3, 4:3 Letterbox, or 16:9.
    Unfortunately, many discs are incorrectly flagged, and you see a lot of
    4:3 Letterbox discs that are flagged as plain 4:3.

    *Some* DVD players (certainly not all, nor even a majority) have the
    ability to read this flag and offer automatic aspect ratio control when
    viewing on a 16:9 TV. In other words, if the TV is 16:9 but the disc is
    flagged as 4:3, the player will automatically scale the image down to a
    540x480 square and put black pillarbox bars on the sides, thus allowing
    you to watch 4:3 content in its correct aspect ratio on your screen,
    though at the cost of lower resolution and less picture detail. Again,
    many discs are incorrectly flagged, so this is not a 100% foolproof
    operation. Additionally, if the disc is correctly flagged as 4:3
    Letterbox, some of these DVD players with aspect ratio control can zoom
    it to fill the screen, but others treat all 4:3 material the same and
    you wind up with a small picture with black bars on all 4 sides.
    The Oppo OPDV971H is a player that offers automatic aspect ratio control
    for upsampled DVDs. The mode is called "Wide/SQZ" in the set-up menu.
    The mode defaults to only pillarboxing the content, but there is a
    manual zoom you can engage for 4:3 Letterbox discs. Unfortunately, there
    is a significant loss in quality when using the zoom.

    Another, though more expensive option, is to use a video processor or
    HTPC that will allow you to manipulate the image in all sorts of ways.
    Joshua Zyber, Dec 9, 2005
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  3. Thank you for your answer. I'm going to assume, based upon what you have
    said, that 4:3 content DVDs which come out stretched on the same DVD
    players which set the aspect ratio correctly on other 4:3 content DVD
    discs does so because these discs either lack the proper flag, or the
    flag is set incorrectly (many of the discs which do this in my
    collection are older from 1998, 1999, etc.
    George Graves, Dec 9, 2005
  4. George Graves

    Joshua Zyber Guest

    Just to clarify, it's not the DVD player doing the stretching. It's the
    TV. The DVD player outputs the original 720x480 pixel video image, which
    the TV is *supposed* to allow you to adjust.
    That's a reasonable assumption.
    Joshua Zyber, Dec 9, 2005
  5. Why don't you take your question up with several of the major hardware
    manufacturers and see what they say rather than ask a forum full of
    novices? Frankly it doesn't sound like you know enough on the subject to
    review any equipment that I'd want to buy.

    Captain Jinks, Dec 11, 2005
  6. George Graves

    Richard C. Guest

    X-No-archive: yes

    My HD set does not do that.
    Many that I have seen do not do that.....................
    Most show the 4:3 frame within the 16:9 screen (in proper ratio).
    You must have your settings wrong.
    Richard C., Dec 17, 2005
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