TMPGEnc and GOP structure

[Internaut]

My understanding of a GOP, is that it is, by definition, a group of frames, commencing with an I-frame, and terminating with the frame immediately prior to the next I-frame. It therefore follows that there can be only one I-frame per GOP.

What, then, is the option in TMPGEnc Plus :-

Setting -> GOP Structure -> "Number of I picture in GOP" ?

Sure you can set it to 2 or 3 etc, and it draws a GOP sequence in the "Gop structure" window at the top of the pane, but if you encode with more than one I-frame, and then go and look at the result in Bitrate Viewer, there is still only one I-frame per GOP.

For example, if you set "Number of I picture in GOP" = 2, then with P/B = 3/2, TMPGEnc indicates IBBPBBPBBPIBBPBBPBBP, but BRV indicates two GOP's IPBBPBBPBB, IPBBPBBPBB, which makes more sense.

My questions therefore are :-
1) What is the meaning of 2 or more I-frames per GOP ?
2) Why does Bitrate Viewer report a different frame order within the GOP ?

[Kika]

1. You are right and wrong. A standard GOP always uses only 1 I-Frame. But MPEG2 requires Sequence-Headers for each GOP, and if you set I-Frames per GOP bigger than 1 it is similar to one Sequence Header any second GOP.

2. There are two Ways to Show the GOP-Structure. IBBPBB... is the "true" structure, IPBBPBBP is the way, a GOP has to be decoded to show you the Video.