WARNING TECHNICAL: DVD Posterization

Discussion in 'Archived Threads 2001-2004' started by JosephMoore, Oct 24, 2002.

  1. JosephMoore

    JosephMoore Stunt Coordinator

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    http://www.12south.com/posterization/normal.jpg
    http://www.12south.com/posterization/dithered.jpg
    I've read a lot of reviews and a lot of posts that indicate that a lot people just don't understand a fundamental limitation of DVD's.
    Oftentimes an area of "gradated" color (a smooth transition from one color to another) will look "posterized," which is to say that individual steps of color are discernable. This phenomena is sometimes refered to as "banding" and sometimes incorrectly as "polarization" (I can only assume that they mean "posterization."
    Reviewers regularly attribute this flaw to compression artifacts. This is almost always not the case. This is important to note because it is not a problem with the transfer, encoding or compression, it's a limitation of the fact that MPEG-2 uses an 8-bit sample for each channel (YUV) and therefore, can never resolve more than 256 "steps" between pure black and pure white.
    For instance, If a gradation has to cover 400 pixels, but there are only 50 steps of color available, then each step will be 8 pixels wide. At a certain point, the steps in the transition will be visible.
    In fact, the more pristine a transfer, the more likely that this limitation will be noticable because there is no noise to hide the banding. Its the same reason it is most likely to be noticed on animated titles ... the colors tend to be flat and pure.
    Now, some DVD players (software and hardware) are smart enough to "dither" these steps. In effect, introducing controlled noise to give the illusion of a smoother transition.
    To illustrate all of this, I grabbed a frame of sky from the Toy Story Ultimate Box Set. This is widely regarded as about as perfect of a transfer/encode as is available. I took the luminence channel, blew up a section of the grab and then increased the contrast so that the individual bands would be clearly visible. Then I grabbed the same frame from a player that "dithers" and did the same. It's easy to see the benefits.
    This is all a big long explanation just to say don't blame the transfer.
     
  2. Paul W

    Paul W Second Unit

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    I see these posterized effects on ShoHD and HBO-HD al the time. I thought that ATSC(HD) was capable of higher than 8bit sampling.
     
  3. RickardL

    RickardL Second Unit

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    Which players are doing this dithering?
     
  4. Kevin M

    Kevin M Producer

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    Yes, which player are?
    Yamaha?
    Sony?
    Pioneer?
    ...could you tell that those are players that I personally own? [​IMG]
     
  5. JosephMoore

    JosephMoore Stunt Coordinator

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    I don't know specifically what dedicated hardware players dither/interpolate. It would most likely be a feature of a decoder chipset. Because all of the different brands use chips from only a handful of manufacturers, though, if you can find a chipset that does, you should be able to find a selection of different players with the feature.

    It's also possible for the display device to have similar corrective circuitry. A PJ with a high quality onboard scaler may be a good candidate.
     

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