GOP structure out of sync

[Chordmeister]

When capturing using a VideOh PCI, I've discovered after analysis of captured video, that the GOP seems to be out of sync.
Each GOP is 15 frames long (IBBPBBPBBPBBPBB), but the second GOP starts 13 frames into into the first one, and thus the rest are 2 frames out of sync. This means that GOPs aren't starting with I-frames, i.e from the second GOP onwards: BBIBBPBBPBBPBBP. Any ideas why its doing this?

[flaninacupboard]

the majority of MPEG encoders have the option to automatically detect scene changes. If a scene change is detected, it closes a GOP, complete or not, and starts a new one with an I-frame. A GOP can -only- start with an I-frame.
your observation is nothing to worry about, unless you need frame accurate chaptering, or multi angle DVD's, but i'm guessing that's not the case.

[Sulik]

It is not unusual for the first GOP of a sequence to only be 13 frames while the following GOPs are 15 frames.

The display order of a IBBPBBPBB GOP is BBIBBPBBP etc, but the first frames of a sequence can't use bidirectional prediction because there is nothing before, so you will usually see the first GOP having either:
- only 13 frames (first 2 B frames missing)
- broken_link set to 1
- closed_gop set to 1 (B-frames do not use forward prediction)

or any combination of the above.

Hope this answers your question.