How do I control GOP size in Womble MPEG-VCR?

[bamdvd]

I have Womble MPEG-VCR 3.14. In the “MPEG-VCR Save” dialog box, I select the “MPEG Video Elementary Stream (Video Only)” radio button, and then the “Video” button at the bottom of the dialog box. In the subsequent “MPEG Video Encoder Setup” dialog box, I select “DVD” button, “MPEG-2 Format”, 720/480 width/height, and 29.97 frame/sec.

(1) What do the N and M “GOP Size” parameters refer to? For example, I have a series of still JPG images that I want to string together in a chain, each JPG to have a duration of 12 frames per GOP (thus, 5 GOPs would correspond to two seconds) -- this is to match the 12 frames/GOP as recognized by TMPGEnc DVD Author in an existing MPEG2 file I have (TMPGEnc allows edits at GOP boundaries but not at frame boundaries).

(2) The still images won't have any audio when converting the JPGs to MPEG2 in Womble. Will that be a problem when I subsequently use TMPGEnc to append the non-audio MPEG2 to an existing DVD-Video MPEG2 which has both video and audio?

Thanks for any insights.

[FOO]

I think N is total frames in a GOP and N is
spacing of P frames. You could set it to something and
look at the video with VdubMod which shows the frame type

[bamdvd]

Thanks. Yes, after trial and error, it looks like N is the # of frames per GOP and M is the spacing for P-frames. The GOP structure also seems to depend on the Bit Rate Control option in Womble MPEG-VCR. If I set M=12, N=1, and Bit Rate Control to "constant quality (variable bit rate)" (instead of "constant bit rate") with max bit rate = 8900000, then the resulting GOP structure is 12 frames/GOP with one I-frame and 11 P-frames per GOP (which is what I want). So now with MPEG-VCR I can insert new GOPs in an existing MPEG2 file and maintain the GOP structure in TMPGEnc.

The next trick will be to figure out how to insert blank frames (#<12) within a GOP to serve as a breakpoint between two scenes in the original MPEG2 file that transition within a GOP (e.g., TMPGEnc can insert a chapter mark only at an I-frame).

With respect to audio, unfortunately it looks like when I insert non-audio JPG still images in an existing MPEG2 file with AC3 audio, it converts the audio stream to MPEG-1 Audio Layer-2 in the new MPEG2 file (I was hoping to maintain AC3 audio).

[bamdvd]

Update/confirmation based on information from Womble Tech Support:

N = GOP size
M = 1 + number of B-frames between consecutive I/P frames

Thus, for 1 I-frame and 11 P-frames per GOP with 12 frames/GOP, set N=12 and M=1.