What is GOP?

[smokingweed3000]

Hey
I have TMPGEnc 2.53 Plus, and I wanted to know what does the GOP does to the movie? And if it affects the quality then how do u optimize it for better quality?
Thankx

[Triffid]

GOP is Group Of Pictures, a key MPEG concept (so you can find out a lot be reading about MPEG).

In a tool like TMPGEnc, it allows you to set the frequency of the 3 frame types I, P and B.

As to how to optimise with this, I think it is generally a black art!

Having more P and B frames per I frame means more redundant information is removed and so more bit rate is available for the 'main' information. So, this can reduce blocking, if you are bit rate limited (as the quantisation levels should drop). However, nothing is perfect, and going overboard can produce distortions of it's own. So it's something to play with.

For DVD I believe there are imits to the total GOP length (15 for PAL if my memory is correct), but I believe you can do what you like with VCD/SVCD (although I've never tried being really radical). Most I've done is bump up the number for the B field ration by 1, and it did seem to help.

Got to catch a bus and go home now, so good luck!

[smokingweed3000]

can u explain that agian please? is the change in quality really worth the time of playing aroung with it? do u just bump up the B number by one or r u doing anything else to the GOP settings?
Thanks

[Triffid]

I'm not really sure it's worth any effort at all....but personally I like to play around with things.

Anyway, yes, just bump up the B number! (thats the most effective for reducing the amount of data...although there are many who feel that B frames aren't very good!)

I also set the scene changes manualy (but, using the automatic detection function) to "I frame, new GOP" (under the 'force picture type' settings). This, to me, suggests that I get 'I' frames where I need them, and I don't get too many references (P, and B frames) to unrelated scenes.

I like the results I get, but you need to play around with it and see if you feel the result is good or worth the effort, yourself.

(When I'm doing DVD format encoding, I stick with the standard GOP structure, but still use manual scene setting)

Have fun!

[kinneera]

Please, please use the search function of this forum...you will find many previous discussions, some of them fairly technical that should tell you everything you would ever want to know about this part of MPEG encoding. To get you started...

http://forum.vcdhelp.com/viewtopic.php?t=78056&highlight=gop

[SatStorm]

Inventing the weel by yourself is cool, but when I did it, I realise that I waste a lot of my encodings..... Don't do the same mistake!
The standard TMPGenc plus GOP stacture, produce the best possible quality.

With previus tmpgenc versions, non plus, there were some "tricks" to increase the quality. There are plenty of thems on this forum (old posts).

About the scene change: Manual detect is good for CQ_VBR. If you use 2 pass VBR, it is useless. The reason is simply: The first pass stamps the scene changes.
It is one more reason you don't have blocks with 2 pass VBR, like the way you have with CQ_VBR or CBR...

[Triffid]

SatStorm,

I agree, you can waste a lot of time (and sometimes get some bad results) playing around, but I think you also get a much better feel for what your doing (even if the conclusion is that where you started was really the best place to be). Besides, it's fun.

About the scene change settings, I use CBR. I have checked with the log function and it definately obeys the manual frame settings (at least related to GOP, I haven't tried many of the other settings, and I'm sure that some, such as setting different frame rates, probably only work with CQ_VBR or similar) I also truly believe it helps to improve the quality of the the finished product. As far as I can tell, in CBR mode, the 'auto scene detect' (or what ever it is called) does nothing - this can result in a new scene starting on a P or B frame, which can't be a good thing. You are probably correct that 2-pass VBR does this correctly, but I am impatient !

[SatStorm]

Ha ha ha ha ha

Well, there are so few things to discover, so let us re - discover them for fun!!! I know exactly what you mean!!!

Ha ha ha ha ha

Well, I never use CBR. I used to encode with CQ_VBR but after I discover that 2 Pass vbr on Motion estimate search mode need the same time as CQ_VBR at highest quality (very slow) mode, I never used CQ again!
The quality of 2 Pass VBR vs CQ_VBR is like day and night...
In the matter of fact, I suggest to use that option if your PC has SSE commands! For xSVCD/CVD is the best way!

For impatient people, CBR is probably the best solution. For this mode, manual detect scene is a must. The auto fuction do almost nothing!
But try once 2 Pass VBR... It is so coooolllll!!!!!