GOP standards and Intra-Only Encoding, 'mjpeg' style Mpeg2

[video_magic]

Hello, I'd be grateful if anyone could advise me and correct me if I have got any of this wrong!

From what I have read, each GOP can have only one I Frame, the rest of the GOP will be made up of P frames and maybe also B Frames.

An I Frame is an 'Intra Frame' which is encoded entirely from the video frame of the movie, whereas the P frames and B frames are encoded from other frames and are probably going to be less picture quality than I frames.

Each GOP can only have one I frame, but there is an option with some Mpeg2 encoders to have Intra-Only encoding. This could give higher quality encoding if one were prepared to do a high-bitrade encode. I guess this could be MJPEG style with every frame being encoded from the source frame.

But here is my query. What is the valid minimum GOP size for Mpeg2 if I wanted to be compliant for DVD? I was trying Nuenc where I put 1 for GOP size and it said 'please select an integer between 3 and 50'. So I then thought perhaps 3 is the minimum GOP allowed for the specs, in which case I would not be able to do an Intra-only encode for my mpeg2 for DVD (because of only being allowed 1 I-Frame per GOP).

Also, is it better to close GOP to be DVD compliant? I read somewhere that it was only for Multi-Angle DVDs that it was really required.

I ask these questions in particular because I am trying to encode a Wedding Video to DVD as a quite precious (to us) archive copy, and I want it to be done as good as I can, so we won't have issues trying to play it in the future. It is quite important for us to preserve the quality of the original picture as much as possible and pause it at frames.

Thanks very much for any advice

[mean]

If you want to stay within DVD spec you can't play with gop size
From memory it is 15 for Pal and 18 for NTSC
and 2 bframes are required.

Furthermore, dvd has a max bitrate of ~ 9.8 Mbs
Doing intra only will break that

[mean]

If you are not satisfied with the quality of encoding, it is probably a problem of interlacing.

[video_magic]

so GOP 15 and B-frames 2, thanks again mean, that is great info :thanks:

I had just been playing around with different things to see if I can 'tweak' extra quality :) , but I want it to all be as compliant as possible too so there won't be a problem playing the DVD disc.

Because of wanting to pause the video at different frames, that is why 'intra only' was an attractive idea to me, and I have read in a few places that B-Frames can result in lowered quality, like here
http://forum.doom9.org/showthread.php?s=&threadid=88268 which is why I was worried, but I suppose high bitrate encoding will lessen that problem I am doing about 7200

[stephanV]

first of all: ignore that thread, it does not apply to your situation. that guy is decimating his frame rate which indeed may lead to lowered quality if you use b-frames (the temporal difference will be bigger). also, he is using a decoder that cannot deblock b-frames but can deblock p-frames, which gives very annoying results.

secondly: I-frame only is not a solution for back-up even if it were DVD-compliant. I-frames are very bit consumptious frames and are in principle not any better than p-frames or even b-frames (but they have a higher quant by nature, so thats not a fair comparison).

I-frame only will normally give very bad results, unless you use a really high bitrate.

O yeah, pausing the video at any moment has nothing to do with I-frame only or not. You should be able to pause at any frame in a file. Only seeking to p and b frames is slower, but not impossible. Having said that, the GOP-size is so small in MPEG2/DVD that you probably cant tell the difference.

[ronnylov]

The maximum GOP size is 15 for PAL and 18 for NTSC to be DVD compliant but smaller GOP size is allowed. For instance is the default GOP size 12 in many encoder softwares. And some encoders can break a GOP and put an I-frame at scene changes. I don't know if B-frames are required but I have no problems making DVD with a GOP without B-frames and it works in my player. I have tried IPPP and it worked but I did not really gain any quality even at 8000 kbit/s mpeg2.