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Old June 14th, 2005, 03:46 AM   #16
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Well, who knows when a 720p60 cam will be out.

"12 GPO give somewhat better results"

better than what?
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Old June 14th, 2005, 04:49 AM   #17
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The longer GOP, the lower compression. Keeping bit rate constant, if GOP is longer, compression is lower and picture better.

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Old June 14th, 2005, 12:50 PM   #18
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If the compressor can handle the bit rate, a lower GOP ALWAYS produces a better picture. ALWAYS!
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Old June 14th, 2005, 01:04 PM   #19
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Sure, but I said at constant bit rate.
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Old June 14th, 2005, 01:12 PM   #20
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All of the HDV cams use contant bit rate.
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Old June 20th, 2005, 01:27 PM   #21
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Of course. I ment comparing different systems at same bit rare. At 25 Mbps, GOP 12 will mean less compression and better look than GOP 6.

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Old June 20th, 2005, 09:37 PM   #22
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The GOP structure is a form of compression. The longer the GOP the less bandwidth used and the worse the picture. Sometimes Radek I wonder where you get your logic. HDV uses the same compression scheme no matter what the cam, mpeg2ts, so given your variable of 25Mb/s with one cam at 12GOP and one at 6GOP, the 6GOP is going to look superior. Always.
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Old June 21st, 2005, 12:07 AM   #23
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If you don't believe, ask David Newman or Frederic Haubricht in HDV Editing forum. They already answered before.

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Old June 21st, 2005, 09:41 AM   #24
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Please direct me to the thread.
Thanks.
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Old June 21st, 2005, 09:47 AM   #25
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Ken, Radek is actually correct here. If you keep bit-rates the same the longer GOP will produce less image distortion as there will be more of the efficient B and P frames as compared with the I frame. The issue with long GOP comes if there are dramatric scene changes within a GOP (like filming a closeup on fire) the image may not be able to visually encode all the scene changes introducing visual blocking. The more often the I frame the more quickly the frame can fully reconstruct. So the longer GOP will have the higher quality for most sequences, but if rare situation the short GOP can be better. A simple example why a longer GOP wins : 25Mb/s DV has a GOP of 1, yet the same bitrate with a GOP of 12 has 4.5 times the resolution in 1080i HDV.

On the issue whether the 720p60 is a 12 frame GOP or a 6, no camera supports it yet so I don't know. I would very much favor the 12 frame GOP.
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Old June 21st, 2005, 09:46 PM   #26
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It is a shame they haven't figured out to switch GOPS for different scenes, or do variable compression for hard disk recording (as Firewire and internal hard disks, and flash, alternatives to tape are taking off). It is a pity that they don't get this message, so we don't have to tell people "great camera but for filming your kids sports games".
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Old June 21st, 2005, 10:17 PM   #27
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"great camera but for filming your kids sports games". -- Wayne

Funny you should say that -- really this type of footage is where MPEG will have more trouble. MPEG works best for locked down pro shoots, with shollow DOF, and good lighting, the end result will look pretty nice. It is free-form consumer shoot that MPEG stuggles with. :)
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Old June 21st, 2005, 11:02 PM   #28
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Would the free form consumer shoot be better with a 6 or 12 GOP?

Back to your reply David, why did Sony settle on a 12/15 GOP and not go even higher if that is what it takes to make it better? As well why has JVC stuck with 6GOP on their latest HD100? The largest complaint about HDV stems from fast motion that results in blurring, or complex scenes like choppy water, not holding up. This has never been a complaint of the the 6 GOP cams but is quite a common issue brought up with the Sony cams. 12/15 GOP seems very long for a constant bit rate, single pass compressor. It may have worked out well on the Sony's, but I can't see 12/15 GOP being prefered over 6GOP by any serious film maker, brand preference and camera model aside.
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Old June 21st, 2005, 11:22 PM   #29
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Longer GOP is alway better for a fixed bit-rate -- just if all hell breaks loose the short GOP will recover quicker (but neither will look good in that situation.) Compare Sony's 60i to JVC's 30p as way to understand long GOP compression is like trying work out DVD playback by putting a disk up against a bright light. The quality of the compression has more to do with interlaced vs progressive and encoder implemention. Sony is having to encode 60 fields of 1440x540 into 25Mb/s (avg 0.53 bits per pixel) and the JVC only 30 frames 1280x720 into 19Mb/s (avg 0.68 bits per pixel). So the Sony camera has to work harder anyway (and it looks better at 12/15 than it would at 6.)
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