I never really messed around with frames but I have noticed this:

When capturing mpeg2 in MMC 8.1 the default is:
P frames = 4
B frames = 2

My questions is, should the frames in the TMPGEnc templates match this? Because they don't match at all.

In fact, in the SVCD template the frames are:
P frames = 10
B frames = 2

Should I mess with this, or am I opening a whole can of worms?

Last: Is it an advantage or dis-advantage to capturing with I frames only? I have played with this and I think the quality is worse then with default.

Thanks,
gammer

They are Different in Mpeg Captures, because the Gop Length is Different In Captures..A smaller GOP is easier to Compress and Capture Programs needs to be able to compress the frames as fast as they can, but the Quality of the compression is not as good.. So the answer is No your Tmpgenc"s Gop should not be the same as your MMC Gop if you want to retain Quality...

MMC is not wrong. If your TMPGENC template has 10 P-frames and 2 B-frames per P-frame, it will not be DVD compliant.

Total frames in GOP = (P+1)*(B+1) = 15 for P=4, B=2
With P=10 and B=2, this is (10+1)*(2+1) = 33 frames in a GOP, which is much higher than the limit for most standards which is up to 18 frames per GOP for NTSC (I believe VCD and SVCD have the same limit).

I frames is similar to jpg images. P frames is showing the difference only in the picture since the last P or I frame. B frames contain motion vectors describing how parts of the image moves and can be used to calculate a new image if you have all the frames in the GOP. A GOP is a Group of pictures with I P and B frames. Normally there's only one I-frame in a GOP.

DVD compliance requires maximum 15 frames in a GOP for PAL and max 18 for NTSC. But this limit is only for DVD. It is possible to create longer GOP's in VCD and SVCD but you can't use that mpg with long GOP's to create DVD later without reencoding. I recommend to stay within this limits for compliance reasons.

An I-frame needs more bits than a P frame which needs more bits than a B-frame.

A long GOP with many B-frames requies less bitrate than a short GOP with few or no B-frames. So a general rule is to use longer GOP with more B-frames when encoding at a low bitrate and to use a shorter GOP with a high bitrate.

It is not strange that you get bad results when using only I-frames. The I-frames requires much bitrate and even at 15 Mbit/s the quality will suffer when using only I-frames. You could try something like IPPP or IBPBPB if you want a short GOP at a high bitrate.

The default GOP in MMC is good enough in most cases, especially at DVD bitrates. It is not necessary to use the same GOP structure when reencoding with TMPGEnc.