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Old September 23rd, 2005, 01:15 PM   #1
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All about the GOP

Ok, Mac peeps, I've got a question: WHAT is the airspeed velocity of an unladen swallow?

No...wait... I mean what is the proper GOP for DV? It's been a question that burns in my mind ever since I started using Compressor 8 months ago. I've been exporting .m2v files for use on DVDs, and the GOP menu options have always boggled me. Even after a furious Googling I was none the wiser. There's an article in one of my DV mags that touch on it, but it was for HDV and I'm not too sure if or how the two correlate.

Any advice? (it's 16:9, 24p 2:3:3:2 stuff if that helps)
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Old September 23rd, 2005, 02:29 PM   #2
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NTSC uses GOPs between 4 and 18 frames long. I don't know why 18 is the max, but I imagine 4 is the minimum to get a single IBBP frame group. 15 is an average default GOP length, but I learned recently that the GOP length changes throughout the MPEG-2 stream in most encodes. It's possible to do fixed-length GOPs, which are necessary for multi-angle discs, but otherwise it will vary. You can also use open or closed GOPs with Compressor. The default is closed, meaning that the frames in each GOP refer only to each other, which is supposed to decrease compression efficiency but keep random access time faster than open GOP. Open GOP is supposed to increase compression efficiency but make random access time slower. I haven't found much of a difference in my own discs.
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Old September 23rd, 2005, 04:43 PM   #3
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Nathan, yes, 18 is the max. That's 1 I-frame and some combination of P- and B-frames in each GOP. That'll get you the most mileage out of an MPEG-2 stream, and best overall quality for general use.

You can use a shorter GOP if you have lots of motion in your footage, but most encoders will start a new GOP with a fresh I-frame if they detect lots of motion, anyway.

Hey, are you making sure you are generating a true 23.976 progressive MPEG-2 file, since you have 24p footage? Make sure it's ditching the pulldown and doing full frames. It's much better quality if the encoder is spreading the bitrate across just 24 frames, instead of 30 frames, with the split frames. No use encoding frames that should be thrown out anyway.
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Old September 24th, 2005, 11:16 AM   #4
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DV is not using GOP, GOP is a characteristic of mpeg (1/2/4).
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