Cannot author with 24p assets from TMPGEnc Xpress 4

Question

I'm working on a project that involves the creation of 5 exercise-type DVDs. Each DVD contains from 2 to 5 programs (29.97), a copy warning, series intro (the same is used on each DVD), a single motion menu, and an extra feature (23.976). It's the extra feature that's giving me fits...
As there is a great deal of content on each DVD (two of them are 3+ hours), I'm having to use 2-pass VBR encoding, and some relatively low bitrates. Where I thought I could save a little space is on the extra feature, which is a 15 minute interview that will be included on each disc. The interview was shot in 24p anamorphic DV, and edited as such. I've not been happy with the quality of the encode from AME--used 2-pass VBR at about 3Mbps target--so I picked up TMPGEnc Xpress 4, and encoded using that. Ahh... much better. So far, so good.
The problem comes during authoring. I import all my assets, set up my menus and timelines, go about linking and checking end jumps, and everything is fine on preview. I begin the authoring process, and all is well and good until the very end, when the authoring fails and I get a generic "File is not valid MPEG". Some sleuthing has helped me determine that it is, indeed, the 24p file from TMPGenc.
I found that if I burn the 24p file to a disc without menus and set it to autoplay, it plays fine. However, when I link to the file from a menu, I get the error during authoring. The VIDEO_TS folder structure is built (I've just been testing to the hard drive), but once that track is hit, everything comes screeching to a halt.
Now, a little more info: I've compared the file structure of the M2V produced by AME, and the one produced by TMPGEnc--and there are some differences. Most notably, the AME version's N frame value is 12, whereas the TX4 version's N frame value is 13 (!?!). Something definitely doesn't seem right, here. Also, I've tested TX4 with CBR single-pass and ended up with the same error in Encore. And another thing: the main programs mentioned above were encoded using TX4, CBR single-pass (this is the smallest disc, and I could get away with CBR), and those files author perfectly fine.
Has anyone been able to successfully encode and author with a TMPGEnc'ed 24p MPEG-2 file? Or should I just give up and accept lower quality output from AME? I welcome your advice.

Answer 1

Hey Jeff--thanks for the reply. Here's your answers:
1) Yes, they all import as "Don't Transcode". The files are untouched by Encore, all the way through the Build process.
2) I tried both linking in the original project (with all the other assets), and in a new project that contained only the 24p file. I built a quick menu, linked to the 24p timeline, and hit Build... kablooie. Build failed, once again giving me the faulty MPEG error.
3) I checked all the files I've created in GSpot, and they all report the same information as you've described above. It's the GOP information that leaves me scratching my head. Observe:
AME CBR 1-pass
AME VBR 2-pass
TX4 CBR 1-pass
TX4 VBR 2-pass
AME produces results that I would expect--TX4 has those weird N values of 13 and 14.
4) My source footage is indeed NTSC DV, and is going to an NTSC DVD. What I had found in my earlier research (the source eludes me at the moment) was that for a true 24p MPEG-2 file, with pulldown flags inserted, is that the N value should be 12. The rationale was that you want 2 GOPS per second (24 fps / 2 GOPs per sec = 12). I'll have to find that article/site where I found that information--it made sense to me at the time. I do know that both the AME encoded files work perfectly with this value--the N values in the TX4 files (13 and 14) are not controllable when you use the DVD-Video preset. You can change to an MPEG-2 preset where you can change the GOP, but I haven't done this... although I'm considering trying that next.
Thanks for any insight you can provide!