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DVD size vs. move time length (discrepancy)

Discussion in 'DVDR' started by cyclist, Sep 12, 2004.

  1. cyclist

    cyclist Member

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    I have a a very basic question that I just can't seem to find an answer for.

    I understand what the capacity of a DVD is. I "sort of" understand how many bytes there should be for different durations movies (I have read the FAQs), but I am perplexed at a seeming paradox.

    1. The movie "Starman" is 1 hour and 54 minutes long.

    2. The movie "Fat Man and Little Boy" is 2 hours and 6 minutes long.

    DVD Decrypter and DVD Shrink tell me:

    The DVD of Starman (main title) is about 3.8 GBs.

    The DVD of "Fat Man ..." is about 7.7 GBs.

    Neither movie has extras. They are both only the widescreen version (Starman is a flip-over disc for the full screen version). The above sizes are for the main titles (which differ by only 12 minutes in length).

    Why is there such a big difference (2x) in the size of these two movies?

    Is Starman compressed? Does "Fat Man and Little Boy" have some kind of higher resolution?

    Shouldn't a 2 hour and 6 minute movie ("Fat Man ...") be closer to 4 GBs? (That's what my research of all the DVD FAQs seems to indicate.)

    Can anyone shed some light on this for me, as I've read the FAQs and can not figure out why "Fat Man ..." is so much bigger.

    Thanks in Advance for responding to this very basic question.

    Chuck
     
  2. vurbal

    vurbal Administrator Staff Member

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    There are a lot of reasons for different movies being different sizes, some of which probably apply to the specific movies you're talking about, and some of probably don't.

    First off, it's important to understand that the length of a movie is a poor indicator of what size it will be. DVDs use 2 types of compression, intraframe (within each picture) and interframe (between frames). Intraframe compression doesn't vary between discs, but interframe compression does. Interframe compression means only 1 out of 12 frames on a typical DVD is a full picture. The other frames only store the differences between one full picture and the next. If there are a lot of changes between full pictures (lots of motion) it will require more information in the partial pictures in between, and therefore the movie will take more space on the DVD.

    Other differences include how many audio streams are used (and how many channels/what type of compression), how high the quality of the encode is, and often the age of the DVD. Most flipper discs (like Starman) are older DVDs that were encoded at whatever bitrate is required to make them fit on a single side of a single layer disc. That means that they are often encoded at a lower quality than they would be if they had been encoded later to put on a dual layer disc. You sometimes see the same thing with longer movies that are encoded to fit on a single dual layer disc.
     
  3. cyclist

    cyclist Member

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    I appreciate you taking the time to explain that to me. It appears that DVDs are have more complexities than I had thought. They can be created using a variety of resolutions (bitrates) and encoding schemes.

    I'll eventually get all this. I should probably sit down some day and do the math. I understand audio encoding - samples sizes and bit rates. I suppose it all comes down to the same thing for video, too.

    Thanks,
    Chuck
     
  4. vurbal

    vurbal Administrator Staff Member

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    You're welcome. I know how complex it is just to wrap your brain around the concepts, let alone understand how to apply (or explain) them. If you want a good starting point for more technical information I'd suggest reading (and re-reading and re-reading) the FAQ at Berkeley:

    http://bmrc.berkeley.edu/frame/research/mpeg/mpeg2faq.html

    Lots of good information there, and at the very least it should give you an idea of what you need to learn. Of course, understanding MPEG compression is only part of the battle with DVDs. You also have to understand the DVD standards as well, but IMO MPEG is the best place to start.
     

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