weird mpeg-1 muxing issue

[DeusExMachina]

I have two mpg files from a split VCD which I wished to join onto one DVD. The second file overlaps a few frames of the first, so I used MPEGstreamclip to cut the surplus, and then used ffmpegX to join the two files. Everything seemed to work great. Then I demuxed and imported into DVDSP and things went haywire. On playback, the file stutters until it gets to the join, and DVDSP crashes. Also, the video file is about 7 minutes longer than the audio, resulting in VERY bad sync issues.
I have tried everything I can think of, doing the join in MPEGStreamClip using cut and paste, demuxing in MPEGStreamClip, joining in mpegtx, joining in QT…. I have used every app I have at my disposal with every combination of functions, and no luck.
Also, I noticed that when I try to play the joined file in QT it plays fine, but when I play it in VLC it flashes black every few seconds. Not sure if this is even relevant. Perhaps they are added black frames and that is the cause of the sync issue?
Any clue what is going on?

[tbluewater]

I cannot tell what you are trying to accomplish. It seems that you are using the word 'demuxing' wrong. Joining two video clips means that you put one video clip after the other in a single track. This procedure is not muxing. Therefore, splitting them is not demuxing. In the meantime, it sounds like what you are trying to do is put one video clip in one track and another clip in a different track so that you can re-encode the entire timeline to make a single video clip.

[DeusExMachina]

The situation is very straight forward. I have two mpg files I wish to join. since the beginning of the second file overlaps the end of the first by about 15 frames or so, I want to edit the second prior to the join, which I have done. DVDSP does not accept the finished mpg, which is often the case, so I need to break it into its elementary streams, thus I demux to m1v or m2v and mp2. Whereas the muxed file plays just fine in QT, the demuxed parts play unsynced in DVDSP, and the file length is over 7 minutes apart. Also the muxed file plays oddly in VLC, and the demuxed streams crash DVDSP when it reaches the join. Obviously there is something screwed up in the header of t he first file.
In addition,I have a number of other VCD files that I would like to get to DVD without re=encoding, but sometimes DVDSP refuses to accept them, whether they have been demuxed or not.
Hopefully this is clearer. I am using the terms mux, demux, and join appropriately.

[DeusExMachina]

Well, further investigation implies that this may be a GOP issue in the second file. Stillworking on it.

[tomlee59]

I don't know if the following will help, but it works for me every time. Not every step is absolutely necessary, but if you follow the procedure, you should enjoy the same successes I do.

First, StreamClip is a *great* app. When using it to join two files (and, of course, the two files must have the same framerates, bitrates, and other attributes), make sure that you have cut the files only at keyframe boundaries (use cmd-K to locate them). If you don't, then you screw up the GOP sequence, which will be flagged as a data break, and probably crash the next app to use the merged file. Re-authoring is then necessary to fix things up. If you don't mind making your cuts with the crude resolution set by keyframe locations, you can avoid re-authoring. I'm too impatient to re-author, so I generally live with the coarse editing resolution this method produces. I generally trim some off the end of the first file, as well as trim the beginning of the second file, ensuring that the splice truly occurs at a proper keyframe boundary.

After joining the files, make sure to fix timecode breaks, then save the resulting fixed, merged file.

Next -- and this is not always necessary, but I always do it anyways -- demux, then remux. Doublecheck A/V sync throughout, and especially before and after the splice point.

The resulting file should be a standards-compliant MPEG1 file. At that point, it should provoke no problems different from any other mpeg1 file.

[DeusExMachina]

well, I also did it with SimplemovieX,which ONLY cuts at keyframe boundaries, and mgptx, so I think that may not be the full issue. Just as a check, instead of just cutting the extra bit off the one file, I cut half the difference off both, cutting at keyframes and joining. After importing, I was met with the same issues as before.

Also, any idea why certain VDC compatible files will not import in to DVDSP and how I might fix this, absent re-encoding, which I am loath to do?

[tomlee59]

I haven't used DVDSP before, but I'm curious how it handles the audio resampling part of the re-authoring process. Since VCD encodes audio at 44.1ks/s, while DVD needs it at 48ks/s, resampling has to take place somewhere. The 7 minute discrepancy you mention is suspiciously close to the ~10% error you'd expect if you had a typical (shortish) movie, and the audio wasn't resampled. The samples would be played back too fast, resulting in the video outlasting the audio.

As I said, I've never used that tool, so sorry if this is off base. But if you haven't looked at the audio resampling question, that's probably what I'd do next at this point.

[DeusExMachina]

Actually, the audio was already in 48k mp2, so that was not the issue. Also, I did try resampling into AC3, but still had the same problem.
The files work fine if I leave them separate and just make the DVD autojump from the first clip to the second. It is only if I try to join them and then import them that the problem arises. They joined file plays fine in QT, has occasional black frames (every few seconds or so) on playback in VLC, but crashes DVDSP at the join, or loses sync if one starts playing after the join. All very odd.

[tomlee59]

Since you were joining VCDs, the original audio should have been sampled at 44.1ksamp/s. I assume that you resampled the audio stream in a separate operation to get it to 48ksamp/s? What tool did you use to do that?

And does DVDSP contend with the combination of VCD-compliant video, but non-VCD-compliant audio? If it's not well-written, it might not handle this case very well. I still can't get over the seeming numerical coincidence implied by the 7 or so minute discrepancy (about what you'd expect from a resampling problem for a ~90 minute movie). Just out of curiosity, what is the duration of the video stream?

[DeusExMachina]

No, I think the file always had 48k audio. Although non-standard, many players that are K(S)VCD compatible accept it. Also, I am pretty sure the problem is not the audio. First, it plays fine when muxed, second, the weird video artifacts when the demuxed/remuxed files are played in VLC make me suspicious of the video stream.
DVDSP does not care if the video is VCD compliant or if the audio isn't. It wants things to DVD spec, and if it is not, can use Compressor to convert. Compressor is VERY slow, however, and makes very poor quality files at low bit-rates, as it is optimized for professional DVD generation, and so I chose not to use it for these archiving tasks.
Plus, everything works fine if I just don't join them. I link the one track to the second track, so the DVD player just plays them sequentially.
Both streams are approximately 1:08:00.
At this point I have pretty much given up, and have moved on to my other issue, which is why certain VCD compliant files won't load while others will. I would rather not resample unless I have to, since the low bit-rate of the VCD files means that there is not enough data to make a decent quality file. Suspicious it is a GOP problem (Damned Republicans! .)

[tomlee59]

Sorry I couldn't help! But if you ever figure out the problem (and solution), do post back. I'm sure I'll learn something very useful from your pain and suffering.

[DeusExMachina]

Well, part of the solution, for me, involved giving in to the inevitable and resampling. I think part of the problem is that the DVD spec is far more unforgiving than the VCD spec. And whereas many players (including DVD Player) will play nonstandard spec video, DVDSP will not accept it. So while a VOB in a Video_TS from a 320x240 source will play in DVD Player, it will not import into DVDSP unless letterboxed to spec-compliant 352x240. All this is just speculation, for the moment, however. This also relates to GOP. Certain GOP formats are not copacetic for DVD, but work just fine in VCD. Without any utility to investigate GOP formats in video files, however, it is impossible for me to investigate further.