GOP lenght of 26 detected on original R2 DVD

[GOPPER]

Using different tools I detetected a GOP size of even 26 on a R2 PAL dvd...


That shouldn't be possible, but it's an original rip....

Indeed there are (most of the time) one I frame followed by 25 P,B frames ???

Can anybody explain ?

Used bitrate-viewer and Mpegvalid, which both gives gop-size 26 !

Peter.

[jdobbs]

My guess is that it's probably a field based interlaced MPEG-2 source rather than frame-based. When that happens a picture_start_code is embedded in the stream for each field and they are presented to the decoder individually. The programs might be checking for picture_start_code and incorrectly assuming this is 26 frames as opposed to 26 fields (13 frames).

If you open the source with VOBEDIT and follow the stream you will probably see a sequence something like this:

I - P - B - B - B - B - P - P - B - B - B - B...

You could also look at the picture_structure indicator in the picture coding extension following the picture_start_code/extension_start_code and it will show "bottom_field" or "top_field" as opposed to "frame_picture".

Since virtually all (more than 99%) of the available sources are frame based, it isn't unusual for software to misinterpret it. DVD-RB should read it correctly, though.

But, as I said, that's just a guess. People at authoring houses can make mistakes too.

[Edit] Corrected the "I - I" by replacing it with "I - P"

[GOPPER]

That's strange.

It's an R2 PAL disc, and opened it with VOB-editor.

It's name "Riding The Bus with My Sister"

You see one I frame followed bij 25 P/B frames in VOB-editor.

And in beteween some audio packs......

However PICTURE_STRUCTURE = TOP FIELD......, so indicating field based.

But however not double I frames.

In Vobeditor the sequences seems 1 I follwed bij 25 P/B's (in LBA sequence)

So this would be contradictory , or is there another explanation ????

thanx

Peter

[jdobbs]

you will probably see The fact that is it showing the picture_structure as TOP_FIELD confirms exactly what I said. The software you are using is reporting fields as frames...

[GOPPER]

Yep, probably ok, well I know now this can happen too, makes live easier, and I learned something again.

So I would be happy if there were some tools I could really trust, without becomming a guru on the bits..... (this stuff would cost me few 100 'dreds hours to study)

But it seems difficult to get things right......

But if I see this kind of structure, this would mean about 1/2 of the I frames regarded to a 'normal' DVD ?

Which would mean space saving.....or worser quality....

Peter

[jdobbs]

Is the I frame also listed as picture structure TOP_FIELD?

[hank315]

@GOPPER
But however not double I frames.In field based encodings only the first field is I, the second field is P which is predicted from the first field, these two form the first I frame.

But if I see this kind of structure, this would mean about 1/2 of the I frames regarded to a 'normal' DVD ?
Which would mean space saving.....or worser quality....Worse quality... depends on the source.
If the source is progressive it's not the right way to do it, for (heavily) interlaced stuff it improves quality.

[GOPPER]

@JDOBBS, yes I was taken the info from the I frame as I rembered correctly.
(and I now even checked VOB1, and indeed it is)
Source is interlaced and even has GOP-timestamps that are incorrect, but I see this al the time, so I think time info is retrieved another way.
The timing howewer is sequnetial as far as I looked.
The SRC start at 0 with the first pack (Navigation pack)
The SRC is 00000146.086 for this I frame start.


Some other info:

Frame-type= 1 --> I-frame
Closed Gop =0 (I thought the first GOP should be closed ?, prob. wrong again)
Broken Gop=0


@Hank315, Yep it's Interlaced. And indeed three P frames follow this I frame.

And I demuxed it and (VOBEDIT is smart !, taking the remaning VOB's in account) and discovered in the M2V:

Stream is CBR (again another CBR original...., happens ofter to me)
Bitrate is: 5100 (the lowest I've ever seen on an original CBR source)
GOP were: 26 (according to bitrate-viewer)

GOP structure is the same for every GOP: I PPP BBBB PP BBBB PP BBBB PP BBBB

Peter.

[jdobbs]

This note is just to keep the terminology straight for anyone reading this. Instead of saying "three P frames followed this I frame"... they should be referred to as "pictures" -- and the picture could be either a field or a frame. But, as I said, the vast majority of DVD sources are frame based anyway.

[TomBrooklyn]

What does GOP mean? What is an I-Frame and P-Frame etc.? Is there something I can read that outlines this whole thing?

[DK]

the abbreviation GOP stands for GroupOfPictures and is considered the basis of mpeg-2

gop contain i-, p- and b-frames

c.f. here for more:
http://en.wikipedia.org/wiki/MPEG-2