I know nothing about I frames, but isn't it a bit of a coincidence that all the chapters are exactly one frame late?

Hey Phil,
I tried it a couple of times and they were a lot more than just one frame off. But it doesn't bother me now because I actually prefer setting them in DVDA.
Randy

Can't give proper credit, forgot where I dug this up, anyhow interesting.

MPEG Frame Types:

Sequences of MPEG video comprise GOPs. Each GOP (Group of Pictures) comprises video frames of three different types. These are:

I-frames (Intra coded frames) use DCT encoding only to compress a single frame without reference to any other frame in the sequence. Typically I-frames are encoded with 2 bits per pixel on average. Since the initial data comprises 4 bytes of Y, 1 byte of U and 1 byte of V (total 6 bytes = 48 bits) per pixel, this gives a compression ratio of 24:1. For random playing of MPEG video, the decoder must start decoding from an I-frame not a P-frame. I-frames are inserted every 12 to 15 frames and are used to start a sequence, allowing video to be played from random positions and for fast forward/reverse. Decoding of video can start only at an I-frame.

P-frames (Predicted frames) are coded as differences from the last I or P frame. The new P-frame is first predicted by taking the last I or P frame and 'predicting' the values of each new pixel. P-frames use Motion Prediction and DCT encoding.

As a result P-frames will give a compression ratio better than I-frames but depending on the amount of motion present. The differences between the predicted and actual values are encoded. Most prediction errors will be small since pixel values do not have large changes within a small area. The error values will therefore compress better than the values themselves. Quantization of the prediction errors further reduces the information.

B-frames (Bidirectional frames) are coded as differences from the last or next I or P frame. B-frames use prediction as for P-frames but for each block either the previous I or P frame is used or the next I or P frame. P-frames use Motion Prediction and DCT encoding.

Because B-frames require both previous and subsequent frames for correct decoding, the order of MPEG frames as read is not the same as the displayed order. This gives improved compression compared with P-frames, because it is possible to choose for every macroblock whether the previous or next frame is taken for comparison.
These frames are interleaved in a sequence such as IBBPBBP.. or IBPBPBPBP. The former is more difficult to encode but provides a higher compression ratio than the latter.

I'm sure its perfectly clear now why chapters start at a "I" frame. <wink>

Thanks for that - I've been trying to find a good description of I-Frames etc!!

So am I to assume it IS a coincidence that each of these I-Frames are exactly one frame away from wherever I stick a marker??! I find that hard to believe.

Does Vegas 'instruct' the MainConcept encoder to add I-Frames at the marker points? If not, would it be possible for it to do so?

More crucially, if I add a chapter point in DVDA (instead of V4) at a point where there ISN'T an I-Frame in the source MPEG-2, what does DVDA do about it??

I'll give it a try... but I don't see how that will solve the problem unless all scene changes are on exact (whole) seconds.

Am I missing something? One frame off?

1/30th of a second difference. How much of a problem can that really be?

Sounds pretty accurate to me.

If you open the custom settings you can fiddle with the frequence of the "I" frames and other stuff for the MC encoder. You're getting in the tall grass and unless you know up front WHY you would want to do it, and what changes you want to make, the best advice is don't.