Gop Structure, What is it???

[Frumble]

Hi,

I have read ChickenMans Tutorials, and came across "Gop Structure", mentioned and altering it 5 for ntsc and 4 for pal...


Ok the actual template that comes with tmpgenc actualy sets this figure to ten!

so I know how to knock out great SVCD and VCD's but now i really want to understand what I am doing. So could somebody nice please explain with a little detail what GOP structure in the MPEG or DAT actually is and why we are altering it in the template, as per the tutorial? I have noticed bigger GOP, bigger file size....

after all understanding is the key.....


Thanks in advance Guys

 

[mb1]

GOP means "Group of pictures" which means I-, P- and B-Frames which are part of a normal mpg-stream.

Only I-frames (Intra-Frame) contain complete picture informations to restore and therefor are the biggest in size.

P-frames (predictive frames) always refer to an I-frame or a P-frame and are using their motion compensated prediction. So compression is quite good in comparison to I-frames.

B-frames (bidirectionally predictive frames) use both past and future I- or P-pictures for motion compensation, and offer the highest degree of compression.
To enable backward prediction from a future frame the coder re-orders the pictures from natural display order to 'transmission' (or 'bitstream') order so that the B-frame is transmitted after the past and future pictures which it references.
This introduces a delay which depends upon the number of consecutive B-frames.

For example if you have the following GOP-order for display
I1 B2 B3 P4 B5 B6 P7 B8 B9 I10 ...

the transmission order is
I1 P4 B2 B3 P7 B5 B6 I10 B8 B9 ...

Hope that helps a bit

 

[ChickenMan]

Well done mb1, good post.

GOP is a difficult one understand and I'm sure I dont fully understand it either. The TMPGEnc standard template for VCD is 1 5 2 while for SVCD they now say 1 10 2.

For a PAL VCD is suggest dropping the P gop down by one to increase the I gops for a stighlt better pic. File size would actually be a bit bigger this way but that does not influence the ammount that can be fitted to a CD really.

As for SVCD, I would recommend the DVD2SVCD program using CCE as the encoder using 3 or 4 pass VBR (TMPGEnc cannot do this, 2 pass only) for supperior result. I am in the middle of a Tutorial explaining all this actually. Hopefully wont be to long,