Smaller GOP Size

[olyteddy]

(no, not the US Senate)...
I'm gonna rip some old VHS movies I have and have a Hauppauge PVR USB2. The latest version of WinTV2000 lets me set the GOP to 6, 12 or 15. Two quick questions:
1) Will the smaller GOP significantly increase picture quality of my rip?
2) Will the smaller GOP significantly increase file size of my rip?
I'll probably encode as VBR 8,000 peak and run them thru DVD Shrink, if I have to.

[reboot]

The only way is to try it.
Smaller GOP can significantly increase quality of a recording, but for a VHS source, there's no point.
Leave it at 15.

[Scorpion King]

My 2cents worth....

A GOP structure consists of an I-frame, P-frames, and B-frames. A normal GOP looks like IBBPBBPBBPBBPBBPBB where there is 1-I frame followed by 2-B frames then a P-frame. The total for NTSC is 18, or 1:5:2 (1-I, 5-P, and 2-B frames per GOP). (Tmpgenc Plus DVD-template defaults). My ATI AIW defaults @ 1:4:2 or 15 frames per GOP, and the later versions of ATI MMC defaults @ 1:3:2, or 12 frames per GOP. The difference in visual quality in my experience is very small, if noticeable at all, between 18,15, or 12. You may be able to see a difference.

I frames compresses the least, followed by P frames, then B frames which uses the least amount of bits of the three. A video encoded with I-frames only requires a higher bitrate to retain the video quality due to the lower compression value of the I frame compared to the compression achievable using P and B frames.

In my opinion..... a shorter GOP (which results in more I-frames) will result in lower overall video quality if you are going to use the same bitrate for each GOP example. But the difference in quality may not be noticeable....

Also, you are better off if you use a bitrate calculator and determine the proper bitrate to use instead of guessing at the bitrate then transcoding (shrinking) the video later, which looses quality.

Good luck.

[olyteddy]

My thinking on using Shrink was that I'm using a hardware encoder, which by its nature has limited look behind and probably no look ahead and therefore can't totally optimize the encoding. DVDShrink, otoh, can look as far forward and back as it needs to optimize.

[SatStorm]

IMO, shorter GOPs with correct scene detection changes, helps on VBR encodings. CBRs won't benefit.

I always use short GOPs, because I might edit my mpeg 2's one day.