Why not use 18 GOP?

[DVD-RBNewbie]

Greetings all! Long time lurker (and DVD-RB user).

I've finally gotten around to "playing" with DVD-RB's settings like VBR_Bias and Quality_prec to learn what effect they have on the final output. I have been reading, searching, and learning alot (a big Thank You to all the poseters here!) but have a question I can't find good a answer for:

Why should I *not* use a max of 18 GOP for NTSC DVD's? It seems to give better compression and quality (as it should in theory) than the default of 15, and it seems to work fine in the 3 or 4 players I have tested it in. I don't care about compatibility with players other than my own, so if 18 GOP works for me is there any reason I should't use it? If there is a particular type of source that will not work with this setting will DVD-RB or CCE automatically detect that and give me an obvious error message?

I case it has anything to do with the answer I should also note that I encode only with CCE and watch DVD's only on players with progressive output and very large screens (60+ inches).

[jdobbs]

Better compression yes, but not necessarily better quality. An 18 frame GOP uses a higher percentage of P and B frames and fewer I frames.

For NTSC it is fine to use 18 frame GOPs -- as long as the encoder is strict enough to never go larger. The DVD standard limits GOP length to 18 for NTSC and 15 for PAL.

[bigotti5]

I encode only with CCE ...

CCE cant create 18 frame GOPs..

[jdobbs]

Actually it can. True enough the GUI will limit you to 15... but you can set the necessary parameters in the ECL file and it will encode with 18 frames per GOP.

It's one of those tricks you learn along the way I guess. ;)

[DVD-RBNewbie]

I'm using CCE SP v2.70.02.00 and it creates 18 frame GOP's for me just fine. I *think* CCE Basic won't...

@Jerry - Thanks, and please excuse my "newbie-ness" as I try to get my head around how this stuff all works...

I understand how an "error" introduced early in the GOP will propogate further if the GOP length is 18 rather than 15, but wouldn't this small trade-off be more than made up for by the much better compression? I know there are no absolutes in this hobby, but wouldn't this be true most of the time?

(BTW, I think I am wrong about this because if i were right then I would see advice to use GOP 18 everywhere... I haven't seen such advice and don't think I've ever seen a "pro" NTSC DVD done with GOP 18, so there is probably a good reason - i just want to understand exactly what that reason is so i can determine if it applies to *me* and the type of backups that *I* do).

Thanks again for any info.

[jdobbs]

Sometimes encoders make little decisions on their own and may "stretch" a GOP by adding a frame. If that happens to one that has a GOP of 18 -- it immediately violates the standard. I'm not sure if that ever happens with CCE.

You might also run into problems with the size limits of VOBUs. It all depends on the encoder and how smart it is.

I'd recommend staying with 15 for NTSC without pulldown and 12 for PAL or NTSC FILM (23.976fps) unless you have a specific reason for not doing so. DVD-RB automatically chooses those unless you override it.

[Trahald]

18 gop could potentially be dangerous on an ntsc FILM title as with pulldown (rff) flags added the gop will be > 36 fields which is illegal for dvd .. for non-film video ntsc its ok.

[jdobbs]

Trahald is absolutely right. I knew there was something missing in my answer... ;)

[DVD-RBNewbie]

Thank you guys!

Film? Is that stuff still around? :-)

Just kidding - I was aware of the potential problem with pulldown flags but always check my source during pre-processing.

Ok, last question, promise :-)

If CCE did occasionally go over 18 GOP, would that show up as a momentary "glitch" during playback, or would it cause some more drastic problem, such as refusal to play past that point at all?

Or, even better, is there an easy way for me to *force* CCE to create an "oversized" GOP so I can stick it in my players and see what it does?

[jdobbs]

You can modify the ECL and make CCE create a 21 or even a 24 frame GOP if you wanted to.

In the ECL file you have these settings that would create a GOP size of 15:

gop_m=3
gop_nm=5
gop_hdr=15

Giving: IBBPBBPBBPBBPBB

To get 18:

gop_m=3
gop_nm=6
gop_hdr=18

Giving: IBBPBBPBBPBBPBBPBB

21 would be:

gop_m=3
gop_nm=7
gop_hdr=21

Giving: IBBPBBPBBPBBPBBPBBPBB

I'm not sure how far it will go...

As far as the playback is concerned, I think it depends on the firmware of the player. One player will accept a momentary "lapse of judgement" without any complaint. But on another it may simply stop. Just remember -- standards are there for a reason... and you may not be using the same player in a year or two, so sticking with the standard is very important.