GOP length 15 instead of 12

[91638]

I've searched the forum but i haven't found anything about this. Maybe i'm to blind. My question is: Why do you use GOP length of 12 instead the GOP length of 15 for PAL DVD's with CCE. Is there a reason for or can i change the GOP length to 15 without problems?

Thanks for the answer or a link with a description (if i was to blind to find).

P.S.: My english is not the best but i hope you can understand my question.

[jdobbs]

The default for all PAL DVDs is 12 now (in DVD Rebuilder). The only time a length of 15 is used is on NTSC interlaced sources.

[DK]

being able to choose between any such values would be a good idea though

not too few user prefer 15 over 12

[Sir Didymus]

Hi 91638, :)

This type of tweaking [together a lot of other interesting encoding options] is available using RB-OPT, the nice add-on application by robot1...

Personally, almost never adopting it and doubtfully I would recommend its usage...

From the video quality point of view, using 15 frames per GOP instead of 12 allow by sure the user to gain something on the compressibility side (even though don't think this gain is huge), on the other hand reducing the I frame frequency in the stream makes harder the encoder job, may lower the whole similarity with the original stram and may lead to some compliancy troubles in the authoring task...

So, IMHO, it is debatable to quantify if it is really benefical to go to the limit of 15 frames per GOP (for PAL or 18 for NTSC)...

Cheers,
SD

[jdobbs]

Further complicating... with many (most?) encoders there are instances where the value you give can be stretched a little. For example, if you set it to 15 you may occasionally get GOPs that are, say, 17 long. That becomes a problem... because in PAL 15 is the maximum and when that happens the DVD is out of compliance... the same applies to the 18 frames per GOP usage in NTSC.

[archaeo]

from RB-Opt Readme file:
Gop Length; Default 12. 15 is suggested for low bitrates (it's my default for a backup).

Yes, robot1 mentions using this value. I've often wondered if choosing GOP=15 for all backups would get me into any trouble...

[Boulder]

Originally posted by jdobbs
Further complicating... with many (most?) encoders there are instances where the value you give can be stretched a little. For example, if you set it to 15 you may occasionally get GOPs that are, say, 17 long. That becomes a problem... because in PAL 15 is the maximum and when that happens the DVD is out of compliance... the same applies to the 18 frames per GOP usage in NTSC.
This doesn't apply to CCE, but I've seen it with NuEnc sometimes so it might apply to QuEnc as well. CCE keeps the max length at 15 frames. I've used 15-frame GOPs for all my DVDs and have had zero problems because of that.

[archaeo]

I've used 15-frame GOPs for all my DVDs

a newb question, but what are the benefits of using 15 frames over 12 frame GOP's for backups? (I use CCE as well)

[Boulder]

Higher compressibility, there are less I-frames. Encode a sample OPV at Q30 with both GOP size 12 and 15 and compare the filesizes and you'll see for yourself.

[archaeo]

thanks Boulder

[jdobbs]

Originally posted by Boulder
Higher compressibility, there are less I-frames. Encode a sample OPV at Q30 with both GOP size 12 and 15 and compare the filesizes and you'll see for yourself. There's a delicate balance involved. Fewer I-Frames also means a greater likelihood of drifting from the true picture because the predictive nature of B and P frames... but now we're getting esoteric. :)

[Boulder]

Originally posted by jdobbs
There's a delicate balance involved. Fewer I-Frames also means a greater likelihood of drifting from the true picture because the predictive nature of B and P frames... but now we're getting esoteric. :)
That's true..and I usually like the KISS approach;) Fortunately the difference isn't that big unless we are discussing percentage here. Still a valid point:)

EDIT: Let's say that if you need the extra compression, go for 15. If the movie compresses well or doesn't need much compression to fit a 4.7GB disc, no need to change anything as you probably won't see any difference whatsoever. Anything that's truly interlaced needs 15, that I can guarantee;)