GOP length questions.

[WolfPac_Ite]

I read a post by Adam here: https://forum.videohelp.com/viewtopic.php?t=222898

That says:

That sets the length of the GOP. When you encode to 23.976fps (with pulldown so it plays back at 29.97fps) you should use a GOP setting of N=3 N/M=4. This results in less frames per GOP and is required for DVD compliancy. When you encode to 29.97fps you can get away with a longer GOP length since you have more frames, so you can use N=3 N/M=5.

N=3 N/M=4 would still be compliant for 29.97fps footage, its just not as efficient as with the longer GOP setting. The longer the GOPs are the better the quality at the same bitrate, but of course the DVD standard only supports a maximum of 18 frames per GOP when using NTSC.

That had me wondering. I encoded a 23.976 FPS video in CCE (which was then pulled down after to play back at 29.97) using a setting of N=3 N/M=5 as per instructions on the Doom9 guide (I suppose he could have intended that for use on real 29.97 video...?). That ended up giving me "The video GOP is too long" message TMPGenc DVD Author, but as a test I went with it and it plays, so I know my players work with it.

My question is, if a higher GOP length (N/M=5 in my case) is used for 23.976 video, then does "The longer the GOPs are the better the quality at the same bitrate..." still pertain? I am not educated on GOPs, as you can see...

Is GOP length proportionate to the number of frames? If yes, then how so?

Not sure if I made sense with the above. If not I will reword it.

[manono]

Hi-

Use N=3, N/M=4 for 23.976fps encoded frames followed by pulldown. Use N=3, N/M=5 when encoding for 29.97fps. To get the number of frames per GOP, multiply N by N/M.

If you use N=3 N/M=5 for 23.976fps followed by pulldown, you create a non-compliant NTSC DVD. Why? Because the max GOP size is 36 fields. N/M=5 is already 15 frames=30 fields. Pulldown increases the number of outputted fields by a ratio of 5:4 (29.97/23.976=5:4=1.25). 30 fields x 1.25 = 37.5. So, you'll get some GOPs of as many as 38 fields or 19 frames. Too many. The fact that your player played it OK is coincidence, I guess. You do want to do it correctly, don't you?

I've read Doom9's guide many times before and also questioned his suggestion of keeping it at 5. So, either your guess is right, that he meant 29.97 encoded frames per second, or as a PAL guy, he just didn't know what he was talking about.

[WolfPac_Ite]

Thanks for the helpful explanation. I understand now.