GOP settings.......need input from you guys......

[wildcatfan]

OK well I have been messing around with the GOP settings in Tmpgenc 12a a bunch today and have come across something that seems to work well. I have been testing on a high action scene from "The Rock" (beginning of the car chase scene in case anyone wants to try it out). After using the default of IBBPBBPBBPBBPBBPBB I was getting what seemed to me a bunch of blocks so I started fiddling. I went through the encoded scene frame by frame comparing I to P to B frames and noticed that the B frames were by far the worst block wise (of course since they are the most compressed). First of all I reduced the GOP to IBBPBBPBBPBB to get closer to what the VCD standard calls for of an I frame every ~.5 seconds (I am doing NTSC Film, so 23.97 fps). Then I added a B frame to get IBBBPBBBPBBBPBBB. Looked worse, so I went to IBBBPBBBPBBB to get back close to the "optimal" sequence of 12 frames (once again ~.5 sec). Not much change, still seemed to look worse than the default. So then I decided to remove B frames. That game me a seq of IBPBPBPB and tried it. Slightly better but less than .5 second per GOP (although shouldn't really be a problem I guess). Then I tried IBPBPBPBPBPB (once again back to ~.5 sec) and voila! Blocks seemed reduced without noticeable effect on the crispness of the P or I frames. So now I have a question, well 2 actually.

1) Has anyone tried any different combinations of IPB frames and found something that seems to work well?

2) (more important) For the experts, Will this still be a valid mpeg stream for making a VCD or does it even matter? I assume all I frames or I and P only is bad (not to mention looks like crap at vcd bitrates) but is using only a single B frame bad for compliance? I ask bcuz I always see 2 B frames in every encoder and wondered that was the only way to have a "compliant" vcd mpeg stream.

Thanks for any responses.....

[kinneera]

There is a lot of theory to the selection of GOP sequences. Apparently, the very inclusion of B frames was somewhat controversial. In any case, what you have discovered is certainly reasonable: B-frames (average of forward and reverse prediction based on the surrounding frames) causes the most image spoilage, while providing the best compression. Reducing the number of B-frames will produce better quality, in theory, but will result in fewer bits being available for I and P frames, assuming an unchanged bitrate. You've probably found a reasonable sweet spot for your video, but it may vary from one to another. In any case, there isn't any real "standard" for the GOP format, assuming the MPEG decoder is properly written.

[glinckens]

Great work wildcatfan.
Don't worry about getting close to ~.5 sec per GOP, the VCD standard doesn't ask for a specific GOP length. I encode VCD's using a GOP length of 30 frames using my PV-231 capture card and it plays back perfectly on all DVD players I tested.
For the use of only I, B or P frames: it depends very much on the player you're planning to use. I don't think all DVD players accept a VCD with only I-frames. Mine does (apex 600a), and it plays every GOP structure I make.

I tried the following settings for the GOPs:

IBBB : nice for very high motion but not good for slow camera panning.
IPPPPPPPPPPP : gives surprisingly good quality, dont know about this one yet.
IBBPBBPBB.... (30 frames from I to I) : best quality I get so far.

[glinckens]

Great work wildcatfan.
Don't worry about getting close to ~.5 sec per GOP, the VCD standard doesn't ask for a specific GOP length. I encode VCD's using a GOP length of 30 frames using my PV-231 capture card and it plays back perfectly on all DVD players I tested.
For the use of only I, B or P frames: it depends very much on the player you're planning to use. I don't think all DVD players accept a VCD with only I-frames. Mine does (apex 600a), and it plays every GOP structure I make.

I tried the following settings for the GOPs:

IBBB : nice for very high motion but not good for slow camera panning.
IPPPPPPPPPPP : gives surprisingly good quality, dont know about this one yet.
IBBPBBPBB.... (30 frames from I to I) : best quality I get so far.

[wildcatfan]

After a little more trial and error I have come to the conclusion that IBPBPBPBPBPB seems to work well for wide screen formats, while IBBPBBPBBPBBPBBPBB seems to work better for full screen. The larger screen area seems to need too many bits and so all frames get a tad worse. Also for very still areas the latter is better too. So my conclusion is for widescreen fast paced movies the 1st selection works well, for slower paced or for full screen the default in tmpgenc works better.....

<font size=-1>[ This Message was edited by: wildcatfan on 2001-12-04 01:17:23 ]</font>

[wildcatfan]

Now I am thinking I just may start doing my movies in sections. Kinda like my own mpeg1 version of the high and low motion divx codecs. Use the IBPBPB... version for the high action scenes and the default for the low action scenes and splice em together at the end....