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Difference between open GOP and closed GOP

Discussion in 'Other video questions' started by Konran, Apr 19, 2004.

  1. Konran

    Konran Member

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    Hi everbody,

    for quite a log time I've been cut-editing MPEG-2's with various cutting tools. Sometimes these tools get crashed with specific MPEG streams that I've checked as OK with MPEG-VCR. Now I realized that it's sometimes cause by a difference in GOP format.

    So, what is the internal difference in a GOP that is closed for edit and a GOP declared as open in a MPEG stream? Both follow the coding rules e.g. IBBPBBPBBPBB etc., so there must be another thing which makes the difference. Any Idea?

    Best regards, Konran
     
  2. vurbal

    vurbal Administrator Staff Member

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    To understand the difference you need to understand how B frames work. B frames are bidirectional frames, meaning they depend on frames that are before and after them. With a closed GOP, the B frames don't depend on any frame that's not in the same GOP. Some GOPs should be closed (like scene changes) because it does no good to look at the information in the previous GOP, which is from a completely different picture.
     
  3. Konran

    Konran Member

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    Thanks Vurbal. I knew the bidirectional behaviour of B frames but I did not knew that B frames can be monodirectional.

    Is it a known issue for MPEG encoders like MainConcept or TMPGEnc to crash on errors in the open/closed GOP structure?

    I again and again have MPEG-2 files meanwhile (704x576 PAL, from Panasonic VRO file, demuxed and cut/edit adverts with MPEG-VCR) with which both encoders crash with either memory fault (read error at 0x00000000 or silently rebooting the PC). What I'm wondering at is the fact that I've got these files working after splitting them into chunks of 2 ... 13 minutes with MPEG-VCR, encoding each separately and re-joining them with MPEG-VCR.

    Any idea, what happens here?
     
  4. vurbal

    vurbal Administrator Staff Member

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    I've never used MPEG VCR, and I only encode with CCE (I learned about closed GOPs from CCE's manual), so I can't really say. CCE doesn't accept MPEG-2 files as input so I've never had to worry about it. I suppose it's possible that those encoders don't know about closed GOPs and MPEG VCR changes them back to open GOPs when it joins them.
     
  5. Konran

    Konran Member

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    I don't use CCE because when I tried once to feed it with an AVI stream via VirtualDubMod (from MPEG-2, what is always the source for my complete video stuff) it didn't work out. The CCE demo also was not able to configure to VBR for MPEG-2 encoding, so I cancelled further testing.

    Yes indeed, MPEG-VCR places open GOP's wherever it has to re-encode GOP's in a cut/edit project. That is i.e. when I make an intra-GOP splicing. The rest of the source files in the cut list remain unchanged, so the DVD hard disk recorder saves both GOP types in its VRO files.
     

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